- GCN Circular #8245
Adam Goldstein (UAH) and Alexander van der Horst (NASA/MSFC) report on
behalf of the Fermi Gamma-ray Burst Monitor Team:
"At 00:12:45 UT on 16 September 2008, the Fermi Gamma-Ray Burst Monitor
(GBM) triggered and located the bright GRB 080916C (trigger 243216766 /
080916.009). The on-ground calculated location, using the GBM trigger data,
is RA = 121.8, Dec = -61.3 (J2000 degrees, equivalent to J2000 8h 07m,
-61d 18'), with a statistical uncertainty of less than 1 degree (radius,
1-sigma
containment) and an additional systematic error which is currently estimated
to be 2 to 3 degrees. The angle from the Fermi Large Area Telescope (LAT)
boresight is 52 degrees.
This GRB has several peaks, with T90 (50-300 keV) = 66 s and
T50 (50-300 keV) = 33 s. This burst is detected up to several MeV;
spectral analysis is ongoing."
- GCN Circular #8246
H. Tajima (SLAC), J. Bregeon (INFN Pisa), J. Chiang (SLAC), G. Thayer (SLAC)
on behalf of the Fermi LAT team:
We report a detection by the Fermi Large Area Telescope (LAT) of
emission from the long GRB 080916C, which was triggered by the Fermi
Gamma-ray Burst Monitor (GBM) at 00:12:45 UT on September 16th 2008 (GCN
8245). The angle of the GBM best position (RA, Dec=121.8,-61.3) with
respect to the LAT boresight was 52 degrees at the time of the trigger,
which is close the edge of our field of view.
The data from the Fermi LAT shows a significant increase in the event
rate within 10 degrees of the GBM location after the GBM trigger that is
spatially and temporally correlated with the GBM emission with high
significance. More than 10 photons are observed above 1 GeV during this
time.
The best LAT on-ground localization is found to be (RA,Dec=119.88,
-56.59) with a 90% containment radius of 0.13 deg (statistical; 68%
containment radius: 0.09 deg, preliminary systematic error is less than
0.1 deg) which is consistent with the GBM localization.
This circular is an official product of the Fermi LAT team.
- GCN Circular #8251
K. Hurley and J. Goldsten, on behalf of the MESSENGER NS GRB team,
S. Golenetskii, R. Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, and
T. Cline on behalf of the Konus-Wind team
E. Bellm, D. M. Smith, R. P. Lin, J. McTiernan, R. Schwartz, C. Wigger, W.
Hajdas, and A. Zehnder, on behalf of the RHESSI GRB team,
A. von Kienlin, G. Lichti, and A. Rau, on behalf of the
INTEGRAL SPI-ACS GRB team, and
E. Del Monte, I. Donnarumma, Y. Evangelista, M. Feroci, I. Lapshov,
F. Lazzarotto, L. Pacciani, M. Galli, and M. Marisaldi, on behalf
of the AGILE Team,
report:
GRB080916C (Goldstein and van der Horst, GCN 8245; Tajima et
al., GCN 8246) was also observed by AGILE (MCAL, SuperAGILE, and ACS -
but not localized), RHESSI, INTEGRAL (SPI-ACS), Konus-Wind,
and MESSENGER. A preliminary triangulation gives a long,
narrow error box centered at RA, Dec = 119.830, -56.790 degrees
(0.2 degrees from the center of the LAT error circle), whose
corners are:
RA(2000) DEC(2000)
119.486 -62.130
120.126 -62.702
119.901 -50.632
120.321 -51.274
A figure is posted at
ssl.berkeley.edu/ipn3/080916C.
Using the RHESSI data between 100 keV and 17 MeV, the time-integrated
spectrum for the 62-second interval beginning at 00:12:46 UT can be described
by a cutoff power law with alpha ~ -1.2 +/- 0.3, Epeak ~ 1100 +/- 500 keV,
and fluence ~ (9.0 +/- 1.6) x 10^-5 ergs/cm^2 (100 keV - 10 MeV).
Both the triangulation and the energy spectrum can be substantially
improved.
- GCN Circular #8253
Jamie A. Kennea reports on behalf of the Swift Team:
At 17:08 UT, September 16th 2008 Swift began TOO observations of the Fermi
GBM and LAT detected GRB 080916C (GCN 8245,8246). Preliminary analysis of
downlinked data from this observation reveals a single point source
detected in the field of view at RA, Dec = 119.8459, -56.63891 which is
equivalent to:
RA (J2000): 07h 59m 23.0s
Dec (J2000): -56d 38' 20.1"
with an estimated uncertainty of 8 arcsec (radius, 90% confidence). We
note that this point source is uncatalogued and 189 arcseconds from the
center of the Fermi LAT error circle (GCN 8246), within the LAT 90%
confidence error circle. Currently we cannot determine if this source is
fading, and therefore cannot confirm if this source is associated with GRB
080916C. Observations of this source are on-going.
- GCN Circular #8255
P.A. Evans, J.P. Osborne, A.P. Beardmore and M.R. Goad, (U. Leicester)
report on behalf of the Swift-XRT team.
Using 1310 s of XRT Photon Counting mode data and 1 UVOT images, we find an
astrometrically corrected X-ray position (using the XRT-UVOT alignment and
matching to the USNO-B1 catalogue): RA, Dec = 119.84706, -56.63788 which is
equivalent to:
R (J2000): 07 59 23.29
Dec (J2000): -56 38 16.4
with an uncertainty of 2.1 arcsec (90% confidence).
This position may be improved as more data are received. The latest position
can be viewed at http://www.swift.ac.uk/xrt_positions. Position enhancement is
described by Goad et al. (2007, A&A, 476, 1401
http://www.swift.ac.uk/xrt_positions/Goad.pdf), the current algorithm is an
extension of this method.
This circular is an official product of the Swift-XRT team.
- GCN Circular #8257
C. Clemens (MPE), A. Rossi (Tautenburg Obs.), J. Greiner and S. McBreen (both
MPE) report on behalf of the GROND team:
GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI
telescope at La Silla Observatory (Chile) observed the field of GRB 080916C,
which triggered Fermi GBM (Goldstein et al., GCN #8245) and for which
significant correlated emission was detected by the Fermi LAT (Tajima et al.,
GCN #8246). The field was followed up by Swift XRT (Kennea et al., GCN
#8253).
Observations started at 07:57 UT on September 17th, 2008, 31.7 hr after the
GBM trigger. They were performed at an average seeing of 1.5" and at an
average airmass of 1.7 as well as under bright moon.
We found a faint source at the edge of the astrometrically corrected 2.1''
Swift-XRT error circle reported by Evans et al. (GCN #8255) at
RA (J2000.0) = 07h 59m 23.32s
DEC (J2000.0) = -56d 38' 18.0''
with an uncertainty of 0.5".
Based on the first 50 min of effective exposures, we estimate a preliminary
magnitude of
z' ~ 21.74 mag +/- 0.17.
Given magnitude is calibrated against USNO-B1 field stars.
No statement about variability can be made at this point. Further observations
are planned.
Please note, that no correction for the Galactic foreground reddening of
E(B-V) = 0.32 mag (Schlegel et al., 1998) has been applied.
- GCN Circular #8258
S. Golenetskii, R.Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, P.
Oleynik, M. Ulanov, D. Svinkin, and T. Cline on behalf of the Konus-Wind
team, report:
The long GRB 080916=F3 (Fermi GBM trigger 243216766/080916.009; Goldstein=
and van der Horst, GCN 8245; Tajima et al., GCN 8246; Hurley et al., GCN=20
8251) triggered Konus-Wind at T0=3D764.632 s UT (00:12:44.632).
The burst light curve shows the main part with a duration of ~70 s=20
followed by a weak tail seen at least till ~T0+200 s.
As observed by Konus-Wind the burst
had a fluence of (1.24 +/- 0.17)x10^-4 erg/cm2,
and a 256-ms peak flux measured from T0+3.168 s
of (1.19 +/- 0.30)x10^-5 erg/cm2/s
(both in the 20 keV - 10 MeV energy range).
The time-integrated spectrum of the burst
(from T0 to T0+71.424 s) is well be fitted (in the 20 keV - 10 MeV=20
range) by GRBM (Band) model for which:
the low-energy photon index is alpha =3D -1.040(-0.058, +0.065),
the high energy photon index beta =3D -2.26(-0.40, +0.21),
the peak energy Ep =3D 505(-70, +79) keV (chi2 =3D 75.6/84 dof).
All the quoted errors are at the 90% confidence level.
The Konus-Wind light curve of this GRB is available
at http://www.ioffe.rssi.ru/LEA/GRBs/GRB080916_T00764/
- GCN Circular #8261
M. Perri, B. Preger, G. Stratta (ASDC) report on behalf of the
Swift XRT team:
The Swift XRT started observing the field of the FERMI GRB 080916C
(Goldstein et al., GCN Circ. 8245) at 2008-09-16 17:11:28 UT, about
17 hours after the trigger.
We confirm that the X-ray source reported by Kennea (GCN Circ. 8253) is
the afterglow of the GRB.
Using 2614 s of XRT Photon Counting mode data and 2 UVOT images, we find
an astrometrically corrected X-ray position (using the XRT-UVOT
alignment and matching to the USNO-B1 catalogue): RA, Dec = 119.84684,
-56.63801 which is equivalent to:
RA (J2000): 07 59 23.24
Dec (J2000): -56 38 16.8
with an uncertainty of 1.9 arcsec (90% confidence). This position is
consistent with the XRT position reported by Evans et al. (GCN Circ. 8255)
using less data.
The X-ray light curve from T+61 ks to T+102 ks can be fit with a single
power-law model with a decay index of 1.7 (-0.7) (+1.0).
A 7.6 ks exposure X-ray spectrum from T+61 ks to T+102 ks can be well
fit by an absorbed power-law model with a photon index of 2.1 (+0.9 -0.7)
and a column density of 3.7 (+3.3) (-2.1) x 10^21 cm^-2. We note that
the Galactic column density value in the direction of the burst is
1.5 x 10^21 cm^-2.
The observed 0.3-10.0 keV flux is 8.6 x 10-13 erg cm^-2 s^-1 which
corresponds to an unabsorbed flux of 1.5 x 10^-12 erg cm^-2 s^-1.
Providing the source continues to decay at the same rate, we predict an
observed flux of about 3 x 10^-13 erg cm^-2 s^-1 at T+2 days.
This circular is an official product of the Swift-XRT team.
- GCN Circular #8262
E. A. Hoversten (PSU), P. Schady (MSSL-UCL), and M. Perri (ASDC)
report on behalf of the Swift UVOT team:
The Swift/UVOT began observations of the field of GRB 080916C 17.0
hours after the Fermi GBM dectection (Goldstein & van der Horst, GCN
8245). No afterglow is detected within the XRT error circle (Evans, et
al., GCN 8255) in any of the observed UVOT filters.
UVOT magnitude 3-sigma upper limits are reported in the following
table:
Filter T_start T_stop Exp(s) Mag (3-sigma upper limit)
-------------------------------------------------------------
v 101253 101857 590 > 19.75
u 96469 97132 645 > 19.99
uvw1 66675 67517 829 > 20.34
uvw1 95562 96462 886 > 20.38
uvm2 61130 66667 1090 > 20.31
uvm2 94655 95555 886 > 20.20
uvw2 100347 101246 886 > 20.43
The quoted upper limits have not been corrected for the large expected
Galactic extinction along the line of sight of E_{B-V} = 0.32 mag
(Schlegel et al., 1998). All photometry is on the UVOT flight system
described in Poole et al. (2008, MNRAS, 383, 627).
- GCN Circular #8272
C. Clemens (MPE Garching), A. Rossi (Tautenburg Obs.), J. Greiner, S. McBreen,
T. Kruehler, A. Yoldas (all MPE Garching), A. Kupcu Yoldas (ESO) and G.
Szokoly (Eoetvoes Univ., Budapest) report on behalf of the GROND team:
We observed the field of GRB 080916C for a second time with GROND at 08:04 UT
on September 19th, 2008, 3.3 d after the GBM trigger with 54 min of effective
exposures. Observations were performed at an average seeing of 1.7" and at an
average airmass of 1.7.
We do not detect the afterglow candidate first reported by Clemens et al. (GCN
#8257) with the following 5-sigma upper limits:
z' > 22.7 mag.
These measurements strongly indicate a fading of the source compared to our
first epoch observations. We therefore propose this is to be the afterglow of
GRB 080916C.
- GCN Circular #8273
C. Clemens (MPE Garching), A. Rossi (Tautenburg Obs.), J. Greiner, S. McBreen,
T. Kruehler, A. Yoldas (all MPE Garching), A. Kupcu Yoldas (ESO) and G.
Szokoly (Eoetvoes Univ., Budapest) report on behalf of the GROND team:
We observed the field of GRB 080916C for a second time with GROND at 08:04 UT
on September 19th, 2008, 3.3 d after the GBM trigger with 54 min of effective
exposures. Observations were performed at an average seeing of 1.7" and at an
average airmass of 1.7.
We do not detect the afterglow candidate first reported by Clemens et al. (GCN
#8257) with the following 5-sigma upper limits:
z' > 22.7 mag.
These measurements strongly indicate a fading of the source compared to our
first epoch observations. We therefore propose this is to be the afterglow of
GRB 080916C.
- GCN Circular #8274
T. Nagayama (Kyoto University) reports on behalf of the IRSF/SIRIUS
team:
We have imaged the field of GRB 080916C (van der Horst, GCN 8245)
based on the localization by Fermi/LAT (Tajima et al, GCN 8246) in J, H,
and Ks with the Simultaneous three-color InfraRed Imager for Unbiased
Survey (SIRIUS) on the Nagoya-SAAO 1.4m telescope (IRSF) starting at
2:53 on September 17 UT for 50 min.
The optical afterglow candidate suggested by Clemens et al. (GCN 8257,
GCN 8272) was detected marginally in H and Ks bands.
The preliminary magnitude was Ks= 18.6 +- 0.5.
- GCN Circular #8278
Alexander van der Horst (NASA/ORAU) and Adam Goldstein (UAH) report on
behalf of the Fermi Gamma-ray Burst Monitor Team:
"We have performed spectral analysis of GRB 080916C (GCN 8245, 8246).
The time averaged spectrum, from 8 keV up to 30 MeV, of the main emission
up to 66 seconds after the burst is best fit by a Band function with
Epeak = 424 +/- 24 keV, alpha = -0.91 +/- 0.02, and beta = -2.08 +/- 0.06.
The fluence (8 keV - 30 MeV) is 1.9e-4 erg/cm^2.
These spectral analysis results are preliminary; the final results will
be published in the GBM GRB Catalog."
- GCN Report 166.1
GCN_Report 166.1 has been posted:
http://gcn.gsfc.nasa.gov/reports/report_166_1.pdf
by G. Stratta
at ASDC
titled: "Final Swift report on Fermi GRB 080916C"
- GCN Circular #10019
Aquib Moin (Curtin Institute of Radio Astronomy / Australia Telescope
National Facility), Steven Tingay (Curtin Institute of Radio Astronomy),
Chris Phillips (Australia Telescope National Facility), Gregory Taylor
(University of New Mexico), Mark Wieringa (Australia Telescope National
Facility) and Ralph Martin (Perth Observatory) report:
We observed the XRT position of the GRB080916c (GCN 8261) at 5.5 GHz
with the Australia Telescope Compact Array (ATCA) between 14:00:00 UT
and 20:00:00 UT on July 01, 2009.
We did not detect a radio source at the XRT position of the GRB080916c
(GCN 8261). The radio flux density at the afterglow position found out
to be 0.230 +/- 0.180 mJy/beam (1-sigma). The compact array was in its
most compact configuration.
The Australia Telescope Compact Array (/ Parkes telescope / Mopra
telescope / Long Baseline Array) is part of the Australia Telescope
which is funded by the Commonwealth of Australia for operation as a
National Facility managed by CSIRO.
See the 5.5 GHz field image at:
http://cira.ivec.org/dokuwiki/doku.php/grb/grb080916c_field_image
- 1002.0228 from 2 Feb 10
Masanori Ohno et al.: Fermi-LAT observations of GRBs with weak LAT emission
We present the analysis results of three Gamma-Ray Bursts (GRBs) detected by the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope
(LAT) onboard Fermi: the two long GRB 080825C and GRB 090217, and the first short burst with GeV photons GRB 081024B. The emission from GRB
081024B observed by the LAT above 100 MeV is delayed with respect to the GBM trigger, and significantly extends after the low-energy episode.
Some hints for spectral hardening was observed in this burst as well as in GRB 080825C, possibly related to a separate and harder component
showing up at late times. Conversely, GRB 090217 does not exhibit any noticeable feature. Together with the other bright LAT detected bursts
(e.g. GRB 080916C and GRB 090510), these observations help to classify the GRB properties and give new insight on the acceleration mechanisms
responsible for their emission at the highest energies.
- 1006.2440 from 15 Jun 10
Akira Mizuta et al.: Thermal Radiation from GRB Jets
The light curves and spectrum of the photospheric thermal radiation from ultrarelativistic gamma-ray burst (GRB) jets are calculated, using the
results of the 2D relativistic hydrodynamic simulations of the jets from a collapsar. The density around the head of the jet decreases, as the
jet proceeds, and the Lorentz factor of the jet reaches as high as 200 at the photospheer and as high as 400 inside the photosphere. The shape
of the photosphere for on-axis observer gets concave due to low density and high beaming factor of the jet. The light curve is flare like for
the first phases for all viewing angles. Then the luminosity for on-axis observer varies caused by the quick transition of the position of the
photosphere due to the internal structure in the jet. We compare our results with GRB090902B, and conclude that GRB090902B had a more
collimated jet with higher temperature and/or beaming factor than our simulation. The spectrum for on-axis observer is harder than that for
off-axis observer. There is a few seconds time lag for high energy bands in the light curve, which may be related with the delayed onset seen
in GRB080916C.
- 1011.1197 from 5 Nov 10
Bing Zhang et al.: The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts
The recent Fermi observation of GRB 080916C shows that the bright photosphere emission associated with a putative fireball is missing, which
suggests a Poynting-flux-dominated outflow. We propose a model of gamma-ray burst (GRB) prompt emission in the Poynting-flux-dominated regime,
namely, the Internal-Collision-induced MAgnetic Reconnection and Turbulence (ICMART) model. It is envisaged that the GRB central engine
launches an intermittent, magnetically-dominated wind, and that in the GRB emission region, the ejecta is still moderately magnetized. Similar
to the internal shock (IS) model, the mini-shells interact internally at the traditional internal shock radius. Most of these early collision
have little energy dissipation, but serve to distort the ordered magnetic field lines. At a certain point, the distortion of magnetic field
configuration reaches the critical condition to allow fast reconnection seeds to occur, which induce relativistic MHD turbulence in the
interaction regions. The turbulence further distorts field lines easing additional magnetic reconnections, resulting in a runway release of the
stored magnetic field energy (an ICMART event). Particles accelerated in the ICMART region radiate synchrotron photons that power the observed
gamma-rays. Each ICMART event corresponds to a broad pulse in the GRB lightcurve, and a GRB is composed of multiple ICMART events. This model
retains the merits of the IS and other models, but may overcome several difficulties/issues faced by the IS model (e.g. low efficiency, fast
cooling, electron number excess, Amati/Yonetoku relation inconsistency, and missing bright photosphere). It predicts two-component variability
time scales, and a decreasing Ep and polarization degree during each ICMART event. The model may be applied to most Fermi LAT GRBs that have
time-resolved, featureless Band-function spectra (abridged).
- 1011.1205 from 5 Nov 10
T. Krühler et al.: Photometric redshifts for GRB afterglows from GROND and Swift/UVOT
We present a framework to obtain photometric redshifts (photo-zs) for gamma-ray burst afterglows. Using multi-band photometry from GROND and
Swift/UVOT, photo-zs are derived for five GRBs for which spectroscopic redshifts are not available. We use UV/optical/NIR data and synthetic
photometry based on afterglow observations and theory to derive the photometric redshifts of GRBs and their accuracy. Taking into account the
afterglow synchrotron emission properties, we investigate the application of photometry to derive redshifts in a theoretical range between z~1
and z~12. The measurement of photo-zs for GRB afterglows provides a quick, robust and reliable determination of the distance scale to the
burst, particularly in those cases where spectroscopic observations in the optical/NIR range cannot be obtained. Given a sufficiently bright
and mildly reddened afterglow, the relative photo-z accuracy is better than 10% between z=1.5 and z~7 and better than 5% between z=2 and z=6.
We detail the approach on 5 sources without spectroscopic redshifts observed with UVOT on-board Swift and/or GROND. The distance scale to those
same afterglows is measured to be $z=4.31^{+0.14}_{-0.15}$ for GRB 080825B, $z=2.13^{+0.14}_{-0.20}$ for GRB 080906, $z=3.44^{+0.15}_{-0.32}$
for GRB 081228, $z=2.03^{+0.16}_{-0.14}$ for GRB 081230 and $z=1.28^{+0.16}_{-0.15}$ for GRB 090530. Combining the response from UVOT with
ground-based observatories and in particular GROND operating in the optical/NIR wavelength regime, reliable photo-zs can be obtained from z ~
1.0 out to z ~ 10, and possibly even at higher redshifts in some favorable cases, provided that these GRBs exist, are localized quickly, have
sufficiently bright afterglows and are not heavily obscured.
- 1101.4232 from 25 Jan 11
Omer Bromberg et al.: Sub-Photospheric Emission from Relativistic Radiation Mediated Shocks in GRBs
It is proposed that the prompt emission observed in bursts that exhibit a thermal component originates from relativistic radiation mediated
shocks that form below the photosphere of the GRB outflow. It is argue that such shocks are expected to form in luminous bursts via collisions
of shells that propagate with moderate Lorentz factors $\Gamma\lesssim 500$. Faster shells will collide above the photosphere to form
collisionless shocks. We demonstrate that in events like GRB 090902B a substantial fraction of the explosion energy is dissipated below the
photosphere, in a region of moderate optical depth $\tau\lesssim300$, whereas in GRB 080916C the major fraction of the energy dissipates above
the photosphere. We show that under conditions anticipated in many GRBs, such relativistic radiation mediated shocks convect enough radiation
upstream to render photon production in the shock transition negligible, unlike the case of shock breakout in supernovae. The resulting
spectrum, as measured in the shock frame, has a relatively low thermal peak, followed by a broad, nonthermal component extending up to the KN
limit.
- 1107.5737 from 29 Jul 11
R. Hascoët et al.: Do Fermi-LAT observations really imply very large Lorentz factors in GRB outflows ?
d'Astrophysique de Paris (2) CNRS, UMR 7095, Institut d'Astrophysique de Paris)
Recent detections of GeV photons in a few GRBs by Fermi-LAT imply huge bulk Lorentz factors to avoid a large gamma gamma optical depth at high
energy. Estimates can be as high as Gamma ~ 1000 in the most extreme cases. This puts severe constraints on models of the central engine and
the jet acceleration in GRBs. These estimates are however obtained from a simplified single zone model. We present here a more realistic
calculation which takes into account the time, space and direction dependent photon field existing in an outflow with several relativistically
moving emitting zones. The formalism is general and can be applied to many models of the prompt GRB emission. We present results obtained for a
numerical implementation in the framework of the internal shock model. We show that (i) the minimum Lorentz factor Gamma_min in bright LAT GRBs
is reduced by a factor ~ 2-3 compared to previous estimates if the GeV and MeV emission are produced in the same region, and by an additional
factor ~ 2-8 if the GeV emission is produced at larger radii. We provide an improved approximate formula for Gamma_min which is in good
agreement with our numerical results and can be directly applied to LAT GRB data; (ii) a delayed GeV onset can be due to the time evolution of
the opacity. As an illustration of these two first results, we present a synthetic GRB that reproduces most features of GRB 080916C with a mean
Lorentz factor of ~ 340, an optically thin regime for gamma gamma opacity at 3GeV in bin 'b', a variability timescale of 0.5s in the MeV
lightcurve and a delayed onset of ~ 5s of the GeV emission; (iii) the gamma gamma opacity can smooth the short timescale variability in the GeV
lightcurve. This last result implies that the observed variability at high energy is not necessarily a good test to distinguish between an
internal and an external origin for the GeV emission in GRBs. [abridged]
- 1109.5191 from 27 Sep 11
Robert J. Nemiroff et al.: Limiting properties of light and the universe with high energy photons from Fermi-detected Gamma Ray Bursts
Quantum Cosmology (gr-qc)
An analysis of four Fermi-detected gamma-ray bursts (GRBs) is given that sets upper limits on the energy dependence of the speed and dispersion
of light across the universe. The analysis focuses on photons recorded above 1 GeV for Fermi detected GRB 080916C, GRB 090510A, GRB 090902B,
and GRB 090926A. Upper limits on time scales for statistically significant bunching of photon arrival times were found and cataloged. In
particular, the most stringent limit was found for GRB 090510A at redshift $z \gtrsim 0.897$ for which $\Delta t < 0.00136$ sec, a limit driven
by three separate photon bunchings. These photons occurred among the first seven super-GeV photons recorded for GRB 090510A and contain one
pair with an energy difference of $\Delta E \gtrsim 23.5$ GeV. The next most limiting burst was GRB 090902B at a redshift of $z \gtrsim 1.822$
for which $\Delta t < 0.161$, a limit driven by several groups of photons, one pair of which had an energy difference $\Delta E \gtrsim$ 1.56
GeV. Resulting limits on the differential speed of light and Lorentz invariance were found for all of these GRBs independently. The strongest
limit was for GRB 090510A with $\Delta c / c < 6.09$ x $10^{-21}$. Given generic dispersion relations across the universe where the time delay
is proportional to the photon energy to the first or second power, the most stringent limits on the dispersion strengths were $k_1 <$ 1.38 x
$10^{-5}$ sec Gpc$^{-1}$ GeV$^{-1}$ and $k_2 <$ 3.04 x $10^{-7}$ sec Gpc$^{-1}$ GeV$^{-2}$ respectively. Such upper limits result in upper
bounds on dispersive effects created, for example, by dark energy, dark matter or the spacetime foam of quantum gravity. Relating these
dispersion constraints to loop quantum gravity energy scales specifically results in limits of $M_1 c^2 >$ 7.43 x $10^{21}$ GeV and $M_2 c^2 >$
7.13 x $10^{11}$ GeV respectively.
- 1307.2663 from 11 Jul 13
Andrei M. Beloborodov et al.: On the origin of GeV emission in gamma-ray bursts
The most common progenitors of gamma-ray bursts (GRBs) are massive stars with strong stellar winds. We show that the GRB blast wave in the wind
should emit a bright GeV flash. It is produced by inverse Compton scattering of the prompt MeV radiation (emitted at smaller radii) which
streams through the external blast wave. Some of the prompt photons are scattered and many scattered photons convert to electron-positron
pairs. The inverse-Compton flash is bright due to the huge e+- enrichment of the medium. GeV emission generated by this mechanism lasts much
longer than the prompt GRB because of a broader angular distribution of scattered photons. At late times, the blast wave switches to normal
synchrotron-self-Compton cooling. The mechanism is demonstrated by a detailed transfer simulation. The observed prompt MeV radiation is taken
as an input of the simulation; we use GRB 080916C as an example. The result reproduces the GeV flash observed by the Fermi telescope. It
explains the delayed onset, the steep rise, the peak flux, the time of the peak, the long smooth decline, and the spectral slope of GeV
emission. The wind density required to reproduce all these features is typical of Wolf-Rayet stars. Our simulation predicts strong TeV emission
1 min after the burst trigger; then the cutoff of the observed high-energy spectrum must be shaped by extragalactic background light
absorption. In addition, a bright optical counterpart of the GeV flash is expected for plausible values of the magnetic field; such double
(optical+GeV) flashes may be observed with Fermi and optical robotic telescopes.
- 1307.3037 from 12 Jul 13
W. B. Atwood et al.: New Fermi-LAT event reconstruction reveals more high-energy gamma rays from Gamma-ray bursts
Based on the experience gained during the four and a half years of the mission, the Fermi -LAT collaboration has undertaken a comprehensive
revision of the event-level analysis going under the name of Pass 8. Although it is not yet finalized, we can test the improvements in the new
event reconstruction with the special case of the prompt phase of bright Gamma-Ray Bursts (GRBs), where the signal to noise ratio is large
enough that loose selection cuts are sufficient to identify gamma- rays associated with the source. Using the new event reconstruction, we have
re-analyzed ten GRBs previously detected by the LAT for which an x-ray/optical follow-up was possible and found four new gamma rays with
energies greater than 10 GeV in addition to the seven previously known. Among these four is a 27.4 GeV gamma-ray from GRB 080916C, which has a
redshift of 4.35, thus making it the gamma ray with the highest intrinsic energy (147 GeV) detected from a GRB. We present here the salient
aspects of the new event reconstruction and discuss the scientific implications of these new high-energy gamma rays, such as constraining
extragalactic background light models, Lorentz invariance violation (LIV) tests, the prompt emission mechanism and the bulk Lorentz factor of
the emitting region.
- 1308.2506 from 13 Aug 13
A.R. Rao et al.: Time resolved spectral analysis of the prompt emission of long gamma ray bursts with GeV Emission
We make a detailed time resolved spectroscopy of bright long gamma ray bursts (GRBs) which show significant GeV emissions (GRB 080916C, GRB
090902B, and GRB 090926A). In addition to the standard Band model, we also use a model consisting of a blackbody and a power-law to fit the
spectra. We find that for the latter model there are indications for an additional soft component in the spectra. While previous studies have
shown that such models are required for GRB 090902B, here we find that a composite spectral model consisting of two black bodies and a power
law adequately fit the data of all the three bright GRBs. We investigate the evolution of the spectral parameters and find several generic
interesting features for all three GRBs, like a) temperatures of the black bodies are strongly correlated to each other, b) flux in the black
body components are strongly correlated to each other, c) the temperatures of the black body trace the profile of the individual pulses of the
GRBs, and d) the characteristics of the power law component like the spectral index and the delayed onset bear a close similarity to the
emission characteristics in the GeV regions. We discuss the implications of these results to the possibility of identifying the radiation
mechanisms during the prompt emission of GRBs.
- 1501.0508 from 22 Jan 15
Yunguo Jiang et al.: GRB Spectra in the complex of synchrotron and Compton processes
Under the steady state condition, the spectrum of electrons is investigated by solving the continuity equation under the complex radiation of
both the synchrotron and Compton processes. The resulted GRB spectrum is a broken power law in both the fast and slow cooling phases. On the
basis of this electron spectrum, the spectral indices of the Band function in four different phases are presented. In the complex radiation
frame, the detail investigation on physical parameters reveals that both the reverse shock photosphere model and the forward shock with strong
coupling model can answer the $\alpha \sim -1$ problem. A possible marginal to fast cooling phase transition in GRB 080916C is discussed. The
time resolved spectra in different pulses of GRB 100724B, GRB 100826A and GRB 130606B are investigated. We found that the flux is proportional
to the peak energy in almost all pulses. The phases for different pulses are determined according to the spectral index revolution. We found
the strong correlations between spectral indices and the peak energy in GRB 100826A, which can be explained by the Compton effect in the fast
cooling phase. However, the complex scenario predicts a steeper index for the injected electrons, which challenges the acceleration mechanism
in GRBs.
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