Short time scale variability at gamma rays in FSRQs and implications on the current models

Abstract

We studied the rapid variability at GeV gamma rays of the flat-spectrum radio
quasar PKS 1222+216, which was recently found by the MAGIC Cerenkov telescope
to display very short variability (minutes time scale) at hundreds of GeV. We
analyzed the time period between 2010 April 29 and June 20, when the source
generated a few gamma-ray flares with flux in the MeV-GeV band in excess of
10^-5 ph cm^-2 s^-1 on daily basis. We set tight upper limits on the observed
doubling time scale (about 1 hour on 2010 April 30), the smallest measured to
date at MeV-GeV energies, which can constrain the size of the gamma-ray
emitting region. We also studied the spectra measured during two flares (2010
April 30 and June 17-18). The combination of spectral and variability studies
obtained in the present work favors the hypothesis that gamma rays are
generally produced inside the broad-line region (BLR), but sometimes the
dissipation can occur at larger distances, nearby the infrared torus.

Full text

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arXiv:1110.4471v1 [astro-ph.HE] 20 Oct 2011
2011 Fermi Symposium, Roma, May 9-12
1
Short time scale variability at gamma rays in FSRQs and implications
on the current models
L. Foschini, G. Ghisellini, F. Tavecchio, G. Bonnoli
INAF Osservatorio Astronomico di Brera, 23807 Merate, Italy
A. Stamerra
Universit` a di Siena, 53100 Siena, Italy
We studied the rapid variability at GeV gamma rays of the flat-spectrum radio quasar PKS 1222+216, which
was recently found by the MAGIC Cerenkov telescope to display very short variability (minutes time scale) at
hundreds of GeV. We analyzed the time period between 2010 April 29 and June 20, when the source generated
a few γ-ray flares with flux in the MeV-GeV band in excess of 10−5ph cm−2s−1on daily basis. We set tight
upper limits on the observed doubling time scale (∼ 0.8 hours on 2010 April 30), the smallest measured to date
at MeV-GeV energies, which can constrain the size of the γ-ray emitting region. We also studied the spectra
measured during two flares (2010 April 30 and June 17-18). The combination of spectral and variability studies
obtained in the present work favors the hypothesis that γ rays are generally produced inside the broad-line
region (BLR), but sometimes the dissipation can occur at larger distances, nearby the infrared torus.
1. INTRODUCTION
The variability of the emission from active galac-
tic nuclei (AGN) with relativistic jets viewed at small
angles is amplified because of the Doppler boosting
δ. This measurement can be linked to the size of
the emitting region r, which should be smaller than
τδc/(1 + z), being τ the observed characteristic time
scale, c the speed of light, and z the redshift of the
source. Rather obviously, very short time scales im-
ply very small emitting regions. When the size r is
smaller than the gravitational radius of the central
black hole (rg= GM/c2, where G is the gravitational
constant and M is the mass of the singularity), this
can create some problems in blazar models (cf [1]).
According to the present models, the dissipation re-
gion of jets in blazars is at about 103rg(e.g. [9]). By
taking into account that any perturbation propagate
with a starting size of the order of the gravitational
radius and the jet is a self-similar structure – which in
turn implies that the emitting region is linked to the
distance from the central black hole R through the
aperture of the jet ψ ∼ 0.1 − 0.25 (e.g. [2, 8]) – the
dissipation at R ∼ 103rg requires an emitting region
of the order of (100−250)rg. Such a size can sustain a
variability of about 7−17 hours in the case of a blazar
at z = 1, M = 109M⊙, and δ = 10.
The observation of sub-hour time scales in the high-
energy emission from some blazars ([5, 10, 11, 12])
threatened this scenario. Although a simple increase
of the Doppler factor could be an affordable gross so-
lution in the case of BL Lac Objects (which have a
photon-starved nearby environment) this is no more
reasonable for flat-spectrum radio quasars (FSRQs),
with a richer photon field. Since the external energy
density is proportional to Γ2(Γ is the bulk Lorentz
factor), an increase of δ implies an increase of the en-
ergy density and, hence, of the optical depth for pair
production (cf [8]). Particularly, the observation of
variability with time scales of minutes at hundreds of
GeV in the case of PKS 1222+216 [12] has put very
severe constraints to the existing models (see [21] for
a review and some possible solutions).
Therefore, the search for short time scales in the
emission of FSRQs is an insuperable testing ground
for the existing theories about relativistic jets. The
launch of the Large Area Telescope (LAT, [3]) onboard
the Fermi satellite made it available the state-of-the-
art of the high-energy γ-ray instrumentation. With its
superior performance, it is now possible to probe the
variability in FSRQs on hourly time scale. Some work
has been already published in [6, 7]. Here we focus on
the case of PKS 1222+216 as observed by Fermi/LAT
and reanalyzed the data presented in [7] with a most
recent and improved version of the software.
2. DATA ANALYSIS
The FSRQ PKS 1222+216 (a.k.a. 4C +21.35) is a
high-power blazar at z = 0.432. It was listed in the
catalog of very high energy (E > 100 GeV) sources de-
tected by Fermi/LAT (in a time period of about two
years) [15]. In 2010 April-June, the source underwent
a few strong γ-ray outbursts, with fluxes at energies
greater than 100 MeV in excess of 10−5ph cm−2s−1
[19]. Particularly, on 2010 June 17, it was detected at
energies of hundreds of GeV by the MAGIC Cerenkov
telescope, while displaying very short time scale vari-
ability (8.6+1.1
−0.9minutes, [12]).
The analysis of Fermi/LAT data in the period 2010
April-June revealed short variability: 1.5 ± 0.6 hours
or < 2.3 hours, with a more conservative approach
[7].This analysis was performed by using a pre-
flight version of the instrument response function
(IRF), adapted to overcome some problems in the
100−200 MeV energy range. After the publication of
the work [7], the Fermi/LAT Collaboration released
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2011 Fermi Symposium, Roma, May 9-12
0 1020
Time [MJD − 55315]
3040 50
0
10
20
30
Flux [10−6 ph cm−2 s−1]
Figure 1: Fermi/LAT light curve in the 0.1 − 100 GeV
energy band with GTI time bins (too small to be visible).
The vertical grey dot-dashed line indicates the period of
the MAGIC detection [12] (see the zoom in Fig. 3). Time
starts on MJD 55315 (2010 April 29).
(2011 August 5) a new fully post-flight IRF, together
with improved files for the background subtraction.
Given the importance of this release, we decided to re-
analyze the LAT data, searching for tighter estimates
of the time variability.
Therefore, in this work, we adopt the LAT Science
Tools v. 9.23.1, with the IRF P7SOURCE V6, and
background files iso p7v6source.txt (isotropic) and
gal 2yearp7v6 v0.fits (Galactic diffuse).
lected events of class 2, within a zenithal distance of
100◦and a rocking angle smaller than 52◦. The time
period covered by the analysis is the same of [7], i.e.
2010 April 29 (MJD 55315) and June 20. The re-
maining of the analysis has been done as described in
[7], i.e. by building a light curve with the width of
bins equal to the Good-Time-Intervals (GTI). Then,
we searched for the time scale τ for doubling/halving
the flux as defined by:
We se-
F(t)
F(t0)= 2
−(t−t0)
τ
(1)
where F(t) and F(t0) are the fluxes at times t and
t0, respectively. Fig. 1 shows the γ-ray light curve of
PKS 1222+216 in the analyzed period.
The analysis based on the calculation of τ from two
consecutive points and Eq. (1) results only in an upper
limit of τ < 2.8 hours, similar to the τ < 2.3 hours
obtained with the previous analysis.
However, in some cases like the flares occurred on
2010 April 30 and June 17-18 (see Fig. 2 and 3), it is
possible to perform a fit on a few consecutive points,
thus improving the measurement errors.
estimates can be obtained during the flare of 2010
April 30 15:07 UTC (MJD 55316.6301, Fig. 2), which
reached a peak flux of (2.4±0.4)×10−5ph cm−2s−1.
The best
0 0.51 1.52 2.53
0
10
20
30
Flux [10−6 ph cm−2 s−1]
Time [MJD − 55315]
Figure 2: Zoom on the first flare occurred on 2010 April
30 (MJD 55316). Time starts on MJD 55315 (2010 April
29). See the text for details.
49 49.5 5050.5 5151.5 52
0
10
20
30
Flux [10−6 ph cm−2 s−1]
Time [MJD − 55315]
Figure 3: Zoom on the period of the MAGIC detection
occurred on 2010 June 17 (MJD 55364.908, indicated
with grey dot-dashed vertical lines [12]). It is worth
noting that Fermi detected the highest flux after the
MAGIC observation. Time starts on MJD 55315 (2010
April 29). See the text for details.
The rise time is calculated with four points between
MJD 55136.42 and 55136.63 (i.e. the points betwee
1.42 and 1.63 in the abscissa of Fig. 2 indicated with
the red squares) and resulted in τ = 0.83±0.74 hours.
The decay is sharper and, with six points between 1.63
and 1.94 (i.e. MJD 55136.63 and 55136.94; see the or-
ange stars plus the peak in Fig. 2), it is possible to
calculate a τ = −0.79 ± 0.35 hours.
Another γ-ray flare occurred after the MAGIC ob-
servation [12] (Fig. 3), with a peak of (1.8 ± 0.3) ×
10−5ph cm−2s−1on MJD 55365.6276 (2010 June 18
15:03 UTC). The rise time derived from the fit of four
points (red squares in Fig. 3) is τ = 1.2 ± 1.1 hours,
while the decay is τ = −1.3 ± 0.6 hours (fit with six
points, orange stars plus the peak in Fig. 3).
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2011 Fermi Symposium, Roma, May 9-12
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1022
1023
1024
1025
1026
10−11
10−10
10−9
νFν [erg cm−2 s−1]
Frequency [Hz]
Figure 4: γ-ray spectral energy distribution (SED) of
PKS 1222+216. The MAGIC detection of 2010 June 17
is indicated with black circles (from [12]). Fermi/LAT
detection of 2010 April 30 (1 day integration) is
displayed with red stars (the upper limit is at 5σ level),
while blue triangles are for the detection on 2010 June 18
(1 day integration).
It is worth noting that the MAGIC observation
refers to 2010 June 17 [12]. The two events (April 30
and June 17-18) are not necessarily correlated or simi-
lar. It is not possible to know if on April 30 the source
extended its emission to hundreds of GeV, because
MAGIC started observing PKS 1222+216 on May 3
[12]. However, the spectral analysis of Fermi/LAT
data suggests that the emission drops after a few tens
of GeV (Fig. 4, red star points). The photon with
highest energy detected by LAT at that time measured
∼ 23 GeV. Instead, the spectrum measured during
the flare of 2010 June 17-18 (blue squares in Fig. 4)
seems to extend to match the MAGIC measurements
of [12], with the highest energy of LAT photons of
∼ 63 GeV. Particularly, in the last Fermi bin (31 <
E < 100 GeV), the source is detected with high signif-
icance (TS = 30, equivalent to ∼ 5.5σ). The partial
drop in flux as observed in the two 3 < E < 10 GeV
and 10 < E < 31 GeV bins can be consistent with an
absorption within the BLR according to the theory of
Poutanen & Stern [16], as already noted in [19].
3. FINAL REMARKS
From the variability alone, the two flares of 2010
April 30 and June 17-18 seem similar, with the time
scales consistent each other within the errors. The
value of τ ∼ 1 hour favors the dissipation in the BLR
(cf. Sect. 3 and Eqs. 3 and 4 of [7]). However, the
spectral analysis revealed two different states of the
source: one (2010 April 30) with a spectrum peak-
ing around ∼ 1024Hz and with no detection above
a few tens of GeV; the other (2010 June 17-18) has
a harder spectrum extending to hundreds of GeV, al-
though partially absorbed in the range ∼ 3−31 GeV.
It seems that most of the dissipation generally oc-
curs in the BLR, but sometimes there are episodes
where the region is at larger distances from the cen-
tral spacetime singularity, nearby the infrared torus
(see [12, 19, 21]). The possibility of an outward mo-
tion of the dissipation zone has been already suggested
by Stern & Poutanen [18] by studying 3C 454.3. In
this case, the motion would be on a larger scale, from
the BLR to the infrared torus.
Perhaps, this is the solution of the long-standing
question on where γ-rays are produced, which has
seen several researchersdebating during the past years
mainly on two opposite arrays: on one side, the two
groups led by Marscher [13, 14] and Sikora [17], re-
spectively, favoringthe hypothesis of a location nearby
the infrared torus, i.e. on super-pc scales; on the other
side, a more heterogeneous group supporting the sub-
pc location, within the BLR [4, 9, 16, 20]. The com-
bined spectral and variability analysis presented in
this work suggests that it is not a matter of either
one or the other hypothesis (aut-aut), but that both
hypotheses are possible in the same source, depend-
ing on the time of the event. That is, the dissipa-
tion occurs both in the BLR and in the infrared torus.
Generally in the BLR, but sometimes the blob is only
partially absorbed by the BLR and can move outward
still sufficiently collimated and dissipate nearby the
torus.
This is just one case and more coordinated obser-
vations in the GeV-TeV range are necessary to un-
derstand if this is an isolated anomaly or a typical
behavior of blazars.
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