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Possible Role of Gamma Ray Bursts on Life Extinction in the Universe

December 2014
Physical Review Letters 113(23):231102

DOI:10.1103/PhysRevLett.113.231102

Source
PubMed

Authors:

Tsvi Piran

Hebrew University of Jerusalem

Raul Horacio Jimenez

Autonomous University of Chile

As a copious source of gamma rays, a nearby galactic gamma ray burst (GRB) can be a threat to life. Using recent determinations of the rate of GRBs, their luminosity function, and properties of their host galaxies, we estimate the probability that a life-threatening (lethal) GRB would take place. Amongst the different kinds of GRBs, long ones are most dangerous. There is a very good chance (but no certainty) that at least one lethal GRB took place during the past 5 gigayears close enough to Earth as to significantly damage life. There is a 50% chance that such a lethal GRB took place during the last 500×10^{6} years, causing one of the major mass extinction events. Assuming that a similar level of radiation would be lethal to life on other exoplanets hosting life, we explore the potential effects of GRBs to life elsewhere in the Galaxy and the Universe. We find that the probability of a lethal GRB is much larger in the inner Milky Way (95% within a radius of 4 kpc from the galactic center), making it inhospitable to life. Only at the outskirts of the Milky Way, at more than 10 kpc from the galactic center, does this probability drop below 50%. When considering the Universe as a whole, the safest environments for life (similar to the one on Earth) are the lowest density regions in the outskirts of large galaxies, and life can exist in only ≈10% of galaxies. Remarkably, a cosmological constant is essential for such systems to exist. Furthermore, because of both the higher GRB rate and galaxies being smaller, life as it exists on Earth could not take place at z>0.5. Early life forms must have been much more resilient to radiation.

The fraction of stars in the Galaxy within a distance d form Earth for an exponential disc with scale-length of 2.15 kpc (left y-axis) as a function of energy of a life-extinction GRB for different locations of the life harbouring planet (2.15, 4, 8.5 and 16kpc). Also plotted (right y-axis) are the luminosity functions as a function of energy for LGRB and sGRB.

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Draft version September 10, 2014

Preprint typeset using L

X style emulateapj v. 5/2/11

ON THE ROLE OF GRBS ON LIFE EXTINCTION IN THE UNIVERSE

Tsvi Piran1and Raul Jimenez2,3

Draft version September 10, 2014

ABSTRACT

As a copious source of gamma-rays, a nearby Galactic Gamma-Ray Burst (GRB) can be a threat

to life. Using recent determinations of the rate of GRBs, their luminosity function and properties of

their host galaxies, we estimate the probability that a life-threatening (lethal) GRB would take place.

Amongst the diﬀerent kinds of GRBs, long ones are most dangerous. There is a very good chance (but

no certainty) that at least one lethal GRB took place during the past 5 Gyr close enough to Earth as

to signiﬁcantly damage life. There is a 50% chance that such a lethal GRB took place during the last

500 Myr causing one of the major mass extinction events. Assuming that a similar level of radiation

would be lethal to life on other exoplanets hosting life, we explore the potential eﬀects of GRBs to life

elsewhere in the Galaxy and the Universe. We ﬁnd that the probability of a lethal GRB is much larger

in the inner Milky Way (95% within a radius of 4 kpc from the galactic center), making it inhospitable

to life. Only at the outskirts of the Milky Way, at more than 10 kpc from the galactic center, this

probability drops below 50%. When considering the Universe as a whole, the safest environments for

life (similar to the one on Earth) are the lowest density regions in the outskirts of large galaxies and

life can exist in only ≈10% of galaxies. Remarkably, a cosmological constant is essential for such

systems to exist. Furthermore, because of both the higher GRB rate and galaxies being smaller, life

as it exists on Earth could not take place at z > 0.5. Early life forms must have been much more

resilient to radiation.

Subject headings: galaxies:evolution — galaxies: statistics — galaxies: stellar content — gamma-ray

burst: general

1. INTRODUCTION

Gamma Ray bursts (GRBs), short and intense bursts

of γ-rays, are the brightest explosions known. The co-

pious ﬂux of γ-ray photons with energies above 100keV

from a galactic GRB could destroy the ozone layer mak-

ing them potentially damaging to life on Earth. This has

led to the suggestion4(Thorsett 1995; Dar & De Rujula

2001; Scalo & Wheeler 2002; Melott et al. 2004; Thomas

et al. 2005a,b) that events of massive life extinction were

caused by galactic GRBs. This issue depends of course

on the rate of galactic GRBs in the Earth neighborhood.

Once it was realized that long GRBs are preferentially

located at low-metallicity environments it was claimed

(Stanek et al. 2006) that nearby Galactic GRB are rare

and GRBs are unlikely to play any role in life extinc-

tion on Earth (see however, (Melott 2006) who claims

that metallicity won’t protect life on Earth from GRBs).

Given the recent signiﬁcant progress in quantifying the

main ingredients that determine whether GRBs have any

eﬀect on Earth: their rate, luminosity function and de-

pendence on metallicity it is therefore timely to re-asses

this issue, extending the discussion to GRBs eﬀects on

life in the whole Milky Way and in the whole Universe.

GRBs are traditionally divided in two groups accord-

ing to their duration: long (>2s) GRBs (LGRBs) and

short (<2s) GRBs (sGRBs). This division follows to a

1Racah Institute of Physics, The Hebrew University,

Jerusalem 91904, Israel; tsvi.piran@huji.ac.il

3ICREA & ICC, University of Barcelona, Marti i Franques 1,

Barcelona 08024, Spain; raul.jimenez@icc.ub.edu

4Institute for Applied Computational Science, Harvard Uni-

versity, MA 02138, USA.

4See (Ruderman 1974) for an earlier discussion of nearby Su-

pernovae as the cause of life extinction.

large extend5the origin of these events. LGRBs are asso-

ciated with the death of massive stars (see e.g. Woosley

& Bloom 2006, for a review) while sGRBs have a diﬀer-

ent origin, most likely compact binary mergers (Eichler

et al. 1989). Recently, it was realized that there is a third

group characterized by low luminosity (L≈1046−48 erg

s−1) and denoted llGRBs. These events are also asso-

ciated with the death of massive stars, but they origi-

nate from a diﬀerent physical mechanism (Bromberg et

al. 2011). A fourth type of a related explosion - giant

SGR ﬂares might also relevant. Such a ﬂare took place

in the Milky Way on 27 Dec 2004, releasing ≈4×1046

ergs (Palmer et al. 2005). This ﬂare, that was suﬃciently

powerful to disturb the Earth magnetosphere, is the only

known object outside the solar system to have a direct

clear impact on Earth. Giant SGR ﬂares are diﬀerent

phenomenon than GRBs but as their rates could be as

high as once per thirty years in the Galaxy we explore

their possible role as well.

Wanderman & Piran (2010) have recently recon-

structed, in a model independent way, the rate of LGRBs

as a function of redshift and their luminosity function.

One of their most interesting ﬁndings is that the LGRB

rate is not reproduced by the star formation rate of the

global galaxy population. This discrepancy is statisti-

cally highly signiﬁcant, particularly at low (<3) red-

shifts, which is relevant here. This is, at ﬁrst, surpris-

ing as there is ample evidence that long duration GRBs

originate from the collapse of very massive stars and one

would expect that LGRB follow the SFR. Jimenez &

5We note in passing that some GRBs that are shorter than

2s do arise from collapsing massive stars Bromberg et al. (2013).

However this is unimportant for this work.

arXiv:1409.2506v1 [astro-ph.HE] 8 Sep 2014

Piran (2013) have shown that the LGRB rate and the

galaxy derived SFR agree for a special class of galaxies:

low mass (stellar mass <1010 M) and low metallicity

(∼

<1/10 solar). This is, of course, done in a statisti-

cal sense and does not exclude that few outliers to this

trend exist. But it is clear that the LGRB host popula-

tion is a special subclass of the general galaxy population.

These results are in agreement with earlier observations

that indicate that LGRBs take place in dwarf (Natarajan

et al. 1997), low metallicity (Fynbo et al. 2003) galax-

ies. They are also consistent with direct observations of

LGRBs host metallicities (e.g. Savaglio 2013; Levesque

2014; Cucchiara et al. 2014) and with the ﬁndings of

Fruchter et al. (2006) who have shown that the local SFR

in the vicinity of LGRBs is much higher than expected

if they simply follow the SFR (see also Svensson et al.

2010).

Short GRBs have very diﬀerent host environments and

they clearly arise from diﬀerent progenitors (see e.g.

Nakar 2007; Berger 2013, for reviews). They are sig-

niﬁcantly weaker than LGRBs and as such are observed

to much shorter distances than LGRB. sGRBs are be-

lieved to originate in compact binary mergers (Eichler

et al. 1989) but a direct proof for that is still lacking. As

sGRBs are weaker, fewer GRBs have been observed than

LGRBs. However their current overall rate is about ﬁve

times larger than the rate of LGRBs. In the following we

use a recent determination of the sGRBs global rate and

luminosity function by Wanderman & Piran (2014), (see

Cohen & Piran 1995; Ando 2004; Guetta & Piran 2005,

2006; Nakar et al. 2006; Guetta & Stella 2009; Coward

et al. 2012; Siellez et al. 2013, for earlier work).

llGRBs are signiﬁcantly weaker with energies of

1047−49erg (as well as smoother and softer) than both

LGRBs and sGRBs. Like LGRBs they are associated

with the death of massive stars but they arise due to a dif-

ferent physical mechanism (Bromberg et al. 2011). While

less than half a dozen llGRBs have been observed so far

they are more numerous than both LGRBs or sGRBs

(Soderberg et al. 2006). Because of their low luminosi-

ties they are observed only up to relatively short (but

still cosmological) distances.

We use the very recent determination of GRB rates

and luminosity function to estimate the ﬂux of Galactic

GRBs on Earth and compare it with the ﬂux needed

to destroy the ozone layer. Given that LGRBs are the

most powerful and hence most dangerous, and given their

dependence on metallicity we begin with an exposition

of the Milky Way metallicity distribution. We continue

estimating the life threatening eﬀect of LGRBs, turning

later, using the same formalism to sGRBs, llGRBs and

giant SGR ﬂares. We conclude summarizing the results

and their implication to life extinction on Earth. We also

explore the implications to life extinction on exoplanets

elsewhere in the Milky Way and in the whole Universe.

2. THE MILKY WAY METALLICITY DISTRIBUTION

LGRB rate estimates derive the expected rates of

LGRBs per unit volume per unit time. When trans-

lating this volumetric rates to event rate per galaxy and

more speciﬁcally to the rate within the Milky Way, one

has to consider the type of galaxies in which the events

take place. Our earlier analysis (Jimenez & Piran 2013)

shows that LGRB hosts are dwarf low metallicity galax-

Figure 1. The percentage of stars as a function of metallicity in

the Milky Way disk with ages 1 <Gyr <5 (solid orange line) and

with ages <1 Gyr (solid black line) as obtained by Casagrande

et al. (2011). The distribution of LGRBs metallicity as obtained

by Jimenez & Piran (2013) from matching the RGB global rate

to the global star formation rate of galaxies (solid green line) and

that from direct metallicity determinations of LGRBs (dashed line)

(Savaglio 2013) and Cucchiara et al. (2014) from DLA (solid red

line). The overlap between the LGRB and Milky Way stars distri-

butions is only at the few % level.

ies that are very diﬀerent from the Milky Way. There

are outliers and some LGRBs has been found in higher

metallicity galaxies (Savaglio 2013; Levesque 2014).

Casagrande et al. (2011, and references therein) de-

termine the ages and metallicities of stars in the Milky

Way disk. Fig. 1 depicts the percentage distribution of

stars in the Milky Way for ages <1 Gyr (solid black

line) and stars older than 1 Gyr but younger than 5 Gyr

(solid orange line). Stars that are older than the Sun

and that therefore trace the chemical conditions of the

star forming gas at earlier epochs are not relevant for the

question of life destruction on Earth. In the same plot we

also show (solid green line) the percentage distribution of

LGRB hosts derived from Jimenez & Piran (2013) using

the mass metallicity relation from Panter et al. (2008).

Note that due to the metallicity bias for the LGRB host

galaxies, there is very little overlap with the distribution

of stars in the Milky Way disk. In fact they only overlap

at the 10% level.

Also shown in Fig. 1 are the distributions of LGRB

hosts with direct metallicity determinations (dashed blue

lines) as compiled by Savaglio (2013) and those of GRB

hosts metallicities derived from Damped Lyman−αmea-

surements (red line) as reported by Cucchiara et al.

(2014). The percentage of overlap of direct hosts metal-

licities with those of stars in the Milky Way is 10%. We

conclude that the metallicity bias will reduce the prob-

ability for LGRB within the last 5 Gyrs in the Milky

Way by a percentage between 5% (from the metallic-

ity determination by Jimenez & Piran (2013)) and 10%

(from direct metallicity determinations (e.g. Savaglio

2013; Levesque 2014; Cucchiara et al. 2014)), resulting

in a reduction factor between 10 and 20 as compared to

the volumetric rate of LGRBs. In what follows we will

assume a conservative 10% value for a metallicity bias

for LGRB above solar.

3. LIFE THREATENING GRBS IN THE MILKY WAY

Following Wanderman & Piran (2010, 2014) we write

the current (z= 0) luminosity function as:

φ(L) = n0(L/L∗)−ˆαLmin < L < L∗

(L/L∗)−ˆ

βL∗< L < Lmax.(1)

The parameters of the luminosity functions6are given

in table 3 and the functions are shown in Fig. 2. This

luminosity and rate are the isotropic equivalent (namely

disregarding the poorly constrained beaming), which are

the quantities needed for our estimates here. In the fol-

lowing we need the total energy and not the peak lumi-

nosity. A good but rough estimate is obtained by assum-

ing a typical duration of 20s (1s) for LGRBs (sGRBs).

Multiplying by the average (∼half) of the peak ﬂux we

obtain ELGRB = 10Land EsGRB = 0.5L. In what fol-

lows we adopt the cosmological volume occupied by a

Milky Way type galaxy as 10−7Gpc3(see e.g. Panter et

al. (2007) Fig. 3 where we use 6×1010 Mas the stellar

mass of the Milky Way (McMillan 2011)).

Assuming that GRBs follow the stellar distribution,

they are distributed in the exponential disk of the

Milky Way with a radial density proﬁle given by ρ∝

exp(−r/rd), with rd= 2.15 ±0.14 kpc (a number that,

surprisingly, has only been accurately determined re-

cently (Bovy & Rix 2013)). Using this density proﬁle

we calculate p[d, R], the fraction of the Galaxy within a

distance dfrom a position R(see Fig. 2). The expected

number of GRBs, with a ﬂuence exceeding Fat a loca-

tion at distance Rfrom the Galactic center is:

hNi=ZLmax

Lmin

φ(L)p[d(E, F), R]dL. (2)

To estimate the eﬀect of a GRB on life on Earth we

need to know what the dangerous radiation doses are.

Ruderman (1974), who considered at the time the ef-

fect of a nearby SNe on Earth, realized that the most

damaging eﬀect would be the depletion of the Earth pro-

tective Ozone layer for a period of months. This would

happen via formation of stratospheric nitric oxide that

destroys the Ozone. The Ozone depletion would lead

to enhancement of UBV solar radiation that, in turn,

would be harmful to life. Note that the UBV ﬂuence on

the surface of the ocean will destroy surface marine life

(as described in detail in Thomas et al. 2005b) among

them plankton, which will deprive (marine) life of their

main nutrient. In 1995, after it was realized that GRBs

are cosmological and their rate was estimated, Thorsett

(1995) applied these ideas to Galactic GRBs. A decade

later Thomas et al. (2005a,b) carried out the most exten-

sive, to date, calculation of the eﬀects of the gamma-ray

ﬂux on the Earth atmosphere. They ﬁnd that a ﬂuence

of 10kJ/m2will cause a depletion of -68% of the ozone

layer on a time scale of a month. Fluences of 100kJ/m2

and 1000kJ/m2will cause depletions of -91% and -98%

respectively. One has to realize that these are average

quantities. The exact amount of depletion depends on

the direction of the GRB as well as on the season when

the GRB takes place and may vary from one latitude to

6The luminosity function deﬁned here, φ(L), is per dL/L∗. As

such it diﬀers from that given in (Wanderman & Piran 2010, 2014)

that is per dlog10 (L). The power law indices are marked byˆ to

denote this diﬀerence. Clearly, ˆα=α+ 1 and ˆ

β=β+ 1.

Figure 2. The fraction of stars in the Galaxy within a distance

dform Earth for an exponential disc with scale-length of 2.15 kpc

(left y-axis) as a function of energy of a life-extinction GRB for

diﬀerent locations of the life harbouring planet (2.15,4,8.5 and

16kpc). Also plotted (right y-axis) are the luminosity functions as

a function of energy for LGRB and sGRB.

another. Following Thomas et al. (2005a,b) we estimate

that a ﬂuence of 10kJ/m2will cause some damage to

life, while 1000kJ/m2will wipe out nearly the whole at-

mosphere causing a catastrophic life extinction event; we

consider F= 100kJ/m2as our canonical life threatening

ﬂuence. We don’t consider here other sources of damage,

such as cosmic rays associated with the GRBs that could

lead to enhanced radioactivity in the atmosphere (Dar &

De Rujula 2001).

Integrating over the luminosity functions in eq. 2 we

estimate hNi, for both long and short GRBs. These val-

ues are listed in Table. 2. To estimate the signiﬁcance

of these numbers taking into account the errors in the

luminosity function, burst duration and the Milky Way

disk scale length, we carry out a Monte Carlo simulation

of 1000 realizations for both long and short GRBs. We

calculate the distribution of hNiand the overall proba-

bility of more than one life threatening GRB taking place

within the last 5 Gyr, 1 Gyr and 500Myr.

Inspection of Fig. 2 reveals that the maximal danger

arises from weak but not extremely weak events, namely

those around 0.01L∗. Lower luminosity bursts are more

abundant but their covering fraction of the Galaxy is

too small. Higher luminosity bursts can destroy life on

a large fraction of the Galaxy but those are extremely

rare. From the point of view of computational certainty

these results are reassuring as the conﬁdence in our de-

termination of the rate of events around L∗is good. This

is also important from another point of view. Spatially

GRBs are concentrated within regions of the highest SFR

(Fruchter et al. 2006; Svensson et al. 2010). The domi-

nance of strong GRBs whose radius of inﬂuence is a few

kpc implies that we can ignore this spatial inhomogeneity

and the approximation that the distribution of LGRBs

follow the distribution of matter in the galaxy holds.

We ﬁnd that the probability of a LGRB, in the past 5

Gyr, with ﬂuence 100kJ/m2on Earth to be higher than

90% and in the last 0.5 Gyr this probability is 50%. It

is somewhat surprising that this result (50% chance of a

biospherically important event in a half Gyr) is so similar

to the original calculation in Thorsett (1995). At lower

Table 1

Parameters of the LGRBs and sGRBs luminosity functions from Wanderman & Piran (2010, 2014). Note that the upper and lower limits

are not well determined but this is unimportant for our estimates here.

n0ˆαˆ

β L∗Lmin Lmax

Gpc−3yr−1ergs s−1ergs s−1ergs s−1

LGRB 0.15+0.7

−0.81.2+0.2

−0.12.4+0.3

−0.61052.5±0.21049 1054

sGRB 0.04+0.023

−0.019 1.9±0.12 3.0+1

−0.81052.3±0.25×1049 1053

Figure 3. The probability distribution function, p, of the average

number of lethal LGRBs (top panel) and sGRBs (bottom panel)

that irradiated Earth in the past Gyr with enough ﬂux to cause

severe life extinction (100 kJ/m2). For LGRBs we show the case

where we applied a 10% metallicity bias.

ﬂuence, 10kJ/m2, these probabilities are higher than

99.8% (95%) for 5 Gyr (0.5 Gyr) and thus nearly cer-

tain. However, the chances of a truly catastrophic event

with a ﬂuence of 1000kJ/m2, are at most 25% thus mak-

ing it unlikely. These probabilities are of course much

larger (see Table. 2) if we ignore suppression of GRBs in

the Milky Way due to large metallicity.

sGRBs are weaker and as such, even though their rate

is larger than the rate of LGRBs (and particularly so

in the Milky way, because of the metallicity bias) their

life threatening eﬀect is negligible as can be seen from

Table 2. As llGRBs are even weaker their eﬀect is com-

pletely negligible. For completeness we mention that a

giant SGR ﬂare would have to be within ∼1−2 pc from

Earth to produce a 100kJ/m2ﬂuence. This is compa-

Figure 4. The probability, hNi, of having on average more than

one lethal GRB in the past Gyr for an exoplanet at a distance r

from the centre of the Milky Way. The grey line shows the fraction

of mass in the Milky encompassed within a radius r. The dashed

line is for LGRB assuming no metallicity correction.

Table 2

Probability, in %, of at least one GRB having occurred in the

past time twith enough ﬂux to produce signiﬁcant life extinction.

For LGRB we show in parenthesis the probability when we use a

10% metallicity bias. We consider three cases of the GRB ﬂuence

on Earth (10,100 and 1000 kJ/m2).

t < 5 Gyr t < 1 Gyr t < 0.5 Gyr

10kJ/m2

LGRBs 99.8 (99.95) 98.7 (99.90) 95 (99.80)

sGRBs 80 37 22

llGRBs <1<1<1

100kJ/m2

LGRBs 90 (99.8) 60 (96) 50 (90)

sGRBs 14 3 2

llGRBs <1<1<1

1000kJ/m2

LGRBs 25 (80) 7 (40) 4 (25)

sGRBs 10−22×10−310−3

llGRBs 0 0 0

rable to the distances between stars in the solar neigh-

bourhood. Consequently giant SGR ﬂares are unlikely

to cause any signiﬁcant damage to life.

4. GRBS AND LIFE IN THE GALAXY

We turn now to explore the possible threat caused by

GRBs to life elsewhere in the Milky Way, turning to the

whole Universe in the next section. Clearly to do so

one must assume the lethal radiation dose that will be

threatening to life elsewhere. While life can take nu-

merous other forms and could be much more resilient

to radiation than on earth, we make here the conserva-

tive assumption that life is rather similar to the one on

Earth. This common assumption is the basis for searches

of Earth like exoplanets as places that harbour life. Un-

der this assumption, we explore what is the likelihood

that a nearby GRB results in a dose of 100 as well as 10

and 1000 kJ/m2in various regions of the Milky Way.

The stellar density is signiﬁcantly larger towards the

center of the Galaxy and hence the threat to life on

most exoplanets, that reside in this region, are much

larger. Fig. 4 depicts the probability of having one life

threatening event within the last71 Gyr as a function

of the distance rof an exoplanet from the Galactic cen-

ter. Also shown is the fraction of the stellar population

of the Milky Way within this radius. A lethal GRBs,

for 100kJ/m2, would be more likely than 95% up to a

distance of 2 kpc from the Galactic center in which 25%

of the MW stars reside. When considering F= 10 and

1000kJ/m2we ﬁnd 12 and 0.5 kpc respectively. In agree-

ment with the speciﬁc estimates for Earth, events around

the Solar distance from the Galactic could be signiﬁcant

but rare and only at a distance >10kpc the threat from

GRBs becomes small. Therefore, life can be preserved

with certainty only in the outskirts of our Galaxy. Over-

all GRBs would destroy life on 40%, 90% and 5% of the

Milky Way exoplanets for F= 100,10 and 1000kJ/m2

respectively.

Finally, given the LGRBs luminosity function there are

practically no lethal events with a distance larger than

30kpc. This implies that nearby small satellite galaxies

with a large SFR, like the LMC, are too far to inﬂuence

life in the Milky Way. The fact that the local group

is such a low density region containing only two large

galaxies (Andromeda and the Milky Way) and with the

nearest cluster of galaxies, Virgo, at 16 Mpc, i.e. much

farther away than the typical inter-galactic distance of

1 Mpc, seems to provide the required environment to

preserve life on Earth. There is no threat from nearby

extragalactic bursts.

5. GRBS AND LIFE IN THE UNIVERSE

Before concluding we turn now to consider the condi-

tions elsewhere in the Universe. We already mentioned

that the local neighbourhood of the Milky Way has a

lower density of star forming dwarf galaxies making the

Milky Way a more friendly neighbourhood for life. We

can take our calculation one step further and compute

the eﬀective volume in the Universe protected from GRB

explosions for life proliferation. This happens for galax-

ies that produce enough metals so that their metallicity

is at least 1/3 solar and their stellar disks are larger than

4 kpc. Using the mass-metallicity relation in Panter et

al. (2008, their Fig. 6) such galaxies must have stellar

masses larger than 1010M. This corresponds to a co-

moving abundance of 10−3galaxies per Mpc3(see Fig. 3

of Panter et al. (2007)). This is a factor 10 less than the

abundance of most common galaxies. Galaxies friendly

to harbor and preserve life will preferably inhabit low

density regions in voids and ﬁlaments of the cosmic web.

Turning to earlier epochs we may wonder whether life

7We use 1 Gyr as a round number to estimate life extinctions

that could have cause a massive extinction that terminated life and

thus made it unlikely that we ﬁnd signs of life today.

could have existed in the earlier universe? We recall that

the age of the Universe at z=1 is about 6 Gyr so in

principle there was enough time for life to evolve even

before this redshift; here we note that the LGRB rate is

signiﬁcantly larger in the past making the GRB threat

much more signiﬁcant. Furthermore, galaxies at high-z

are smaller than current ones by a factor of 2−4 in radius

and as such have less room for isolated safe regions like

the outskirts of the Milky Way. We conclude that it is

impossible to harbor life at z > 0.5 as LGRBs will always

be suﬃciently nearby to life-harboring planets and thus

cause life extinctions. It seems the survival of life, as

we know it on Earth, was only a recent phenomenon in

the history of the Universe caused by the growth of large

galaxies. Life forms that might have existed earlier or

that exist today in other regions of the Universe that are

much more susceptible to signiﬁcant GRB bombardment

must have been much more resilient to radiation than life

on Earth.

6. CONCLUSIONS

We have used the latest determination of GRB rates

and luminosities to estimate the likelihood of them be-

ing the source of life extinction on Earth. Using also the

latest determinations of metallicity of stars in the Milky

Way and those of LGRB hosts, we concluded that the

likelihood of a GRB producing life extinction on Earth

is high. Taking the same lethal dose for extraterrestrial

life as for life on Earth we have found that GRBs and in

particular LGRBs are life threatening in a large part of

the Milky Way as well as in many other locations in the

Universe. The safest environments to preserve life are

the outskirts of large galaxies in low density regions (so

that these galaxies don’t have “dangerous” low metal-

licity dwarf satellites). It is curious to point out that

a cosmological constant is essential for the Universe to

grow large galaxies and also preserve low density regions

at late times z < 0.5.

TP thanks the Institut Lagrange de Paris for hospital-

ity while this work was being completed. This research

was supported by the ERC grant GRBs, by the ISF I-

Core center of excellence and by an Israel-China grant.

RJ thanks the Royal Society and the ICIC at Imperial

College for ﬁnancial support and hospitality while this

work was being completed. We thank Chris Flynn and

Luca Casagrande for discussions on the age-metallicity

relation of stars in the Milky Way.

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The Resilience of Life to Astrophysical Events

Preprint

Jul 2017

Much attention has been given in the literature to the effects of astrophysical events on human and land-based life. However, little has been discussed on the resilience of life itself. Here we instead explore the statistics of events that completely sterilise an Earth-like planet with planet radii in the range

0.5-1.5 R_{Earth}

and temperatures of

\sim 300 \; \text{K}

, eradicating all forms of life. We consider the relative likelihood of complete global sterilisation events from three astrophysical sources -- supernovae, gamma-ray bursts, large asteroid impacts, and passing-by stars. To assess such probabilities we consider what cataclysmic event could lead to the annihilation of not just human life, but also extremophiles, through the boiling of all water in Earth's oceans. Surprisingly we find that although human life is somewhat fragile to nearby events, the resilience of Ecdysozoa such as \emph{Milnesium tardigradum} renders global sterilisation an unlikely event.

Solar irradiance changes and phytoplankton productivity in Earth's ocean following astrophysical ionizing radiation events

Preprint

Apr 2016

Two atmospheric responses to simulated astrophysical ionizing radiation events significant to life on Earth are production of odd-nitrogen species, especially NO2, and subsequent depletion of stratospheric ozone. Ozone depletion increases incident short-wavelength ultraviolet radiation (UVB, 280-315 nm) and longer ( > 600 nm) wavelengths of photosynthetically available radiation (PAR, 400 -700 nm). On the other hand, the NO2 haze decreases atmospheric transmission in the long-wavelength UVA (315-400 nm) and short wavelength PAR. Here we use the results of previous simulations of incident spectral irradiance following an ionizing radiation event to predict changes in Terran productivity focusing on photosynthesis of marine phytoplankton. The prediction is based on a spectral model of photosynthetic response developed for the dominant genera in central regions of the ocean (Synechococcus and Prochlorococcus), and remote-sensing based observations of spectral water transparency, temperature, wind speed and mixed layer depth. Predicted productivity declined after a simulated ionizing event, but the effect integrated over the water column was small. For integrations taking into account the full depth range of PAR transmission (down to 0.1% of utilizable PAR), the decrease was at most 2-3% (depending on strain), with larger effects (5-7%) for integrations just to the depth of the surface mixed layer. The deeper integrations were most affected by the decreased utilizable PAR at depth due to the NO2 haze, whereas shallower integrations were most affected by the increased surface UV.

Relative Likelihood for Life as a Function of Cosmic Time

Preprint

Jun 2016

Is life most likely to emerge at the present cosmic time near a star like the Sun? We address this question by calculating the relative formation probability per unit time of habitable Earth-like planets within a fixed comoving volume of the Universe, dP(t)/dt, starting from the first stars and continuing to the distant cosmic future. We conservatively restrict our attention to the context of "life as we know it" and the standard cosmological model, LCDM. We find that unless habitability around low mass stars is suppressed, life is most likely to exist near 0.1 solar-mass stars ten trillion years from now. Spectroscopic searches for biosignatures in the atmospheres of transiting Earth-mass planets around low mass stars will determine whether present-day life is indeed premature or typical from a cosmic perspective.

Inflation in Random Landscapes with two energy scales

Preprint

Nov 2017

We investigate inflation in a multi-dimensional landscape with a hierarchy of energy scales, motivated by the string theory, where the energy scale of Kahler moduli is usually assumed to be much lower than that of complex structure moduli and dilaton field. We argue that in such a landscape, the dynamics of slow-roll inflation is governed by the low-energy potential, while the initial condition for inflation are determined by tunneling through high-energy barriers. We then use the scale factor cutoff measure to calculate the probability distribution for the number of inflationary e-folds and the amplitude of density fluctuations Q, assuming that the low-energy landscape is described by a random Gaussian potential with a correlation length much smaller than

M_{\rm pl}

. We find that the distribution for Q has a unique shape and a preferred domain, which depends on the parameters of the low-energy landscape. We discuss some observational implications of this distribution and the constraints it imposes on the landscape parameters.

Viewing short Gamma-ray Bursts from a different angle

Preprint

Oct 2017

The recent coincident detection of gravitational waves (GW) from a binary neutron star merger with aLIGO/Virgo and short-lived gamma-ray emission with Fermi/GBM (called GW 170817) is a milestone for the establishment of multi-messenger astronomy. Merging neutron stars (NS) represent the standard scenario for short-duration (< 2 sec) gamma-ray bursts (GRBs) which are produced in a collimated, relativistically expanding jet with an opening angle of a few degrees and a bulk Lorentz factor of 300-1000. While the present aLIGO detection is consistent with predictions, the measured faint gamma-ray emission from GW 170817A, if associated to the merger event at a distance of 40 Mpc, is about 1000x less luminous than known short-duration GRBs (sGRBs). Hence, the presence of this sGRB in the local Universe is either a very rare event, or points to a dramatic ignorance of the emission properties of sGRBs outside their narrow jets. Here we show that the majority of previously detected faint sGRBs are local, at redshift smaller than 0.1, seen off-axis. In contrast, the brighter sGRBs are seen on-axis, and therefore out to larger distances, consistent with the measured redshift distribution. Examining the observer-frame parameter space of all Fermi/GBM sGRBs shows that the sGRB associated with GW 170817A is extreme in its combination of flux, spectral softness and temporal structure. We identify a group of similar GRBs, one of which has been associated to a bright galaxy at 75 Mpc. We incorporate off-axis emission in the estimate of the rates of sGRBs, and predict that the majority of future GW-detections of NS-NS mergers will be accompanied by faint gamma-ray emission, contrary to previous thinking. The much more frequent off-axis emission of sGRBs also implies a much higher deadly rate of gamma-rays for extraterrestrial life in the Universe.

Atmospheric ionization by high-fluence, hard spectrum solar proton events and their probable appearance in the ice core archive

Preprint

Feb 2016

Solar energetic particles ionize the atmosphere, leading to production of nitrogen oxides. It has been suggested that some such events are visible as layers of nitrate in ice cores, yielding archives of energetic, high fluence solar proton events (SPEs). There has been controversy, due to slowness of transport for these species down from the upper stratosphere; past numerical simulations based on an analytic calculation have shown very little ionization below the mid stratosphere. These simulations suffer from deficiencies: they consider only soft SPEs and narrow energy ranges; spectral fits are poorly chosen; with few exceptions secondary particles in air showers are ignored. Using improved simulations that follow development of the proton-induced air shower, we find consistency with recent experiments showing substantial excess ionization down to 5 km. We compute nitrate available from the 23 February 1956 SPE, which had a high fluence, hard spectrum, and well-resolved associated nitrate peak in a Greenland ice core. For the first time, we find this event can account for ice core data with timely (~ 2 months) transport downward between 46 km and the surface, thus indicating an archive of high fluence, hard spectrum SPE covering the last several millennia. We discuss interpretations of this result, as well as the lack of a clearly-defined nitrate spike associated with the soft-spectrum 3-4 August 1972 SPE. We suggest that hard-spectrum SPEs, especially in the 6 months of polar winter, are detectable in ice cores, and that more work needs to be done to investigate this.

Late Quaternary Supernovae In Earth History

Article

Apr 2025

G. Robert Brakenridge

Supernovae (SNe) may have affected Earth's atmosphere during Late Quaternary (50 ka-present) time and be detectible in cosmogenic isotopic records. Supernova remnants (SNRs) at distances < 2.3 kpc provide a revised chronology of SNe and predicted hard photons received by Earth. Calculated fluences assume X-ray and γ isotropic emissions of 4 × 1049 erg within 2 y. Such are compatible with high energy observations of extragalactic SNe. Earlier values may be unrealistically small given current knowledge. The radiation events associated with nearby SNR are compared to dated records of terrestrial environmental changes. Eight SNe may have produced hard photon fluences of 1–6 × 1024 erg on the terrestrial disc; they were at distances ≤ .6 kpc. The Vela SN (.29 kpc) produced the highest fluence, at ∼13 ka. Its predicted environmental effects include abruptly elevated atmospheric 14C, reductions in upper atmosphere O3 and CH4, increased solar UVB at Earth's surface, possible cooling of the global climate, selective animal extinctions, increased wildfires, and Pt-group dust deposition. All are recorded in terrestrial records commencing at 12.76 ka and the start of the Younger Dryas cold period. Several thousand years earlier, the Hoinga SN (∼.35 kpc, ∼15 ka) may have caused a single year 30‰ Δ14C rise at 14.32 ka and the Older Dryas cool period. The 14C production dropped to its previous level by 14.23 ka but a subsequent increase occurred 14–13.9 ka and may record the arrival of associated cosmic radiation. Δ14C events at 9.126, 7.209, 2.764, 2.614, 1.175 ka, and .957 ka were apparently global and each have plausible SNe candidates of appropriate distances and ages. The nearest SNe appear to be associated with the largest isotope anomalies.

Gamma-Ray Bursts: The Energy Monsters of the Universe

Article

Full-text available

Feb 2025

Franco Giovannelli

Gamma-Ray Bursts(GRBs) are the most violent and energetic astrophysical phenomena, which I dare call “the Energy Monsters of the Universe”. Indeed, they show an enormous emitted isotropic energy ranging from ∼3 × 1046 erg (GRB 170817A) to ∼1055 erg (GRB 221009A) and a duration ranging from ≈milliseconds to ∼104 s. In this review—which I agreed to write as a scientist not directly involved in the field of GRBs—I will present the history of GRBs from the time of their discovery by chance until the new era whose beginning was marked by the detection of gravitational waves coming from the merger of two neutron stars. I will discuss the experimental results and their physical interpretation, which is still a source of heated debate within the scientific community. Due to the reasonable length of this review and especially given my limited knowledge, I do not claim to have exhausted the complicated topic of GRBs, but to have contributed in making this subject easy to read for non-experts, providing a critical contribution that is hopefully useful to the whole community.

Sedimentary rocks from Mediterranean drought in the Messinian age as a probe of the past cosmic ray flux

Article

Dec 2024

Developing Ecospheres on Transiently Habitable Planets: The Genesis Project

Preprint

Aug 2016

Claudius Gros

It is often presumed, that life evolves relatively fast on planets with clement conditions, at least in its basic forms, and that extended periods of habitability are subsequently needed for the evolution of higher life forms. Many planets are however expected to be only transiently habitable. On a large set of otherwise suitable planets life will therefore just not have the time to develop on its own to a complexity level as it did arise on earth with the cambrian explosion. The equivalent of a cambrian explosion may however have the chance to unfold on transiently habitable planets if it would be possible to fast forward evolution by 3-4 billion years (with respect to terrestrial timescales). We argue here, that this is indeed possible when seeding the candidate planet with the microbial lifeforms, bacteria and unicellular eukaryotes alike, characterizing earth before the cambrian explosion. An interstellar mission of this kind, denoted the `Genesis project', could be carried out by a relatively low-cost robotic microcraft equipped with a on-board gene laboratory for the in situ synthesis of the microbes. We review here our current understanding of the processes determining the timescales shaping the geo-evolution of an earth-like planet, the prospect of finding Genesis candidate planets and selected issues regarding the mission layout. Discussing the ethical aspects connected with a Genesis mission, which would be expressively not for human benefit, we will also touch the risk that a biosphere incompatibility may arise in the wake of an eventual manned exploration of a second earth.

The AD775 cosmic event revisited: The Sun is to blame

Article

Full-text available

Feb 2013

Miyake et al. (henceforth M12) recently reported, based on 14C data, an extreme cosmic event ca. AD775. Using a simple model, M12 claimed that the event was too strong to be caused by a solar flare within the standard theory. This implied a new paradigm of either an impossibly strong solar flare or a very strong cosmic ray event of unknown origin occurred ca. AD775. We show that the strength of the event was significantly overestimated by M12. Several subsequent works have attempted to find a possible exotic source for such an event, but they are all based on incorrect estimates by M12. We revisit this event with analysis of new datasets and consistent theoretical modelling. We verified the experimental result for the AD775 event using independent datasets including 10Be series and newly measured 14C annual data. We surveyed available historical chronicles for astronomical observations for the AD770s to identify potential sightings of aurorae or supernovae. We interpreted the 14C measurements using an appropriate carbon cycle model. We show that: (1) The reality of the AD775 event is confirmed by new measurements of 14C; (2) by using an inappropriate carbon cycle model, M12 strongly overestimated the event's strength; (3) The revised magnitude of the event is consistent with different independent datasets (14C, 10Be, 36Cl) and can be associated with a strong, but not inexplicably strong, SEP event (or a sequence of events), and provides the first evidence for an event of this magnitude (the fluence >30 MeV was about 4.5*10^{10} /cm2) in multiple datasets; (4) This is in agreement with increased auroral activity identified in historical chronicles. This point to the likely solar origin of the event, which is the greatest solar event on a multi-millennial time scale, placing a strong observational constraint on the theory of explosive energy releases on the Sun and cool stars.

Causes of an AD 774-775 C-14 increase

Article

Full-text available

Nov 2012
NATURE

Atmospheric 14C production is a potential window into the energy of solar proton and other cosmic ray events. It was previously concluded that results from AD 774-775 are orders of magnitude greater than known solar events. We find that the coronal mass ejection energy based on 14C production is much smaller than claimed, but still substantially larger than the maximum historical Carrington Event of 1859. Such an event would cause great damage to modern technology, and in view of recent confirmation of superflares on solar-type stars, this issue merits attention.

Relativistic ejecta from X-ray flash XRF 060218 and the rate of cosmic explosions

Article

Full-text available

Aug 2006

Over the past decade, long-duration γ-ray bursts (GRBs)-including the subclass of X-ray flashes (XRFs)-have been revealed to be a rare variety of type Ibc supernova. Although all these events result from the death of massive stars, the electromagnetic luminosities of GRBs and XRFs exceed those of ordinary type Ibc supernovae by many orders of magnitude. The essential physical process that causes a dying star to produce a GRB or XRF, and not just a supernova, is still unknown. Here we report radio and X-ray observations of XRF 060218 (associated with supernova SN 2006aj), the second-nearest GRB identified until now. We show that this event is a hundred times less energetic but ten times more common than cosmological GRBs. Moreover, it is distinguished from ordinary type Ibc supernovae by the presence of 1048 erg coupled to mildly relativistic ejecta, along with a central engine (an accretion-fed, rapidly rotating compact source) that produces X-rays for weeks after the explosion. This suggests that the production of relativistic ejecta is the key physical distinction between GRBs or XRFs and ordinary supernovae, while the nature of the central engine (black hole or magnetar) may distinguish typical bursts from low-luminosity, spherical events like XRF 060218.

Reconciling the gamma-ray burst rate and star formation histories

Article

Aug 2013

While there are numerous indications that gamma-ray bursts (GRBs) arise from the deaths of massive stars, the GRB rate does not follow the global cosmic star formation rate and, within their hosts, GRBs are more concentrated in regions of very high star formation. We explain both puzzles here. Using the publicly available VESPA database of the Sloan Digital Sky Survey (SDSS) Data Release 7 spectra, we explore a multi-parameter space in galaxy properties such as stellar mass, metallicity, and dust to find the subset of galaxies that reproduces the GRB rate recently obtained by Wanderman & Piran. We find that only galaxies with present stellar masses below <1010M ☉ and low metallicity reproduce the observed GRB rate. This is consistent with direct observations of GRB hosts and provides an independent confirmation of the nature of GRB hosts. Because of the significantly larger sample of SDSS galaxies, we compute their correlation function and show that they are anti-biased with respect to dark matter: they are in filaments and voids. Using recent observations of massive stars in local dwarfs we show how the fact that GRB host galaxies are dwarfs can explain the observation that GRBs are more concentrated in regions of high star formation than are supernovae. Finally, we explain these results using new theoretical advances in the field of star formation.

Short versus long and collapsars versus non-collapsars: A quantitative classification of gamma-ray bursts

Article

Feb 2013

Gamma-ray bursts (GRBs) are traditionally divided into long and short according to their durations (2 s). It was generally believed that this reflects a different physical origin: collapsars (long) and non-collapsars (short). We have recently shown that the duration distribution of collapsars is flat, namely, independent of the duration, at short durations. Using this model for the distribution of Collapsars we determine the duration distribution of non-Collapsars and estimate the probability that a burst with a given duration (and hardness) is a Collapsar or not. We find that this probability depends strongly on the spectral window of the observing detector. While the commonly used limit of 2 s is conservative and suitable for BATSE bursts, 40% of Swift's bursts shorter than 2 s are Collapsars and the division 0.8 s is more suitable for Swift. We find that the duration overlap of the two populations is very large. On the one hand there is a non-negligible fraction of non-Collapsars longer than 10 s, while on the other hand even bursts shorter than 0.5 s in the Swift sample have a non-negligible probability to be Collapsars. Our results enable the construction of non-Collapsar samples while controlling the Collapsar contamination. They also highlight that no firm conclusions can be drawn based on a single burst and they have numerous implications concerning previous studies of non-Collapsar properties that were based on the current significantly contaminated Swift samples of localized short GRBs. Specifically (1) all known short bursts with z > 1 are most likely Collapsars; (2) the only short burst with a clear jet break is most likely a Collapsar, indicating our lack of knowledge concerning non-Collapsar beaming; and (3) the existence of non-Collapsars with durations up to 10 s imposes new challenges to non-Collapsar models.

A direct dynamical measurement of the Milky Way's disk surface density profile, disk scale length, and dark matter profile at 4 kpc < R < 9 kpc

Article

Sep 2013

We present and apply rigorous dynamical modeling with which we infer unprecedented constraints on the stellar and dark matter mass distribution within our Milky Way (MW), based on large sets of phase-space data on individual stars. Specifically, we model the dynamics of 16,269 G-type dwarfs from SEGUE, which sample 5 kpc < RGC < 12 kpc and 0.3 kpc |Z| 3 kpc. We independently fit a parameterized MW potential and a three-integral, action-based distribution function (DF) to the phase-space data of 43 separate abundance-selected sub-populations (MAPs), accounting for the complex selection effects affecting the data. We robustly measure the total surface density within 1.1 kpc of the mid-plane to 5% over 4.5 kpc < RGC < 9 kpc. Using metal-poor MAPs with small radial scale lengths as dynamical tracers probes 4.5 kpc RGC 7 kpc, while MAPs with longer radial scale lengths sample 7 kpc RGC 9 kpc. We measure the mass-weighted Galactic disk scale length to be Rd = 2.15 ± 0.14 kpc, in agreement with the photometrically inferred spatial distribution of stellar mass. We thereby measure dynamically the mass of the Galactic stellar disk to unprecedented accuracy: M * = 4.6 ± 0.3 + 3.0 (R 0/ kpc – 8) × 1010M ☉ and a total local surface density of of which 38 ± 4 M ☉ pc–2 is contributed by stars and stellar remnants. By combining our surface density measurements with the terminal velocity curve, we find that the MW's disk is maximal in the sense that V c, disk/V c, total = 0.83 ± 0.04 at R = 2.2 Rd. We also constrain for the first time the radial profile of the dark halo at such small Galactocentric radii, finding that ρDM(r; ≈R 0)∝1/r α with α < 1.53 at 95% confidence. Our results show that action-based DF modeling of complex stellar data sets is now a feasible approach that will be fruitful for interpreting Gaia data.

Are Low-luminosity Gamma-Ray Bursts Generated by Relativistic Jets?

Article

Oct 2011

Low-luminosity gamma-ray bursts (ll-GRBs) constitute a subclass of GRBs that play a central role in the GRB-supernova connection. While ll-GRBs differ from typical long GRBs (LGRBs) in many aspects, they also share some common features. Therefore, the question whether the gamma-ray emission of ll-GRBs and LGRBs has a common origin is of great interest. Here we address this question by testing whether ll-GRBs, like LGRBs according to the Collapsar model, can be generated by relativistic jets that punch holes in the envelopes of their progenitor stars. The Collapsar model predicts that the durations of most observed bursts will be comparable to, or longer than, the time it takes the jets to break out of the star. We calculate the jet breakout times of ll-GRBs and compare them to the observed durations. We find that there is a significant excess of ll-GRBs with durations that are much shorter than the jet breakout time and that these are inconsistent with the Collapsar model. We conclude that the processes that dominate the gamma-ray emission of ll-GRBs and of LGRBs are most likely fundamentally different.

Nucleosynthesis, neutrino bursts and ??-rays from coalescing neutron stars

Article

Jul 1989
NATURE

It is pointed out here that neutron-star collisions should synthesize neutron-rich heavy elements, thought to be formed by rapid neutron capture (the r-process). Furthermore, these collisions should produce neutrino bursts and resultant bursts of gamma rays; the latter should comprise a subclass of observable gamma-ray bursts. It is argued that observed r-process abundances and gamma-ray burst rates predict rates for these collisions that are both significant and consistent with other estimates.

The Cosmic Evolution of Gamma-Ray Burst Host Galaxies

Article

Dec 2012

Sandra Savaglio

Due to their extreme luminosities, gamma-ray bursts (GRBs) can be detected in hostile regions of galaxies, nearby and at very high redshift, making them important cosmological probes. The investigation of galaxies hosting long-duration GRBs (whose progenitor is a massive star) demonstrated their connection to star formation. Still, the link to the total galaxy population is controversial, mainly because of the small-number statistics: ~ 1,100 are the GRBs detected so far, ~ 280 those with measured redshift, and ~ 70 the hosts studied in detail. These are typically low-redshift (z < 1.5), low luminosity, metal poor, and star-forming galaxes. On the other hand, at 1.5< z <4, massive, metal rich and dusty, interacting galaxies are not uncommon. The most distant population (z > 4) is poorly explored, but the deep limits reached point towards very small and star-forming objects, similar to the low-z population. This `back to the future' behavior is a natural consequence of the connection of long GRBs to star formation in young regions of the universe.

The host to gamma-ray burst 970508: A distant dwarf galaxy?

Article

Nov 1997
NEW ASTRON

If the optical transient to gamma-ray burst (GRB) 970508 is indeed associated with the 8 May burst and is connected with the z = 0.835 absorption system reported by Metzger et al. (1997) [Nature, 387, 879], then either the GRB originated from an intrinsically very faint galaxy (L ≲ 0.01L∗) or it occurred at a large distance from a host galaxy (≳25 h70-1kpc); a large offset of GRBs from galaxies would tend to favour the merging neutron star-neutron star (NS-NS) model (Mészáros & Rees, 1993 [ApJ, 405, 278]; Narayan et al., 1997 [ApJL, 395, 83]). Here we show that the properties of a suspected host galaxy, particularly its intrinsic brightness and comoving distance from the transient, can be constrained indirectly using the Mg II absorption features detected in the spectrum of the optical transient (Metzger et al., 1997 [IAUC, 6676]) by examining the galaxies in the vicinity of the optical transient from ground-based and Hubble Space Telescope (HST) images. This is an independent test of the brightness of GRB host galaxies irrespective of distances implied from logN-logP distributions. The spectral lines most likely arise from absorption by very underluminous galaxies; in which case the optical transient has revealed the presence of a population of intermediate-redshift dwarf galaxies.