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# Discovery of an Old, Nearby, and Overlooked Supernova Remnant Centered on the Southern Constellation Antlia Pneumatica

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## Abstract

We report the discovery of a supernova remnant (SNR) with an angular diameter of 24°, centered on the southern constellation Antlia Pneumatica. The SNR is detected well in Hα and X-rays. Within the Antlia SNR's outline, a marginally significant feature is detected in the 1.8 MeV gamma-ray line of the radioisotope 26Al. At an estimated distance dA 60-340 pc, the Antlia SNR is perhaps the nearest SNR except for the Local Bubble. Consequently, any associated neutron star or black hole is expected to have a large proper motion. Of the trajectories of nearby pulsars with well-determined proper motions, only B0950+08's passes within the SNR outline. If the SNR and the pulsar B0950+08 indeed both originated from the same supernova, then their age t = 1.8(dA/100 pc) Myr.
L41
The Astrophysical Journal, 576:L41–L44, 2002 September 1
DISCOVERY OF AN OLD, NEARBY, AND OVERLOOKED SUPERNOVA REMNANT CENTERED
ON THE SOUTHERN CONSTELLATION ANTLIA PNEUMATICA
P. R. McCullough,
1,2
Brian D. Fields, and Vasiliki Pavlidou
Department of Astronomy, University of Illinois at Champaign-Urbana, 1002 West Green Street, Urbana, IL 61801;
pmcc@astro.uiuc.edu, bdﬁelds@astro.uiuc.edu, pavlidou@astro.uiuc.edu
Received 2002 May 22; accepted 2002 July 24; published 2002 August 1
ABSTRACT
We report the discovery of a supernova remnant (SNR) with an angular diameter of 24, centered on the
southern constellation Antlia Pneumatica. The SNR is detected well in Haand X-rays. Within the Antlia SNR’s
outline, a marginally signiﬁcant feature is detected in the 1.8 MeV gamma-ray line of the radioisotope
26
Al. At
an estimated distance pc, the Antlia SNR is perhaps the nearest SNR except for the Local Bubble.d60–340
A
Consequently, any associated neutron star or black hole is expected to have a large proper motion. Of the
trajectories of nearby pulsars with well-determined proper motions, only B095008’s passes within the SNR
outline. If the SNR and the pulsar B095008 indeed both originated from the same supernova, then their age
Myr.tp1.8(d/100 pc)
A
Subject headings: gamma rays: observations — supernova remnants
Supernova explosions are one of nature’s most powerful
phenomena. They shape, heat, and ionize the interstellar me-
dium of galaxies (McKee & Ostriker 1977), and they synthesize
the heavy elements (Woosley & Weaver 1995). They leave
behind neutron stars or black holes, and an expanding super-
nova remnant (SNR) composed of both the star’s ejecta and
the interstellar matter swept up by the ejecta. Here we report
the discovery of an SNR with an angular diameter of 24,
centered on the southern constellation Antlia Pneumatica. Our
discussion is organized by passing from the most reliable to
the most speculative inferences. Thus, we ﬁrst present the Ha
and X-ray observations that support the identiﬁcation of an
SNR in Antlia. Then we discuss the tentative evidence for
26
Al
and the possible association of a high proper-motion pulsar
B095008 with the SNR. Finally, we note the possibility of
terrestrial radioisotopic signatures of such a recent, nearby su-
pernova explosion.
The strongest evidence for a new SNR in Antlia derives
from its Haand X-ray morphology. Located near the Vela
region that itself contains several SNRs, the Antlia Haemission
(Fig. 1) exhibits shell structure with an 24diameter, ap-
proximately centered on Galactic coordinates (l,b)p(276.5,
19) or J2000 coordinates (10
h
44
m
,37.2). The ﬁlaments on
the shell’s interior are also consistent with an SNR. In addition,
the ROSAT all-sky X-ray images clearly show a feature (Snow-
den et al. 1997) inside the Hashell that until now has not been
recognized as an SNR (Fig. 2). This X-ray enhancement is
consistent with a supernova origin, representing emission from
interstellar matter heated by the SNR.
Within the boundary of the Antlia SNR, the ROSAT soft X-
ray emission is anticorrelated with the IRAS 100 mm surface
brightness (Fig. 3). Because the 100 mm emission traces in-
terstellar dust and gas, the anticorrelation indicates that a sig-
niﬁcant fraction of the X-rays is absorbed by the gas column
between the Earth and the SNR. The SNR’s mean surface
brightness in the ROSAT keV band is counts s
1
1
3
0.3 #10
4
arcmin
2
larger than the surface brightness of the SNR’s vi-
cinity, which is counts s
1
arcmin
2
. To allow for
3
0.5 #10
1
Cottrell Scholar of Research Corporation.
2
Also at Space Telescope Science Institute, 3700 San Martin Drive, Bal-
timore, MD 21218.
dust emission and X-ray absorption from matter behind the
SNR, we subtract 0.9 MJy sr / from the IRAS 100 mm
1
sin FbF
emission and make a corresponding correction to the ROSAT
data for an extragalactichalo brightness of counts
3
0.4 #10
s
1
arcmin
2
(Snowden et al. 1998).
We then ﬁtted the ROSAT surface brightness within the SNR
outline to the remaining IRAS 100 mm emission. The median
IRAS 100 mm emission from dust between us and the Antlia
SNR is 2.0 MJy sr , corresponding to
1
ANSp3.0 #
H
between the Earth and the Antlia SNR (Schlegel,
20 2
10 cm
Finkbeiner, & Davis 1998). The inferred unattenuated keV
1
4
surface brightness of the Antlia SNR is Ip(1.10 0.04) #
1
counts s
1
arcmin
2
, which is similar to that of the faintest
3
10
SNR observed by ROSAT in the Large Magellanic Cloud (Wil-
liams et al. 1999). The quoted error for is simply the formal
I
1
error from the ﬁt; it does not include systematic errors inherent
to the model.
The distance to the Antlia SNR can be estimated in at least
three ways. First, the column density
20 2
ANSp3#10 cm
H
combined with an estimated mean volume density in the inter-
stellar medium, (Ferriere 1998), implies a dis-
3
AnSp1.0 cm
tance pc. The SNR must be at least somewhat outside
d100
A
the Local Bubble because its low volume density (AnSp
) cannot account for the column density observed be-
23
10 cm
tween us and the Antlia SNR. Second, SNRs have diameters
100 pc, so if the Antlia SNR subtends on the sky,
2v24
A
then its distance pc. Third, as we show below, if thed240
A
pulsar B095008 and the Antlia SNR were both created by the
same supernova, then the intensity of the
26
Al gamma-ray emis-
sion implies for ejected
26
Al mass between60 pc d140 pc
A
and (Timmes et al. 1995). These dis-
54
2#10 4 #10 M
,
tance estimates are self-consistent and indicate that the Antlia
SNR may be the closest isolated SNR to the Sun.
If the Antlia feature is an SNR, its large size indicates great
age. A model of SNR expansion allows one to estimate the
age given the remnant’s radius . For pressure-drivenRdv
AA
snowplow expansion (McKee & Ostriker 1977; Blinnikov,
Imshennik, & Utrobin 1982), we have #tp1.8 Myr
. We see that the SNR will be
5/6 35/6 7/2
E(n/1 cm ) (R/50 pc)
50
more than 1 Myr in age if the SNR thermal energy Ep
50
ergs is 10% of the initial 10
51
ergs explosion
50
E/10
therm
L42 DISCOVERY OF ANTLIA SNR Vol. 576
Fig. 1.—Antlia SNR (24in diameter) in continuum-subtracted Haimage
(Gaustad et al. 2001). The brighter emission is darker in this negative image.
The Galactic latitude and longitude are indicated in the Hammer-Aitoff pro-
jection. The 24diameter circle (in white) approximates the edge of the Antlia
SNR. The circle extends slightly below the SNR at . Also, there is anb10
Hicloud that approximately ﬁlls the SNR’s large indentation at (l,b)p
.(275,27)
Fig. 2.—The keV ROSAT image of the same region as in Fig. 1. The
1
4
outline of the SNR is nearly identical in the Haand keV emissions.
1
4
kinetic energy and if the explosion occurred in a medium of
number density . This implies that Antlia is a very
3
n1cm
old SNR; the oldest known SNR is seen as an H ishell in the
low-density outer Galaxy and has an expansion age of 4.3 Myr
(Stil & Irwin 2001).
Due to its proximity to Earth, the Antlia SNR offers new
possibilities for the study of supernova nucleosynthesis. Namely,
26
Al has a mean life Myr, and its decaytp1.03
produces a 1.8 MeV gamma-ray line. The COMPTEL experi-
ment on the Compton Gamma Ray Observatory found that the
1.8 MeV emission was tightly concentrated toward the Galactic
plane, a strong indiction that short-lived, massive stars are the
dominant nucleosynthesis site of
26
Al (Diehl et al. 1995). It is
thus natural to search for an
26
Al feature in the nearby Antlia
SNR.
The COMPTEL 1.8 MeV all-sky map (Diehl et al. 1995)
shows a feature (Fig. 4) within the Haand ROSAT boundaries
of the SNR. The 1.8 MeV local maximum is 3 jabove back-
ground (Oberlack 1997) and thus is marginally signiﬁcant; its
location, (l,b)p(271,16), is offset (Dl,Db)p(5.5,
3) from the nominal center of the SNR as seen in Ha,
(l,b)p(276.5, 19). We note that the edge of theSNR nearest
the 1.8 MeV local maximum is also the brightest segment of the
outline in Haand is the only segment of the outline that is
detected in radio continuum at 408 MHz (Haslam et al. 1982).
There are eight local maxima at 3jwithin two “control”
regions of sky deﬁned by and .10FbF300l360
Thus, the a priori probability of at least one of those eight having
appeared within the SNR’s outline by chance is 0.22.Therefore,
the 1.8 MeV feature in the Antlia SNR may be real and may
be due to
26
Al produced by the progenitor of the SNR.
The presence of
26
Al would strengthen the case that the Antlia
feature is an SNR. Also, the association of
26
Al with an SNR
would constitute direct evidence that this isotope is indeed
made in massive stars. Finally, an
26
Al signature is a powerful
chronometer. The angular size of the observed feature is close
to the COMPTEL resolution. If we treat the feature as a point
source of
26
Al, at distance and with age t, the expectedd
A
1.8 MeV line ﬂux is
521
Fp5.9 #10 photons cm s
g
2
M100 pc
ej, 26
t/1.03 Myr
#e.(1)
()
5
5#10 Md
,A
Here is the initial mass of
26
Al, and the ﬁducial value isM
ej, 26
from theory (Timmes et al. 1995) and is consistent with the
observed upper limit from Velorum (Oberlack et al. 2000),
2
g
a Wolf-Rayet star, for which the theoretical
26
Al yields (Langer,
Braun, & Fliegner 1995; Meynet et al. 1997) are comparable
to those of a supernova event. In fact, the Antlia feature appears
in the Velorum ﬁeld of view, and from Figure 1 of Oberlack
2
g
et al. (2000), we ﬁnd an observed ﬂux Fp(1.8 0.6) #
A
photons cm
2
s
1
. With a yield range of
55
10 2 #10 M
,
, we determine that 110 pc
t/1.03 Myr 4
Me 4#10 M
ej, 26 ,
d
A
510 pc, consistent with the estimates above.
Although the COMPTEL 1.8 MeV feature is consistent with
a point source, the full
26
Al emission from an SNR this old is
likely to be extended. The angular distribution of the emission
may give a clue as to the origin of the
26
Al. Current expectations
are that massive star
26
Al ejection can occur in Wolf-Rayet
winds prior to the explosion (Langer et al. 1995; Meynet et al.
1997) or in the subsequent explosion (Timmes et al. 1995). In
either case, the Antlia SNR should have its
26
Al in the shell
wall, either as wind material that was then swept by the blast
wave or as ejecta that has by now overtaken the shell. In either
case, one expects extended emission, the geometry and inho-
mogeneity of which would reﬂect that of the production mech-
anism; e.g., the point source might be the brightest of several
26
Al “knots” resulting from aspherical explosion ejecta, of the
No. 1, 2002 McCULLOUGH, FIELDS, & PAVLIDOU L43
Fig. 3.—The keV surface brightness measured by ROSAT compared with
1
4
the 100 mm surface brightness measured by IRAS within the outline of the
Antlia SNR for Galactic latitude (points). For clarity, and to improveb115
the statistical independence of the points selected, only a small random fraction
(10%) of the points is plotted. The points are binned, and the median (squares)
and error bars are plotted, where jis the standard deviation of the N
1j/N
points in each bin. The best two-parameter ﬁt of the form
jN
HH
IpIIe
X01
is shown, with and counts s
1
arcmin
2
(solid
6
Ip545 Ip1120 #10
01
line). The observed intensities of both the IRAS and ROSAT data have been
shifted to correct for emission and absorption behind the SNR; the upper and
lower vectors show the corrections for and 30, respectively.bp15
Fig. 4.—Surface brightness of the 1.8 MeV emission line from
26
Al con-
toured at levels of 1.0, 1.5, 2.0, … #10
4
photons cm
2
s
1
sr
1
(Kno¨dlseder
et al. 1999). The path of the pulsar B095008 is plotted at regular time
intervals in the past, based on its measured distance and proper motion (Brisken
et al. 2000). The coordinate grids of Figs. 1 and 2 are superposed. For assumed
distances from Earth to the Antlia SNR of 100, 150, and 200 pc, the implied
ages and radial velocities of the pulsar B095008 are , 2.7, andtp1.8
3.6 Myr and , 64, and 38 km s
1
, respectively.vp115
r
kind found in, e.g., the Cas A SNR (Hughes et al. 2000).
3
Also,
because the large SNR is expanding into the inhomogeneous
gas of the disk, we expect the shell material to be more con-
centrated and limb-brightened toward the denser regions inthe
Galactic plane, as observed. At any rate, we predict that ex-
tended emission is present at 1.8 MeV, with intensity Ip
g
, where is the column density of
26
Al.N/4pt N
26 26 26
The upcoming International Gamma-Ray Astrophysical Lab-
oratory (INTEGRAL) mission may be able to conﬁrm or refute
the presence of
26
Al in the Antlia SNR. If the Antlia SNR
26
Al
emission can be treated as a point source by INTEGRAL, then
a3jemission can be achieved after an on-target exposure
time of s (N. R. Trams 2000).
4
However, since the
5
2#10
SNR has such a large angular size, INTEGRAL may in fact be
able to resolve the 1.8 MeV emission. In this case, the relevant
background noise is increased, and larger exposure times will
be required. On the other hand, once the detection of the Antlia
feature by INTEGRAL has been achieved, any spatial infor-
mation concerning the gamma-ray emission will give valuable
information on the distribution of
26
Al within the SNR. More-
26
Al, INTEGRAL might (depending on the
actual age of the SNR) detect another long-lived radioisotope,
60
Fe ( Myr), whose decay produces two g-raytp2.2 0.4
lines, one at 1.173 MeV and another at 1.322 MeV.
60
Fe can
serve as an excellent diagnostic because it is coproduced with
3
The emission could be even more complex if
26
Al condensed into grains
that retain a high velocity, as suggested by balloon-borne observations (Naya
et al. 1996) of the Galactic center at 1.8 MeV.
4
See the SPI Observer’s Manual available on-line at http://astro.estec.esa.nl/
Integral/isoc/html/AO_documents.html.
26
Al (Timmes et al. 1995). Were the
60
Fe/
26
Al line ratios mea-
sured, the age of the SNR could be determined, subject only
to theoretical uncertainties in the relative yields. Conversely,
were distance and age independently determined, the line ratios
could be used to derive the yields and their ratios.
Given the proximity of the Antlia supernova to Earth, a
relatively large proper motion is expected for the neutron star
or black hole produced in the explosion. If a neutron star was
produced, it may be observed as a pulsar. Of the eight pulsars
with kpc and well-measured proper motions (Hooger-d!1
werf, de Bruijne, & de Zeeuw 2001), we ﬁnd only one,
B095008 (Pilkington et al. 1968; Brisken et al. 2000), whose
trajectory intersects the boundary of the Antlia SNR (Fig. 4).
In fact, B095008’s trajectory does not cross the center of the
remnant but does intersect with the location of the
26
Al feature.
However, because not all supernovae produce neutron stars,
and because not all neutron stars are observable as pulsars, we
cannot be certain that B095008 is the stellar remnant asso-
ciated with the Antlia SNR.
If we assume that the SNR and B095008 arise from the
same supernova explosion, then their geometry and kinematics
can be used to derive an age. As seen in Figure 5, we have an
“age-distance relation” of , consistenttp1.8 Myr(d/100 pc)
A
with the distance and age estimates above. This relation and
the pressure-driven expansion (McKee & Ostriker 1977; Blin-
nikov et al. 1982) relation yield a distance
50 1/3 31/3
dp340 pc(E/10 ergs) (n/1 cm ) (2)
Atherm
and an age
50 1/3 31/3
tp6.2 Myr(E/10 ergs) (n/1 cm ) . (3)
therm
L44 DISCOVERY OF ANTLIA SNR Vol. 576
Fig. 5.—Antlia SNR (A), Earth (E), and the pulsar B095008 (P), which
form a triangle with angle AEP ( ). The pulsar’s distance EP pvp45
pc and its tangential velocity km s
1
are known from the280 25 vp39
VLBI (Brisken et al. 2000). If we denote the time tsince the supernova
explosion, then the physical scale of the triangle is determined by AB p
and thus the distance to the Antlia SNR , and
vtdpEA p55(t/1 Myr) pc
A
km s
1
.vp(EP dcos v)/t55(280 pc/dcos v)
AA
r
These estimates are within a factor of 2 of the other estimates
mentioned above, an acceptable agreement given the crudeness
of the simpliﬁcations used for the expansion of an old SNR.
Improved age estimates for the SNR and the pulsar would allow
for a better assessment of the likelihood of their association.
Alternatively, combining the age-distance relation with the
observed
26
Al ﬂux provides other limits on and tvia equationd
A
(1). Given an
26
Al yield of (Tim-
5
Mp(2–40) #10 M
ej, 26 ,
mes et al. 1995; Oberlack et al. 2000) and the COMPTEL ﬂux
value of photons cm
2
s
1
, , and
5
1.8 #10 dp62–140 pc
A
thus Myr, consistent with the results using thetp1.1–2.5
expansion law within uncertainties. The inferred age is ap-
proximately 10 times smaller than the mean spin-down age of
B095008 ( Myr), but kinematically determined
˙
P/2Pp17
ages tend to be less than spin-down ages, in some cases by
large factors (Cordes & Chernoff 1998).
It is thus possible that the Antlia feature and B095008 are
an example of an SNR linked to a distant pulsar. Such an
connection is important to current theories of pulsar kicks (Cor-
des & Chernoff 1998; Lyne & Lorimer 1994) and can be further
tested for the Antlia SNR by better determinations of its age
and distance.
It is interesting to consider the effects of such a nearby su-
pernova on the Earth. Nearby supernovae may have deposited
less than that of the Earth (Ellis, Fields, & Schramm 1996). Live
60
Fe has been recently detected in a deep-ocean crust (Knie et
al. 1999), and Benı´tez, Maı´z-Apella´niz, & Canelles (2002) have
suggested its origin in one or more supernova explosions within
the past 11 Myr from the Scorpius-Centaurus OB association.
The Antlia supernova is probably too recent and too distant to
have contributed to the terrestrial
60
Fe. In any case, the Antlia
event would have occurred beyond the “minimum safe distance”
of 10 pc (Ruderman 1975; Ellis & Schramm 1995) and yet
would have been a spectacular event: at maximum light, it would
have appeared nearly as bright as the full Moon.
We thank Uwe Oberlack for providing machine-readable maps
of
26
Al and for helpful discussions. We also thank You-Hua Chu,
Jim Cordes, John Dickel, Icko Iben, Uwe Oberlack, and Beate
Woermann for helpful discussions. We are grateful to the anon-
ymous referee for constructive comments that improved this Let-
ter. This work was supported by the Research Corporation and
NSF grants AST 98-74670 and AST 00-92939.
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... Here we present results of an initial study of SNRs located far away from the Galactic plane using wide field-of-view (FOV) UV images assembled from the Galaxy Evolution Explorer (GALEX) All-Sky survey (Bianchi 2009). We began this research by investigating two unusually high latitude suspected supernova remnants including the exceptionally large Antlia remnant (McCullough et al. 2002). During this work, we also found a new apparent SNR. ...
... The proposed new SNRs include the huge ∼24 • diameter Antlia nebula (G275. 5+18.4;McCullough et al. 2002) and the ∼10 • diameter radio remnant G354-33 (Testori et al. 2008). Due to limited data on it, Antlia is listed only as a possible SNR in the most recent catalogue of Galactic supernova remnants (Green 2019), whereas no mention is made of G354-33 in either this 2019 SNR catalogue or in online SNR lists 4 . Below we discuss GALEX far UV ...
... In the top panel of Figure 13, we present a mosaic of SHASSA Hα images of the Antlia remnant at a higher resolution than the VTTS Hα image (Finkbeiner 2003) which led McCullough et al. (2002) to discover it. It shows a ∼ 20 • × 26 • Hα emission shell with a well determined boundary roughly centered at α(J2000) = 10 h 38 m , δ(J2000) = −37 • 18 corresponding to Galactic coordinates l = 275.5 • b = +18.4 ...
Preprint
Galactic supernova remnants (SNRs) with angular dimensions greater than a few degrees are relatively rare, as are remnants located more than ten degrees off the Galactic plane. Here we report the results of a UV and optical investigation of two previously suspected SNRs that are more than ~10 degrees in both angular diameter and Galactic latitude. One is the proposed G354-33 remnant discovered in 2008 through 1420 MHz polarization maps. GALEX far UV (FUV) emission and H$\alpha$ mosaics show the object's radio emission coincident with a nearly continuous 11 x 14 degree shell of thin UV filaments which surround a broad H$\alpha$ emission ring. Another proposed high latitude SNR is the enormous 20 x 26 degree Antlia nebula (G275.5+18.4) discovered in 2002 through low-resolution all-sky H$\alpha$ images and ROSAT soft X-ray emission. GALEX FUV image mosaics along with deep H$\alpha$ images and optical spectra of several filaments indicate the presence of shocks throughout the nebula with estimated shock velocities of 70 to over 100 km s$^{-1}$. We conclude that both of these nebulae are bona fide SNRs with estimated ages less than 10$^{5}$ yr despite their unusually large angular dimensions. Using FUV and optical spectra and images, we also report finding an apparent new, high latitude SNR (G249.2+24.4) approximately 2.8 x 4.2 degrees in size based on its UV and optical emission properties.
... The Antlia supernova remnant (SNR) is located at ( , ) = (276.5 • , +19 • ) and has a large angular diameter of 24 • (McCullough et al. 2002). Because such high galactic latitude and size are not common among known Galactic SNRs, whether the object is a supernova-driven remnant or not had been suspected since its first discovery by McCullough et al. (2002). ...
... • , +19 • ) and has a large angular diameter of 24 • (McCullough et al. 2002). Because such high galactic latitude and size are not common among known Galactic SNRs, whether the object is a supernova-driven remnant or not had been suspected since its first discovery by McCullough et al. (2002). Only recently, it is confirmed that the remnant reveals shock-driven emission regions in UV and optical lines supporting the SNR origin (Fesen et al. 2021). ...
... This SNR is bright in but weak in radio continuum, suggesting that it is a relatively evolved system. The distance to the SNR is not well constrained, but the large angular size and features arising from interacting with nearby ISM (e.g., Gum Nebula) locate it around 60 − 340 pc away within the Galactic disc (McCullough et al. 2002). Note its striking morphological coincidence with the Magellanic Leading Arms presented in Figure 2. ...
Preprint
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Faraday Rotation Measures (RM) should be interpreted with caution because there could be multiple magneto-ionized medium components that contribute to the net Faraday rotation along sight-lines. We introduce a simple test using Galactic diffuse polarised emission that evaluates whether structures evident in RM observations are associated with distant circumgalactic medium (CGM) or foreground interstellar medium (ISM). We focus on the Magellanic Leading Arm region where a clear excess of RM was previously reported. There are two gaseous objects standing out in this direction: the distant Magellanic Leading Arm and the nearby Antlia supernova remnant (SNR). We recognized narrow depolarised filaments in the $2.3\,\rm GHz$ S-band Polarization All Sky Survey (S-PASS) image that overlaps with the reported RM excess. We suggest that there is a steep gradient in Faraday rotation in a foreground screen arising from the Antlia SNR. The estimated strength of the line-of-sight component of the magnetic field is $B_{\parallel}\sim 5\,\rm\mu G$, assuming that the excess of RM is entirely an outcome of the magnetized supernova shell. Our analysis indicates that the overlap between the RM excess and the Magellanic Leading Arm is only a remarkable coincidence. We suggest for future RM grid studies that checking Galactic diffuse polarisation maps is a convenient way to identify local Faraday screens.
... As remnants age and dissipate back into the interstellar medium, relics of these shells will become elongated along field lines, and may leave filaments aligned with the mean field. Strong filamentary structure of a very old (and nearby) SNR could possibly remain in evidence as long as ∼ 10 6 years (e.g., McCullough et al. 2002), though this is up to a factor of 10 longer than generally accepted (Reynolds et al. 2012). ...
Preprint
We present a simple, unified model that can explain two of the brightest, large-scale, diffuse, polarized radio features in the sky, the North Polar Spur (NPS) and the Fan Region, along with several other prominent loops. We suggest that they are long, magnetized, and parallel filamentary structures that surround the Local arm and/or Local Bubble, in which the Sun is embedded. We show this model is consistent with the large number of observational studies on these regions, and is able to resolve an apparent contradiction in the literature that suggests the high latitude portion of the NPS is nearby, while lower latitude portions are more distant. Understanding the contributions of this local emission is critical to developing a complete model of the Galactic magnetic field. These very nearby structures also provide context to help understand similar non-thermal, filamentary structures that are increasingly being observed with modern radio telescopes.
... These southern hemisphere all-sky views show the |∇P | maps instead of |∇P |/|P | as shown by Iacobelli et al. (2014). This choice is justified by the large number of depolarisation canals in the |P | map. Figure 8 shows a subregion including the old SNR Antlia (McCullough et al. 2002) where many depolarisation canals are present. This effect, caused by beam depolarisation, occurs when the telescope beam is larger than the scale of the RM gradient (Haverkorn & Heitsch 2004). ...
Article
We compare two rotationally invariant decomposition techniques on linear polarisation data: the spin-2 spherical harmonic decomposition in two opposite parities, the $E$- and $B$-mode, and the multiscale analysis of the gradient of linear polarisation, $|\nabla \mathbf{P}|$. We demonstrate that both decompositions have similar properties in the image domain and the spatial frequency domain. They can be used as complementary tools for turbulence analysis of interstellar magnetic fields in order to develop a better understanding of the origin of energy sources for the turbulence, the origin of peculiar magnetic field structures and their underlying physics. We also introduce a new quantity $|\nabla EB|$ based on the $E$- and $B$-modes and we show that in the intermediate and small scales limit $|\nabla EB| \simeq |\nabla \mathbf{P}|$. Analysis of the 2.3 GHz S-band Polarization All Sky Survey (S -PASS) shows many extended coherent filament-like features appearing as 'double-jumps' in the $|\nabla \mathbf{P}|$ map that are correlated with negative and positive filaments of $B$-type polarisation. These local asymmetries between the two polarisation types, $E$ and $B$, of the non-thermal Galactic synchrotron emission have an influence on the $E$- and $B$-mode power spectra analyses. The wavelet-based formalism of the polarisation gradient analysis allows us to locate the position of $E$- or $B$-mode features responsible for the local asymmetries between the two polarisation types. In analysed subregions, the perturbations of the magnetic field are trigged by star clusters associated with HII regions, the Orion-Eridanus superbubble and the North Polar Spur at low Galactic latitude.
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The supernova remnant (SNR) candidate G 116.6-26.1 is one of the few high Galactic latitude ($|b| > 15^o$) remnants detected so far in several wavebands. It was discovered recently in the SRG/eROSITA all-sky X-ray survey and displays also a low-frequency weak radio signature. In this study, we report the first optical detection of G 116.6-26.1 through deep, wide-field and higher resolution narrowband imaging in $\rm H\alpha$, [S II] and [O III] light. The object exhibits two major and distinct filamentary emission structures in a partial shell-like formation. The optical filaments are found in excellent positional match with available X-ray, radio and UV maps, can be traced over a relatively long angular distance (38' and 70') and appear unaffected by any strong interactions with the ambient interstellar medium. We also present a flux-calibrated, optical emission spectrum from a single location, with Balmer and several forbidden lines detected, indicative of emission from shock excitation in a typical evolved SNR. Confirmation of the most likely SNR nature of G 116.6-26.1 is provided from the observed value of the line ratio $\rm [S\, II] / H\alpha = 0.56 \pm 0.06$, which exceeds the widely accepted threshold 0.4, and is further strengthened by the positive outcome of several diagnostic tests for shock emission. Our results indicate an approximate shock velocity range $70-100\, km\, s^{-1}$ at the spectroscopically examined filament, which, when combined with the low emissivity in $\rm H\alpha$ and other emission lines, suggest that G 116.6-26.1 is a SNR at a mature evolutionary stage.
Article
We present a simple, unified model that can explain two of the brightest, large-scale, diffuse, polarized radio features in the sky, the North Polar Spur (NPS) and the Fan Region, along with several other prominent loops. We suggest that they are long, magnetized, and parallel filamentary structures that surround the Local arm and/or Local Bubble, in which the Sun is embedded. We show that this model is consistent with the large number of observational studies on these regions and is able to resolve an apparent contradiction in the literature that suggests that the high-latitude portion of the NPS is nearby, while lower-latitude portions are more distant. Understanding the contributions of this local emission is critical to developing a complete model of the Galactic magnetic field. These very nearby structures also provide context to help understand similar nonthermal, filamentary structures that are increasingly being observed with modern radio telescopes.
Article
Faraday Rotation Measures (RM) should be interpreted with caution because there could be multiple magneto-ionized medium components that contribute to the net Faraday rotation along sight-lines. We introduce a simple test using Galactic diffuse polarised emission that evaluates whether structures evident in RM observations are associated with distant circumgalactic medium (CGM) or foreground interstellar medium (ISM). We focus on the Magellanic Leading Arm region where a clear excess of RM was previously reported. There are two gaseous objects standing out in this direction: the distant Magellanic Leading Arm and the nearby Antlia supernova remnant (SNR). We recognized narrow depolarised filaments in the $2.3\, \rm GHz$ S-band Polarization All Sky Survey (S-PASS) image that overlaps with the reported RM excess. We suggest that there is a steep gradient in Faraday rotation in a foreground screen arising from the Antlia SNR. The estimated strength of the line-of-sight component of the magnetic field is $B_{\parallel }\sim 5\, \rm \mu G$, assuming that the excess of RM is entirely an outcome of the magnetized supernova shell. Our analysis indicates that the overlap between the RM excess and the Magellanic Leading Arm is only a remarkable coincidence. We suggest for future RM grid studies that checking Galactic diffuse polarisation maps is a convenient way to identify local Faraday screens.
Article
Our view of the interstellar medium of the Milky Way and the universe beyond is affected by the structure of the local environment in the solar neighborhood. We present the discovery of a 30-degree-long arc of ultraviolet emission with a thickness of only a few arcminutes: the Ursa Major arc. This consists of several arclets seen in the near- and far-ultraviolet bands of the GALEX satellite. A two degree section of the arc was first detected in the H α optical spectral line in 1997; additional sections were seen in the optical by the team of amateur astronomers included in this work. This direction of the sky is known for very low hydrogen column density and dust extinction; many deep fields for extragalactic and cosmological investigations lie in this direction. Diffuse ultraviolet and optical interstellar emission are often attributed to scattering of light by interstellar dust. The lack of correlation between the Ursa Major arc and thermal dust emission observed with the Planck satellite, however, suggests that other emission mechanisms must be at play. We discuss the origin of the Ursa Major arc as the result of an interstellar shock in the solar neighborhood.
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Many young, massive stars are found in close binaries. Using population synthesis simulations we predict the likelihood of a companion star being present when these massive stars end their lives as core-collapse supernovae (SNe). We focus on stripped-envelope SNe, whose progenitors have lost their outer hydrogen and possibly helium layers before explosion. We use these results to interpret new Hubble Space Telescope observations of the site of the broad-lined Type Ic SN 2002ap, 14 years post-explosion. For a subsolar metallicity consistent with SN 2002ap, we expect a main-sequence companion present in about two thirds of all stripped-envelope SNe and a compact companion (likely a stripped helium star or a white dwarf/neutron star/black hole) in about 5% of cases. About a quarter of progenitors are single at explosion (originating from initially single stars, mergers or disrupted systems). All the latter scenarios require a massive progenitor, inconsistent with earlier studies of SN 2002ap. Our new, deeper upper limits exclude the presence of a main-sequence companion star $>8$-$10$ Msun, ruling out about 40% of all stripped-envelope SN channels. The most likely scenario for SN 2002ap includes nonconservative binary interaction of a primary star initially $\lesssim 23$ Msun. Although unlikely ($<$1% of the scenarios), we also discuss the possibility of an exotic reverse merger channel for broad-lined Type Ic events. Finally, we explore how our results depend on the metallicity and the model assumptions and discuss how additional searches for companions can constrain the physics that governs the evolution of SN progenitors.
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This paper presents new maps of the soft X-ray background from the ROSAT all-sky survey. These maps represent a significant improvement over the previous version in that (1) the position resolution of the PSPC has been used to improve the angular resolution from ~2? to 12', (2) there are six energy bands that divide each of the previous three into two parts, and (3) the contributions of point sources have been removed to a uniform source flux level over most of the sky. These new maps will be available in electronic format later in 1997. In this paper we also consider the bright emission in the general direction of the Galactic center in the 0.5-2.0 keV band, and the apparent absorption trough that runs through it along the Galactic plane. We find that while the northern hemisphere data are confused by emission from Loop I, the emission seen south of the plane is consistent with a bulge of hot gas surrounding the Galactic center (in our simple model, a cylinder with an exponential fall-off of density with height above the plane). The cylinder has a radial extent of ~5.6 kpc. The X-ray emitting gas has a scale height of 1.9 kpc, an in-plane electron density of ~0.0035 cm⁻³, a temperature of ~106.6 K, a thermal pressure of ~28,000 cm⁻³ K, and a total luminosity of ~2 × 10³⁹ ergs s⁻¹ using a collisional ionization equilibrium (CIE) plasma emission model.
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An analytic solution is obtained for the evolution of a supernova remnant during the radiative-cooling phase. The Cygnus Superbubble X-ray source could have been formed by the explosion of a single supernova releasing an energy of 10 to the 52nd - to to the 53rd erg. Analysis of the light curve of the NGC 1058 supernova 1961v demonstrates that it represents an approximately 2 x 10 to the 52nd outburst of a supermassive (approximately 1000 solar masses) star. Giant shell sources may form an evolutionary sequence, exemplified by R136a as the presupernova, supernova 1961v, and the Superbubble as the remnant.
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The Wolf-Rayet binary system γ2 Vel (WR 11) is the closest known Wolf-Rayet (WR) star. Recently, its distance has been redetermined by parallax measurements with the HIPPARCOS astrometric satellite yielding 258+41-31 significantly lower than previous estimates (300-450 pc). Wolf-Rayet stars have been proposed as a major source of the Galactic 26 Al observed at 1.809 MeV. The gamma-ray telescope COMPTEL has previously reported 1.8 MeV emission from the Vela region, yet located closer to the Galactic plane than the position of 72 Vel. We derive an upper 1.8 MeV flux limit of 1.1 10-5γcm-2 S-1 (2 σ) for the WR star. With the new distance estimate, COMPTEL measurements place a limit of (6.3+2.1-1.4) 10-5 M⊙on the 26 AI yield of γ2 Vel, thus constrains theories of nucleosynthesis in Wolf-Rayet stars. We discuss the implications in the context of the binary nature of γ2 Vel and present a new interpretation of the IRAS Vela shell.
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We have completed a robotic wide-angle imaging survey of the southern sky (delta=+15deg to -90°) at 656.3 nm wavelength, the Halpha emission line of hydrogen. Each image of the resulting Southern Halpha Sky Survey Atlas (SHASSA) covers an area of the sky 13° square at an angular resolution of approximately 0.8' and reaches a sensitivity level of 2 R (1.2×10-17 ergs cm-2 s-1 arcsec-2) pixel-1, corresponding to an emission measure of 4 cm-6 pc and to a brightness temperature for microwave free-free emission of 12 muK at 30 GHz. Smoothing over several pixels allows features as faint as 0.5 R to be detected.
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In a deep ocean ferromanganese crust an excess of 60Fe radioactivity was measured by means of high sensitivity accelerator mass spectrometry. The enhanced concentrations measured in the first two of three layers (corresponding to a time span of 0-2.8 Myr and 3.7-5.9 Myr, respectively) suggest the deposition of supernova produced 60Fe on earth. There is even a weak indication that the flux into the crust was higher about 5 Myr ago.
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Details are now given of three of the four pulsating radio sources discovered at Cambridge.
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THEORY predicts1 that radioactive 26A1 (which has a half-life of 0.72 Myr) is released into the interstellar medium by nova and supernova explosions, from the winds of massive stars in the Wolf–Rayet phase, and from less-massive giant stars in very late stages of the asymptotic giant branch phase. Observations of 1,809-keV γ-ray emission line from 26A1 can therefore be used as a tracer of Galactic nucleosynthesis during the past million years2,3. The irregularity of the emission in the plane of the Galaxy4–7 suggests that the dominant sources are likely to be massive stars and supernovae; the other predicted sources are older, and therefore expected to be distributed more uniformly. Here we report the detection of the 1,809-keV emission line from the direction of the Galactic Centre, and we show that the line width is approximately three times that expected8,9 from the effect of Doppler broadening due to Galactic rotation. The high velocities inferred from the line width favour an origin of the 26A1 in supernovae or Wolf–Rayet stars. Moreover, the fact that the 26A1 has maintained such high velocities is difficult to reconcile with our current understanding of the propagation of material in the interstellar medium.
Article
NEUTRON stars are usually born during the supernova explosion of a massive star. Any small asymmetry during the explosion can result in a substantial ‘kick’ velocity1 to the neutron star. Pulsars (rapidly rotating, magnetized neutron stars) have long been known to have high space velocities2,3, but new measurements of proper motion4–6, adoption of a new distance scale for the pulsars7 and the realization that some previous velocities were systematically low by a factor of 2 (ref. 8) have prompted us to reassess these velocities. Here, taking into account a strong selection effect that makes the observed velocities unrepresentative of those acquired at birth9, we arrive at a mean pulsar birth velocity of 450 ± 90 km s–1 This exceeds the escape velocity from binary systems, globular clusters and the Galaxy, and so will affect our understanding of the retention of neutron stars in these systems. Those neutron stars that are retained by the Milky Way will be distributed more isotropically than has been thought10–12, which may result in a distribution like that of the γ-ray burst sources.
Article
We present a full-sky 100 μm map that is a reprocessed composite of the COBE/DIRBE and IRAS/ISSA maps, with the zodiacal foreground and confirmed point sources removed. Before using the ISSA maps, we remove the remaining artifacts from the IRAS scan pattern. Using the DIRBE 100 and 240 μm data, we have constructed a map of the dust temperature so that the 100 μm map may be converted to a map proportional to dust column density. The dust temperature varies from 17 to 21 K, which is modest but does modify the estimate of the dust column by a factor of 5. The result of these manipulations is a map with DIRBE quality calibration and IRAS resolution. A wealth of filamentary detail is apparent on many different scales at all Galactic latitudes. In high-latitude regions, the dust map correlates well with maps of H I emission, but deviations are coherent in the sky and are especially conspicuous in regions of saturation of H I emission toward denser clouds and of formation of H2 in molecular clouds. In contrast, high-velocity H I clouds are deficient in dust emission, as expected.