- GCN NOTICE
TITLE: GCN/INTEGRAL NOTICE
NOTICE_DATE: Sun 18 Sep 11 21:28:23 UT
NOTICE_TYPE: INTEGRAL SPI ACS Trigger
TRIGGER_NUM: 6356, Sub_Num: 0
GRB_INTEN: 23.76 [sigma]
GRB_TIME: 77217.14 SOD {21:26:57.14} UT
GRB_DATE: 15822 TJD; 261 DOY; 11/09/18
COMMENTS: INTEGRAL SPI_ACS GRB Trigger.
COMMENTS: The SPIACS lightcurve can be found at:
COMMENTS: ftp://isdcarc.unige.ch/arc/FTP/ibas/spiacs/2011-09/2011-09-18T21-26-57.0025-01790-00007-0.lc
- GCN Circular #12357
K. Hurley, on behalf of the Mars Odyssey and MESSENGER GRB teams,
S. Golenetskii, R. Aptekar, E. Mazets, V. Pal'shin, D. Frederiks,
D. Svinkin, and T. Cline on behalf of the Konus-Wind team,
I. G. Mitrofanov, D. Golovin, M. L. Litvak, and A. B. Sanin,
on behalf of the HEND-Odyssey GRB team,
W. Boynton, C. Fellows, K. Harshman, H. Enos, and R. Starr, on
behalf of the GRS-Odyssey GRB team,
A. von Kienlin, X. Zhang, V. Savchencko, and A. Rau, on behalf of the
INTEGRAL SPI-ACS GRB team, and
J. Goldsten, on behalf of the MESSENGER NS GRB team, report:
The long-duration, extremely intense GRB110918A was observed by
INTEGRAL (SPI-ACS), Konus-Wind, Mars Odyssey (HEND), and MESSENGER
(GRNS) at 21:26:57 UT. Swift was in the SAA, and Fermi was
Earth-occulted. We have triangulated it to a preliminary, 3 sigma
error box whose coordinates are:
-----------------------------------------------
RA(2000), deg Dec(2000), deg
-----------------------------------------------
Center:
32.575 (02h 10m 18s) -27.281 (-27d 16' 51")
Corners:
32.587 (02h 10m 21s) -27.114 (-27d 06' 51")
32.636 (02h 10m 33s) -27.237 (-27d 14' 14")
32.561 (02h 10m 15s) -27.447 (-27d 26' 48")
32.513 (02h 10m 03s) -27.324 (-27d 19' 27")
-----------------------------------------------
The error box area is about 62 sq. arcmin., and its maximum
dimension is about 20 arcmin. This localization can be
improved.
We note that this is the most intense burst observed in at
least several years of IPN monitoring; details of the time
history and energy spectrum will be given in a forthcoming GCN
Circular.
A Swift ToO observation has been scheduled.
- GCN Circular #12361
M. Jelinek and A. J. Castro-Tirado (IAA-CSIC, Granada), on behalf of a
larger collaboration, report:
"We observed the error box of the extraordinarily intense GRB110918A
detected by the IPN (Hurley et al. GCNC 12357) with the 0.3m BOOTES-1
robotic telescope in southern Spain. A 270x20s unfiltered coadded
exposure taken under non-optimal conditions (low position in the sky,
moonlight and fog) reached a limiting magnitude of ~17.0 at an
exposure mean time of Sep 20, 1.09 UT (i.e. 27.65 hr after the onset
of the high-energy emission). No new object is present within the
entire IPN error box when compared to the DSS. Deeper optical/nIR
observations are encouraged."
This message can be quoted.
- GCN Circular #12362
S. Golenetskii, R.Aptekar, D. Frederiks, E. Mazets, V. Pal'shin,
P. Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf
of the Konus-Wind team, report:
The long exceptionally intense GRB 110918A
(localized by IPN: Hurley et al., GCN 12357)
triggered Konus-Wind at T0=77222.856s UT (21:27:02.856)
The burst is the most intense long GRB event in the history
of Konus-Wind observations since November, 1994.
The light curve of event started with an extremely bright hard short
pulse followed by three weaker partly overlapping peaks in the next 25 seconds.
The emission during this part of the burst is seen up to ~12 MeV.
A spectral lag between hard G3(1400-300 keV) and soft G1(25-90 keV)
instrument's light curves of the initial bright phase of the event
is estimated to a moderate value of ~0.090 s.
A weak decaying emission in the soft energy channel G1(25-90 keV)
continues till at least T0+250s when the measurements stopped.
The Konus-Wind light curve of this GRB is available at
http://www.ioffe.ru/LEA/GRBs/GRB110918_T77222/
As observed by Konus-Wind the burst
had a fluence of 7.5(-0.2,+0.2)x10-4 erg/cm2,
and a 16-ms peak flux, measured from T0+0.368 s,
of 8.7(-0.4,+0.4)x10-4 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The best fit of the time-integrated spectrum of the burst
(measured from T0 to T0+69.376 s) is achieved
(in the 20 keV - 10 MeV range) with the GRB (Band) model, for which:
the low-energy photon index alpha = -1.2 (-0.1, +0.1),
the high energy photon index beta = -2.0 (-0.04, +0.03),
the peak energy Ep = 150(-17, +20) keV,
chi2 = 171/81 dof.
The spectrum near the maximum count rate
(measured from T0+0.256 to T0+0.512 s)
is best fitted in the 20 keV - 10 MeV range
by the GRB (Band) model, for which:
the low-energy photon index alpha = -0.34 (-0.02, +0.02),
the high energy photon index beta = -2.5 (-0.1, +0.1),
the peak energy Ep = 1150(-110, +120) keV,
chi2 = 102/83 dof.
All the quoted results are preliminary.
All the quoted errors are at the 90% confidence level.
- GCN Circular #12363
J-L. Atteia (IRAP-UPS/CNRS)& A. Pelangeon report:
We used the spectral parameters of GRB 110918A provided by Golenetskii et al. (GCNC 12362)
to compute the spectral pseudo-redshift(**) of this burst localized by IPN (Pal'shin et al., GCNC 12358).
We find a pseudo-redshift pz= 0.28 +/- .07
** cf.http://cosmos.ast.obs-mip.fr/projet/v2/calculation_pz.html
- GCN Circular #12364
V. Mangano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/IASFPA), P. A. Evans
(U Leicester), H. A. Krimm (GSFC/USRA) report on behalf of the
Swift-XRT team:
We have analysed 2.5 ks of XRT data for the IPN-detected burst: GRB
110918A (Hurley et al. et al. GCN Circ. 12357), from 107.4 ks to 113.6
ks after the IPN trigger. The data are entirely in Photon Counting
(PC) mode. An new X-ray source is detected close to the edge of the
XRT FoV. Using 2498 s of PC mode data and 6 UVOT images, we find an
enhanced XRT position (using the XRT-UVOT alignment and matching UVOT
field sources to the USNO-B1 catalogue): RA, Dec = 32.53860, -27.10610
which is equivalent to:
RA (J2000): 02h 10m 9.27s
Dec(J2000): -27d 06' 22.0"
with an uncertainty of 1.5 arcsec (radius, 90% confidence). This
position is 640 arcsec from the IPN position, and outside the IPN error
box. The light curve uses only the first 1.5 ks of data, since after
this the source is too near the edge of the field of view for the light
curve extraction tools to work. This light curve is consistent with a
constant source of mean count rate 2.7e-01 ct/sec. A power-law fit
gives an index of -1.320 (+3.525, -0.013). The source count rate is
definitely above the 3 sigma upper limit of 0.08 ct/sec obtained from
the analysis of RASS data.
We believe that such a bright uncatalogued X-ray source close to the
IPN error box is likely the afterglow of the GRB.
A spectrum formed from the PC mode data can be fitted with an absorbed
power-law with a photon spectral index of 2.12 (+0.28, -0.23). The
best-fitting absorption column is 1.5 (+0.7, -0.6) x 10^21 cm^-2, in
excess of the Galactic value of 1.7 x 10^20 cm^-2 (Kalberla et al.
2005). The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is 2.9 x 10^-11 (4.2 x 10^-11) erg
cm^-2 count^-1.
A summary of the PC-mode spectrum is thus:
Total column: 1.5 (+0.7, -0.6) x 10^21 cm^-2
Galactic foreground: 1.7 x 10^20 cm^-2
Excess significance: 3.9 sigma
Photon index: 2.12 (+0.28, -0.23)
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020186.
This circular is an official product of the Swift-XRT team.
- GCN Circular #12365
N.R. Tanvir, K. Wiersema (U. Leicester), A.J. Levan, S. Greiss,=C2=A0
B. Gaensicke (U. Warwick) report on behalf of a larger collaboration:=C2=A0
We observed the field of IPN burst GRB 110918A (Hurley et al. GCN
12357) with the Isaac Newton Telescope on La Palma, using the Wide
Field Camera starting at 20-Sep-2011 02:22 UT, roughly 29 hours
post-burst. =C2=A0Close to the location of the X-ray source reported by M=
angano
et al. (GCN 12364) we detect a bright optical point-source which is not
visible in the DSS2 images of the region.
Very provisional calibration gives a magnitude R~19.0-19.5 (well above th=
e
DSS2 limit) and a position:
RA(2000) = 02 10 09.39
dec(2000)=-27 06 19.6
accurate to ~0.5" in each coordinate. =C2=A0We cannot yet assess variabil=
ity.
- GCN Circular #12366
J. Elliott (MPE Garching), T. Kruehler (DARK/NBI), S. Klose, D. A. Kann
(both TLS Tautenburg), A. Rau and J. Greiner (both MPE Garching) report
on behalf of the GROND team:
We observed the field of the IPN GRB 110918A (Hurley et al., GCN 12357)
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120,
405) mounted at the 2.2m MPI/ESO telescope at La Silla Observatory (Chile).
Observations started at September 20, 2011, 02:37 which is roughly 29.2
hr after the trigger. They consisted of a dithered mosaic of 5 tiles
covering the full IPN error box. The X-ray and optical afterglow
candidate reported by Mangano et al. (GCN #12364) and Tanvir et al. (GCN
#12365) however is only covered by the larger field of view of the NIR
channels.
We confirm the presence of a bright NIR source, and estimate a
preliminary AB magnitude of J = 18.7 +/- 0.1 at a midtime of 08:04 UT,
which has been derived by calibrating the field against stars from the
2MASS catalog.
- GCN Circular #12367
S. B. Cenko (UC Berkeley), N. R. Tanvir (Leicester), E. O. Ofek (Caltech /
Weizmann Institute), K. Wiersema (Leicester), A. J. Levan (Warwick), and
P. E. Nugent (LBNL):
As part of the Palomar Transient Factory (PTF), we have imaged the
location of the afterglow candidate (GCN 12364, GCN 12365) of the IPN GRB
110918A (GCN 12357) with the robotic Palomar 48 inch telescope. Images
began at 8:49 UT (dt ~ 35.4 hours after the burst trigger time) in the PTF
R-band.
Comparing our images with those obtained by the INT approximately six
hours earlier (GCN 12365), we find the optical candidate has faded by 0.45
+/- 0.13 mag. The observed variability thus confirms that this object is
the afterglow of GRB 110918A. Though with large uncertainties, the
inferred power-law decay slope (alpha ~ 2.1) is quite steep for this
period after the trigger.
- GCN Circular #12368
A.J. Levan (U. Warwick), N.R. Tanvir, K. Wiersema (U. Leicester),
E. Berger (Harvard), D. Fox (Penn State) report on behalf of a larger
collaboration:
We observed the candidate optical afterglow of GRB 110918A reported by
Tanvir et al. (GCN12365) with the GMOS-N spectrograph on Gemini-N,
Mauna Kea. =C2=A0Observations began at 20-Sep-2011 12:51 UT. =C2=A0The tr=
ace is
well detected and consistent with being a power-law continuum,
confirming that this source is very likely the afterglow of the burst
detected by the IPN (Hurley et al. GCN12357, see also Cenko et
al. GCN12367). =C2=A0We identify absorption features of MgII (2796A/2804A=
),
MgI (2853A) and CaII (3935A/3970A) at a common redshift of z=3D0.982 (bas=
ed
on a provisional wavelength calibration).
We thank the Gemini staff, particularly Jen Holt, for obtaining
these observations.
- GCN Circular #12369
Arto Oksanen (Hankasalmi Obs., Hankasalmi, Finland), Caisey Harlingten
(Harlingten Observatory, San Pedro de Atacama, Chile), Bradley Schaefer
(LSU), and Matthew Templeton (AAVSO) report the following follow-up
optical observation of the error box of GRB 110918A (Hurley et al., GCN
Circ. #12357):
A. Oksanen (Hankasalmi Obs., Hankasalmi, Finland) reports an observation
of the error box of the intense, long GRB 110918A (Hurley et al., GCNC
#12357; Golonetskii, et al., GCNC #12362) using the Harlingten Observatory
0.5-m Planewave telescope with Apogee Alta-U42D9 CCD located in San Pedro
de Atacama, Chile. Ten, 60-second unfiltered observations of the field
containing the GRB error box were made; the coadded image has a midpoint
time of 2011 Sep 20 07:33 UT (JD 2455824.8146), approximately 34 hours
post burst. We find the point source detected by Mangano et al. (GCNC
#12364) and Tanvir et al. (GCNC #12365) at a preliminary magnitude of
R=19.0 measured relative to the nearby USNO-A2.0 star 0600-00854967
(B=18.8,R=17.7).
Coadded images of the field are available at the following links:
FITS: http://pilvi.dyndns.org/arto/GRB110918A_median.fit
JPEG: http://pilvi.dyndns.org/arto/GRB110918A_median.jpg
The AAVSO International High Energy Network was made possible through
grants from the Charles Curry Foundation and from NASA, and is supported
through the AAVSO Endowment.
- GCN Circular #12370
D. Frederiks and V.Pal'shin on behalf of the Konus-WIND team, report:
Basing on the Konus-Wind detection of GRB 110918A
(Hurley et al., GCN 12357; Golenetskii et al. GCN 12362),
assuming Gemini-N redshift z=0.982 (Levan et al., GCN 12368),
and a standard cosmology model (H_0 = 71 km/s/Mpc, Omega_M = 0.27,
Omega_\Lambda = 0.73), we estimate the following rest-frame parameters
of the prompt gamma-ray emission:
the isotropic equivalent energy release is E_iso ~1.9x10^54 erg,
the peak luminosity is L_iso_max ~ 4.4x10^54 erg/s, and
the peak energy of the time-integrated spectrum is Ep_rest ~300 keV.
This estimation of L_iso_max makes GRB 110918A the most luminous
gamma-ray burst (with known redshift) ever observed by Konus-Wind.
- GCN Circular #12371
M. H. Siegel (PSU), F. E. Marshall (NASA/GSFC), M. De Pasquale (UNLV),
and H. A. Krimm (GSFC/USRA) report on behalf of the Swift/UVOT team:
The Swift/UVOT began settled observations of the field of GRB 110918A
153272 s after the IPN Trigger. A source consistent with the XRT
position (Mangano et al., GCN 12364) is detected in the initial UVOT exposures.
The preliminary UVOT position is:
RA (J2000) = 02:10:09.33 = 32.53887 (deg.)
Dec (J2000) = -27:06:19.6 = -27.10545 (deg.)
with an estimated uncertainty of 0.55 arc sec. (radius, 90% confidence).
Over approximately 18 ks of observation, we do not detect any statistically
significant fading. Additional Swift/UVOT observations are planned.
Preliminary detections using the UVOT photometric
system (Poole et al. 2008, MNRAS, 383, 627) for the early exposures are:
Filter T_start(s) T_stop(s) Exp(s) Mag
white 153272 154107 791 20.36+-0.13
white 158468 159889 1373 20.16+-0.08
white 163169 165670 2419 20.12+-0.05
white 168950 171166 2178 20.21+-0.06
The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.02 in the direction of the burst
(Schlegel et al. 1998).
- GCN Circular #12375
A. de Ugarte Postigo (DARK/NBI), J. Gorosabel, A.J. Castro-Tirado and C.C.
Thoene (IAA-CSIC) report on behalf of a larger collaboration:
We have obtained spectroscopy of the afterglow of GRB 110918A (Hurley et
al., GCN 12357, Mangano et al. GCN 12364, Tanvir et al., GCN 12365) using
OSIRIS at the 10.4m GTC telescope at Roque de los Muchachos Observatory
(Spain). Our data consisted of 3x900s exposures with the R500B grism, with
a resolution of ~500 obtained with an average epoch of Sep. 21.106 UT
(53.09 hr after the burst).
In a preliminary reduction we detect absorption features of ZnII, FeII,
MnII, MgII, MgI and CaII at a common redshift of 0.984+/-0.001, similar to
the one derived by Levan et al. (GCN 12368).
We acknowledge the excellent support of the GTC staff, in particular J.M.
Gonzalez Perez, A. Cabrera Lavers and B. Gerken.
- GCN Circular #12376
V. Mangano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/IASFPA),
P. A. Evans (U Leicester), H. A. Krimm (GSFC/USRA)
report on behalf of the Swift-XRT team:
We have analysed 13.1 ks of XRT data for GRB 110918A, from 107.4 ks
to 211.3 ks after the IPN trigger (Hurley et al. GCN Circ. 12357).
The data are in Photon Counting (PC) mode. The enhanced XRT
position for this burst was given by Mangano et al. (GCN Circ. 12364).
The light curve can now be modeled with a single power-law decay
with index alpha=1.49 (+0.24, -0.25). This confirms the XRT
source is the afterglow of the GRB.
The PC mode spectrum has a photon index of 2.11 (+0.21, -0.19)
and a best-fitting absorption column at redshift z=0.982
(Levan et al. GCN Circ. 12368) of 6.3 (+2.8, -2.1) x 10^21 cm^-2.
The counts to observed (unabsorbed) 0.3-10 keV flux conversion
factor deduced from this spectrum is
3.0 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1.
The results of the XRT-team automatic analysis are available at
http://www.swift.ac.uk/xrt_products/00020186.
This circular is an official product of the Swift-XRT team.
- GCN Circular #12381
A. de Ugarte Postigo (DARK/NBI), C. De Breuck (ESO), A. Lundgren
(ALMA/ESO) and M. Dumke (APEX/ESO) report on behalf of a larger
collaboration:
We have observed the afterglow candidate (Tanvir et al., GCN 12365) of the
bright IPN GRB110918A (Hurley et al., GCN 12357) using APEX/LABOCA at
Chajnantor (Chile). Observations where performed at 345GHz, using the
photometric mode, with mean epoch Sep. 21.184 UT (54.97 hr after the
burst) under good weather conditions (PWV ~ 0.25 mm). No flux is detected
at the position of the afterglow, with a 3-sigma upper limit of 15mJy.
- GCN Circular #12382
C. Guidorzi (U. Ferrara), D. Kopac (U. Ljubljana) and N. R. Tanvir (U.
Leicester), on behalf of a large collaboration report:
We observed the optical afterglow of GRB 110918A (Cenko et al., GCN 12367)
with the 2-m Liverpool Telescope, starting at 01:07 UT on 2011 September
21. We took 6 images in R band with exposure time 300s and we clearly
detect the optical afterglow. Our preliminary photometry yields:
Mid time from Exp Filter Magnitude
trigger (days) (sec)
-------------------------------------------
2.16 1800 R 19.7 +- 0.1
-------------------------------------------
The magnitude is calibrated against nearby USNO-B1.0 stars and is not
corrected for the Galactic extinction.
- GCN Circular #12388
D. A. Perley (Caltech), M. Ganeshalingam, P. Blanchard, and M. Mason (UC
Berkeley) report:
We observed the location of bright IPN GRB 110918A (Hurley et al, GCN
12357) starting at UT 2011-09-22 03:04:29 UT using the Nickel 40-inch
telescope at Lick Observatory, under good seeing conditions at high
airmass. A series of five 180-second exposures in R-band were acquired
in total. The optical afterglow (Tanvir et al., GCN 12365) is
well-detected in the combined stack. Using five nearby USNOB1.0 stars,
we calculate the following photometry:
R = 20.68 +/- 0.13 (t_mid = 3.2406 day)
The uncertainty estimate does not include the uncertainty of the
calibration to USNO (for convenience, our calibration stars are listed
below). The measurement indicates a decay slope of approximately
alpha=2.2 since the report of Guidorzi et al. (GCN 12382) and may
suggest a very early jet break for this burst (see also Cenko et al.,
GCN 12367).
USNO calibration stars:
RA dec R2
032.546809 -27.104117 19.38
032.549492 -27.121698 19.29
032.555214 -27.117650 19.38
032.558045 -27.100853 19.39
032.569403 -27.106270 19.40
- GCN Circular #12458
Arto Oksanen (Hankasalmi Obs., Hankasalmi, Finland), Bradley Schaefer
(LSU), Caisey Harlingten (Harlingten Observatory, San Pedro de Atacama,
Chile), and Matthew Templeton (AAVSO) report the following observations of
GRB 110918A (Hurley et al., GCN Circ. #12357):
A. Oksanen (Hankasalmi Obs., Hankasalmi, Finland) reports observations of
the optical transient associated with the intense, long GRB 110918A at
z=3D0.982 (Hurley et al., GCNC #12357; Golonetskii, et al., GCNC #12362;
Mangano et al. GCN 12364; Levan et al. GCN 12368) using the Harlingten
Observatory 0.5-m Planewave telescope with Apogee Alta-U42D9 CCD located
in San Pedro de Atacama, Chile. The observations were made on 15 nights
from 1.417 to 16.325 days after the burst, all without a filter so that
the color sensitivity is like that of a broad R-band filter. We have
calibrated the optical transient magnitude with the five comparison stars
used by Perley et al. (GCN 12388) for which they quote the R-band
magnitudes from the USNO catalog. Our magnitudes (plus two taken from the
GCNs) are given in the following table:
JD R(GRB) T-T0 (days)
2455824.8105 19.18 =B1 0.04 1.417
2455825.5537 19.70 =B1 0.10 2.160 (Guidorzi et al. GCN 12382)
2455825.7625 19.97 =B1 0.05 2.369
2455826.6343 20.68 =B1 0.13 3.241 (Perley et al. GCN 12388)
2455826.7059 20.51 =B1 0.06 3.312
2455827.7137 20.85 =B1 0.07 4.320
2455828.7067 21.14 =B1 0.09 5.313
2455829.7193 21.26 =B1 0.09 6.326
2455830.7107 21.56 =B1 0.12 7.317
2455831.7025 21.34 =B1 0.09 8.309
2455832.7116 21.62 =B1 0.11 9.318
2455833.6930 21.90 =B1 0.12 10.299
2455834.7065 21.76 =B1 0.12 11.313
2455835.6894 21.58 =B1 0.10 12.296
2455836.7208 21.67 =B1 0.11 13.327
2455837.7159 21.94 =B1 0.12 14.322
2455839.7185 21.99 =B1 0.13 16.325
The flux up to 10 days after the burst is well fit by a power law with an
index of -1.24. After 10 days after the burst, the light curve appears to
flatten (i.e., the opposite of a jet break), for which we expect that the
underlying galaxy is appearing in the light curve.
There is certainly no jet break in the time interval from 1.417 to roughly
10-16 days after the burst. The index over this time interval is typical
for the interval before the jet break, which implies that the jet break is
at a time of greater than 10 days after the burst. The previous
suggestion of an early jet break was simply due to one group looking at
only two magnitudes with relatively large uncertainty (not counting
inconsistencies in the two calibrations) and closely spaced in time and
seeing an apparently steep slope. With our long interval with consistent
magnitudes with small statistical uncertainty, we can be certain that the
index is shallow and no jet break is present.
The light curve of this GRB may be viewed at the following URL:
http://www.aavso.org/sites/default/files/images/110918A_Oksanen.png
The AAVSO International High Energy Network was made possible through=20
grants from the Charles Curry Foundation and from NASA, and is supported=20
through the AAVSO Endowment.
- GCN Report 350.1
GCN_Report 350.1 has been posted:
http://gcn.gsfc.nasa.gov/reports/report_350_1.pdf
by H.A. Krimm
at CRESST/GSFC/USRA
titled: "Final Swift Observations of GRB 110918A"
- 1306.0892 from 5 Jun 13
J. Elliott et al.: The low-extinction afterglow in the solar-metallicity host galaxy of gamma-ray burst 110918A
Metallicity is theoretically thought to be a fundamental driver in gamma-ray burst (GRB) explosions and energetics, but is still, even after
more than a decade of extensive studies, not fully understood. This is largely related to two phenomena: a dust-extinction bias, that prevented
high-mass and thus likely high-metallicity GRB hosts to be detected in the first place, and a lack of efficient instrumentation, that limited
spectroscopic studies including metallicity measurements to the low-redshift end of the GRB host population. The subject of this work is the
very energetic GRB 110918A, for which we measure a redshift of z=0.984. GRB 110918A gave rise to a luminous afterglow with an intrinsic
spectral slope of b=0.70, which probed a sight-line with little extinction (A_V=0.16 mag) typical of the established distributions of afterglow
properties. Photometric and spectroscopic follow-up observations of the galaxy hosting GRB 110918A, including optical/NIR photometry with GROND
and spectroscopy with VLT/X-shooter, however, reveal an all but average GRB host in comparison to the z~1 galaxies selected through similar
afterglows to date. It has a large spatial extent with a half-light radius of ~10 kpc, the highest stellar mass for z<1.9 (log(M_*/M_sol) =
10.68+-0.16), and an Halpha-based star formation rate of 41 M_sol/yr. We measure a gas-phase extinction of ~1.8 mag through the Balmer
decrement and one of the largest host-integrated metallicities ever of around solar (12 + log(O/H) = 8.93+/-0.13). This presents one of the
very few robust metallicity measurements of GRB hosts at z~1, and establishes that GRB hosts at z~1 can also be very metal rich. It
conclusively rules out a metallicity cut-off in GRB host galaxies and argues against an anti-correlation between metallicity and energy release
in GRBs.
- 1308.5520 from 27 Aug 13
J. Elliott et al.: The low-extinction afterglow in the solar-metallicity host galaxy of GRB 110918A
Galaxies selected through long gamma-ray bursts (GRBs) could be of fundamental importance when mapping the star formation history out to the
highest redshifts. Before using them as efficient tools in the early Universe, however, the environmental factors that govern the formation of
GRBs need to be understood. Metallicity is theoretically thought to be a fundamental driver in GRB explosions and energetics, but is still,
even after more than a decade of extensive studies, not fully understood. This is largely related to two phenomena: a dust-extinction bias,
that prevented high-mass and thus likely high-metallicity GRB hosts to be detected in the first place, and a lack of efficient instrumentation,
that limited spectroscopic studies including metallicity measurements to the low-redshift end of the GRB host population. The subject of this
work is the very energetic GRB 110918A, for which we measure one of the largest host-integrated metallicities, ever, and the highest stellar
mass for z<1.9. This presents one of the very few robust metallicity measurements of GRB hosts at z~1, and establishes that GRB hosts at z~1
can also be very metal rich. It conclusively rules out a metallicity cut-off in GRB host galaxies and argues against an anti-correlation
between metallicity and energy release in GRBs.
- 1311.5734 from 25 Nov 13
D.D. Frederiks et al.: The ultraluminous GRB 110918A
GRB 110918A is the brightest long GRB detected by Konus-WIND during its 19 years of continuous observations and the most luminous GRB ever
observed since the beginning of the cosmological era in 1997. We report on the final IPN localization of this event and its detailed
multiwavelength study with a number of space-based instruments. The prompt emission is characterized by a typical duration, a moderare
$E_{peak}$ of the time-integrated spectrum, and strong hard-to-soft evolution. The high observed energy fluence yields, at z=0.984, a huge
isotropic-equivalent energy release $E_{iso}=(2.1\pm0.1)\times10^{54}$ erg. The record-breaking energy flux observed at the peak of the short,
bright, hard initial pulse results in an unprecedented isotropic-equivalent luminosity $L_{iso}=(4.7\pm0.2)\times10^{54}$erg s$^{-1}$. A tail
of the soft gamma-ray emission was detected with temporal and spectral behavior typical of that predicted by the synchrotron forward-shock
model. Swift/XRT and Swift/UVOT observed the bright afterglow from 1.2 to 48 days after the burst and revealed no evidence of a jet break. The
post-break scenario for the afterglow is preferred from our analysis, with a hard underlying electron spectrum and ISM-like circumburst
environment implied. We conclude that, among multiple reasons investigated, the tight collimation of the jet must have been a key ingredient to
produce this unusually bright burst. The inferred jet opening angle of 1.7-3.4 deg results in reasonable values of the collimation-corrected
radiated energy and the peak luminosity, which, however, are still at the top of their distributions for such tightly collimated events. We
estimate a detection horizon for a similar ultraluminous GRB of $z\sim7.5$ for Konus-WIND, and $z\sim12$ for Swift/BAT, which stresses the
importance of GRBs as probes of the early Universe.
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