- GCN Circular #9866
Elisabetta Bissaldi (MPE) and Valerie Connaughton (UAH)
report on behalf of the Fermi GBM Team:
"At 11:05:08.31 UT on 2 September 2009, the Fermi Gamma-Ray Burst Monitor
triggered and located GRB 090202B (trigger 273582310 / 090902462).
The on-ground calculated location, using the GBM trigger
data, is RA = 264.5, DEC = 26.5 (J2000 degrees,
equivalent to 17h 38m, 26d 30'), with an uncertainty
of 1.0 degree (radius, 1-sigma containment,
statistical only; there is additionally a systematic
error which is currently estimated to be 2 to 3 degrees).
The angle from the Fermi LAT boresight is 52 degrees.
Moreover, this burst was bright enough to result in
a Fermi spacecraft repointing maneuver.
The burst was also independently detected by INTEGRAL SPI-ACS.
The GBM light curve consists of a bright pulse
with a duration of about 21 s.
The time-averaged spectrum between 50 keV and 40 MeV
from T0 to T0+22 s is well fit by a Band function
with Epeak = 775 +/- 11 keV, alpha = -0.696 +/- 0.012
and beta = -3.85 (+0.21/-0.31).
The event fluence (50 keV - 10 MeV) in this time interval is
(3.74 +/- 0.03)E-04 erg/cm^2. The 1-sec peak photon flux measured
starting from T0+14 s in the 50 keV - 10 MeV band
is 46.1 +/- 0.3 ph/s/cm^2.
The spectral analysis results presented above are preliminary;
final results will be published in the GBM GRB Catalog."
- GCN Circular #9867
Francesco de Palma (Universit=E0 e INFN Bari), Johan Bregeon (INFN, =20
Pisa) and Hiro Tajima (SLAC) report on behalf of the Fermi LAT team:
At 11:05:15 UT on 2 Sep 2009, the Fermi Large Area Telescope (LAT) =20
detected gamma rays from the long GRB 090902B, which was triggered and =20
located by the Fermi Gamma-ray Burst Monitor (GBM) (trigger =20
273582310/090902462, GCN9866). The angle of the GBM best position (RA, =20
Dec=3D 264.5, 26.5) with respect to the LAT boresight was 51 degrees at =20
the time of the trigger, which is close the edge of our field of view.
The data from the Fermi LAT show a significant increase in the event =20
rate within 1 degree of the GBM location after the GBM trigger that is =20
spatially and temporally correlated with the GBM emission with high =20
significance. More than 200 photons above 100 MeV and more than 30 =20
photons above 1 GeV are observed within 100 seconds. The highest =20
energy photon is a 33.4 GeV event which is observed 82 seconds after =20
the GBM trigger.
The best LAT on-ground localization is found to be (RA,Dec=3D265.00, =20
27.33) with a 90% containment radius of 0.06 deg (statistical; 68% =20
containment radius: 0.04 deg, preliminary systematic error is less =20
than 0.1 deg) which is consistent with the GBM localization. A Swift =20
TOO request has been issued.
Further analysis is ongoing.
The point of contact for this burst is
Francesco de Palma (francesco.depalma@ba.infn.it)
The Fermi LAT is a pair conversion telescope designed to cover the =20
energy band from 20 MeV to greater than 300 GeV.
It is the product of an international collaboration between NASA and =20
DOE in the U.S. and many scientific institutions across France, Italy, =20
Japan and Sweden.
This message can be cited.
- GCN Circular #9868
J. A. Kennea (PSU) and G. Stratta (ASDC) report on behalf of the Swift/XRT
team:
At 23:36 UT, September 2nd, 2009, Swift began a Target of Opportunity
observation of the Fermi GBM/LAT discovered burst GRB 090902B (de Palma et
al., GCN #9867, Bissaldi et al., GCN #9866), approximately 12.5 hours
after the Fermi detection. In early data we detect an uncatalogued point
source at the following location, RA, Dec = 264.93984, 27.32405, which is
equivalent to:
RA (J2000) = 17h 39m 45.6s
Dec (J2000) = +27d 19' 26.6''
with an estimated uncertainty of 4.2 arcseconds radius (90% confidence).
This position is 3.2 arcmin from the reported LAT position, inside the LAT
error radius.
At this time we cannot confirm if the point source is fading. Observations
of this field are on-going.
- GCN Circular #9869
C. A. Swenson (PSU) and M. H. Siegel (PSU) report on behalf of the Swift
UVOT team:
The Swift/UVOT began observations of the field of GRB090902B
approximately 12.5 hours after the Fermi GBM/LAT trigger (de Palma et
al., GCN #9867, Bissaldi et al., GCN #9866). Within the initial 4.2"
XRT error circle (Kennea et al., GCN #9868) we find a very faint source:
Filter T_start(s) T_stop(s) Exp(s) Mag
u 273627410 273634846 2691 20.41 +- 0.20 (5.5-sigma)
This source does not seem to appear in the DSS; however, its proximity
to another DSS source, also within the XRT error circle, makes
identifying this source as the afterglow difficult. We await an
enhanced XRT position and/or ground-based detection to confirm the
nature of this potential afterglow candidate.
The values quoted above are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.04 in the direction of the burst
(Schlegel et al. 1998).
- GCN Circular #9870
D. A. Perley, I. K. W. Kleiser, and J. M. Rex (UC Berkeley) report:
We imaged the location of the Swift-XRT candidate afterglow (GCN 9868,
Kennea et al.) of extremely bright Fermi GRB 090902B (GCN 9866, Bissaldi
et al.) with the Nickel 1-meter telescope at Lick Observatory. We
marginally detect a faint source in individual 600-second exposures at
the western edge of the XRT error circle with a magnitude comparable to
the Digitized Sky Survey limit. This is likely the same source observed
by the UVOT (GCN 9868, Swenson et al.).
We cannot confirm fading behavior at this time, and further observations
are encouraged. A finding chart showing the XRT error circle and the
putative afterglow is available at the following URL:
http://lyra.berkeley.edu/~dperley/090902b/090902b_lick.jpg
- GCN Circular #9871
P.A. Evans (U. Leicester) reports on behalf of the Swift-XRT team.
Using 2.7 ks of XRT Photon Counting mode data and 2 UVOT images for GRB
090902B, we find an astrometrically corrected X-ray position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue): RA, Dec = 264.93859, 27.32448 which is equivalent to:
RA (J2000): 17h 39m 45.26s
Dec (J2000): +27d 19' 28.1"
with an uncertainty of 2.1 arcsec (radius, 90% confidence).
Position enhancement is described by Goad et al. (2007, A&A, 476, 1401)
and Evans et al. (2009, MNRAS, 397, 1177).
This circular is an official product of the Swift-XRT team.
- GCN Circular #9872
F. de Palma (Universit=E0 e INFN Bari), E. Bissaldi (MPE), H. Tajima (SLAC),
S. Guiriec (UAH), N. Omodei (INFN Pisa), V. Vasileiou (NASA GSFC/UMBC)
and V. Connaughton (UAH) report for the Fermi LAT and GBM teams:
Further analysis of GRB 090902B (Bissaldi et al. GCN 9866, de Palma et al.
GCN 9867, Kennea et al. GCN 9868) reveals it is detected in the Fermi
Large Area Telescope (LAT) at least until 300 s after the Fermi
Gamma-Ray Burst Monitor (GBM) trigger time, T0=3D11:05:08.31 UT.
Spectral analysis of the main emission episode, from T0 to T0+25 s, shows
a
deviation from the Band function obtained from fitting GBM data between
50 keV and 40 MeV (GCN 9866). This deviation is apparent both below 50
keV in the GBM and above 100 MeV in the LAT. It is well-fit by a single
power-law, with a well-constrained index that is retrieved from a fit of
the
GBM data alone, the LAT data alone, and when jointly fitting the entire
data set.
The parameters for this multi-component fit are Band_alpha =3D
0.61 =B1 0.01, Band_beta =3D 3.87 =B1 0.16, Band_EPeak =3D 798 =B1 7 keV,
and a power-law index of 1.94 =B1 0.01 that shows no evidence for a
spectral cut-off below 100 GeV. The fluence between 8 keV and 30
GeV is 4.86 =B1 0.06 x 10-4 ergs/cm2.
The points of contact for this burst are:
Francesco de Palma (francesco.depalma@ba.infn.it) and
Elisabetta Bissaldi (ebs@mpe.mpg.de).
The Fermi LAT is a pair conversion telescope designed to cover the
energy band from 20 MeV to greater than 300 GeV.
It is the product of an international collaboration between NASA and DOE
in the U.S. and many scientific institutions across France, Italy, Japan
and Sweden.
This message can be cited.
- GCN Circular #9873
A. Cucchiara, D. B. Fox (PSU), N. Tanvir (U. Leicester),
E. Berger (Harvard U.) report on behalf of a large collaboration.
"On 2009 September 3.27 UT we observed the Fermi/LAT GRB 090902B
(Bissaldi et al., GCN 9866, de Palma 9867) with the Gemini-N
telescope equipped with the GMOS spectrograph.
A single object has been found at the position of the Swift/XRT
afterglow (Kennea et al. GCN 9868, Perley et al., GCN 9870) in
our acquisition image.
A pair of 900 sec exposures spectra were obtained with a wavelength
coverage of 4500-7500A under good conditions.
The spectrum reveals a series of metal absorption features,
MgII[2796,2803], MgI[2853],MnII[2606], FeII[2600], MnII[2594],
FeII[2586], FeII[2260], SiII[1808], SiII*[1816].
All these features are consistent with a redshift of z = 1.822,
which, therefore, we conclude is the redshift of GRB 090902B.
We thank the Gemini-N staff for performing this observation,
in particular Ricardo Schiavon."
- GCN Circular #9874
F. Olivares, P. Afonso, J. Greiner (all MPE Garching), S. McBreen (MPE +
UCD), T. Kruehler, A. Rau, A. Yoldas, G. Kanbach (all MPE), and S. Klose
(TLS Tautenburg) report on behalf of the GROND team:
The field of GRB 090902B, (Fermi GBM trigger 273582310/090902462, Bissaldi
et al., GCN #9866) which was detected by the LAT (de Palma et al., GCN
#9867), was observed simultaneously in g'r'i'z'JHK with GROND (Greiner et
al. 2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at LaSilla
Observatory (Chile).
Observations started at 00:21 UT on September 3, 2009, 13.25 hours after
the GRB trigger. They were performed at a seeing of around 2", at an
average air mass larger than 2.0 and under variable sky conditions. We
detect a NIR source within the Swift/XRT error circle reported by Kennea et
al. (GCN #9868) and on the edge of the Swift/XRT enhanced position (Evans
et al. GCN #9871), at:
RA (J2000.0) = 17h 39m 45.41s
DEC (J2000.0) = +27d 19' 27.1"
with an uncertainty of 0.5" in each coordinate.
This position seems consistent with the object reported by Perley et al.
(GCN #9870) and is very likely the source reported by Swenson & Siegel (GCN
#9869).
The source is not in the field of view of the griz channels, so no
statement can be made about the optical bands.
The afterglow (Cucchiara et al. GCN #9873) has the following preliminary AB
magnitude and shows tentative evidence of fading within the observations.
midtime J_AB mag
-----------------------------------------------
01:09:31 UT 20.26 +/- 0.07
Calibrations was done against 2MASS catalog stars. No Galactic extinction
correction was applied.
- GCN Circular #9875
C. Guidorzi (U. Ferrara), N.R. Tanvir (U. Leicester),
Z. Cano, I. A. Steele, A. Melandri, D. Bersier,
S. Kobayashi, C.J. Mottram, C.G. Mundell,
R.J. Smith (Liverpool JMU), A. Gomboc (U. Ljubljana),
P. O'Brien (U. Leicester) on behalf of a large
collaboration report:
On 2009 September 03 at 07:50:02 UT we began observing
the Fermi GRB 090902B (Bissaldi et al., GCN 9866; de Palma
et al. GCN 9867, 9872) with the Faulkes Telescope North
using the R filter.
Within the Swift UVOT-enhanced XRT error circle (Evans et al.
GCN 9871) we clearly detect the afterglow candidate (Swenson et
al. GCN 9869, Perley et al. GCN 9870, Cucchiara et al. GCN 9873,
Olivares et al. GCN 9874) at the following position (J2000):
RA: 17:39:45.35
Dec: +27:19:27.1
with an uncertainty of 0.7". At the mid epoch of 21.01 hours
post burst we measure R=21.03 +- 0.11 mag.
Calibration was performed with the USNOB-1 star RA=17:39:42.74
Dec=+27:19:13.76 assuming R2=17.46.
- GCN Circular #9876
G. Stratta, V. D'Elia, M. Perri (ASDC) report on behalf of the
Swift XRT team:
At 23:36 UT, September 2nd, 2009, Swift began a Target of Opportunity
observation of the Fermi GBM/LAT discovered burst GRB 090902B (de
Palma et al., GCN 9867, Bissaldi et al., GCN 9866) about 12.5 hours
after the trigger.
We confirm that the X-ray source reported by Kennea et al. (GCN
Circ. 9868) shows a decaying behavior and is likely the afterglow of
GRB 090902B.
The enhanced XRT position for this burst was given by Evans
(GCN. Circ 9871).
The X-ray light curve from T+12.53 hr to T+17.38 hr can be fit with
a single power-law model with a decay index of -1.3+/-0.9 (90% confidence
level).
With 4.8 ks of integration time, the X-ray spectrum from T+12.53 hr to
T+17.38 hr
after the Fermi/GBM trigger can be fit by an absorbed power-law model
with a photon index of 2.1 (+0.3,-0.3) and a rest frame column density of
(3.4+/-0.9) x 10^22 cm^-2 at z=1.822 in addition to the Galactic column
density
in the direction of the burst (3.88 x 10^20 cm^-2, Kalberla et al. 2005).
Errors are at 90% confidence level.
The observed 0.3-10.0 keV flux is 3.2 x 10^-12 erg cm^-2 s^-1 which
corresponds to an unabsorbed flux of 4.1 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 8.7 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 #9877
C. A. Swenson (PSU) and G. Stratta (ASDC) report on behalf of the
Swift/UVOT team:
The Swift/UVOT began observations of the field of GRB 090902B
approximately 12.5 hours after the Fermi GBM/LAT trigger (de Palma
et al., GCN Circ. 9867; Bissaldi et al., GCN Circ. 9866). We can
now confirm the detection of the optical afterglow (Swenson et al.,
GCN Circ. 9869; Perley et al., GCN Circ. 9870; Cucchiara et al.,
GCN Circ. 9873; Olivares et al., GCN Circ. 9874; Guidorzi et al.,
GCN Circ. 9875) coincident with the enhanced Swift-XRT position (Evans,
GCN Circ. 9871). We also observe a decay in the optical afterglow.
The observed magnitudes using the UVOT photometric system (Poole et
al. 2008, MNRAS, 383, 627) are:
Filter T_start(s) T_stop(s) Exp(s) Mag
u 45096 46188 1075 20.26 =B1 0.19
u 50891 58703 3613 20.50 =B1 0.11
u 62457 81106 8756 20.85 =B1 0.10
The values quoted above are not corrected for the Galactic extinction
due to the reddening of E(B-V) =3D 0.04 in the direction of the burst
(Schlegel et al. 1998).
- GCN Circular #9878
S. B. Pandey, W. Zheng, F. Yuan and C. Akerlof (U Mich), report on behalf
of the
ROTSE collaboration:
ROTSE-IIIa, located at the Siding Spring Observatory Australia, responded ~
283 sec after the GBM trigger 273582310 (Bissaldi E. & Connaughton V. GCNC
9866). The Set of images starting ~ 1.4 hours after the trigger covered the
GRB 090902B location. The OT (Perley et al. GCNC 9870, Kennea & Stratta
GCNC 9868) was detected at 15.9+-0.2 mag in the sum of 30x60 sec exposures.
The images taken by ROTSE-IIIc (located naer the H.E.S.S. site at Mt.
Gamsberg, Namibia) around 6.6 hours after the burst marginally detect the
OT, indicating a decay of ~ 2 magnitudes. The unfiltered images were
calibrated relative to USNO B1.0. Further analysis is in progress.
- GCN Circular #9883
A.J. van der Horst (NASA/MSFC/ORAU) A.P. Kamble, R.A.M.J. Wijers
(U of Amsterdam) and C. Kouveliotou (NASA/MSFC) report on behalf of
a large collaboration:
"We observed the position of the GRB 090902B afterglow with the
Westerbork Synthesis Radio Telescope at 4.8 GHz at September 3 12.72 UT
to September 4 0.35 UT, i.e. 1.07 - 1.55 days after the burst (GCN 9866).
We detect a radio source at the 4-sigma level with a flux density
of 111 +/- 28 microJy at the position of the optical counterpart
(GCN 9869, GCN 9874).
We would like to thank the WSRT staff for scheduling and obtaining these
observations."
- GCN Circular #9889
Poonam Chandra (RMC) and Dale A. Frail (NRAO) report on behalf of a
larger collaboration:
"We used the Very Large Array (VLA) to observe the field of view towards
the GRB 090902B (GCN 9866) at a frequency of 8.46 GHz on 2009 Sep.
03.94 UT. We clearly detect the GRB afterglow at the GROND NIR afterglow
position (GCN 9874) at a flux density of 141+/-39 uJy.
Further observations are planned.
The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc."
- GCN Circular #9897
Y. Terada, M. Tashiro, A. Endo, K. Onda, T. Sugasahara,
W. Iwakiri (Saitama U.), K. Yamaoka, S. Sugita (Aoyama Gakuin U.),
M. Ohno, M. Suzuki, M. Kokubun, T. Takahashi (ISAS/JAXA),
Y. E. Nakagawa, T. Tamagawa (RIKEN), N. Ohmori, A. Daikyuji,
E. Sonoda, K. Kono, H. Hayashi, K. Noda, Y. Nishioka, M. Yamauchi
(Univ. of Miyazaki), N. Vasquez (Tokyo Tech.), Y. Urata (NCU),
Y. Hanabata, T. Uehara, T. Takahashi, Y. Fukazawa (Hiroshima U.),
T. Enoto, K. Nakazawa, K. Makishima (Univ. of Tokyo), S. Hong
(Nihon U.), on behalf of the Suzaku WAM team, report
The Fermi detected long GRB 090902B (Bissaldi et al., GCN 9866,
de Palma et al. GCN 9867) triggered the Suzaku Wide-band All-sky
Monitor (WAM) which covers an energy range of 50 keV - 5 MeV at
11:05:08.446 UT (=T0). The observed light curve shows a multi-peaked
structure lasting from T0-0.5 s to T0+24.5 s with a duration (T90) of about
19 seconds. The fluence in 100 - 1000 keV was 1.45 (-0.04, +0.03)x10^-4 erg/cm^2.
The 1-s peak flux measured from T0+14.5 s was 21.3 (-1.3, +0.9) photons/cm^2/s
in the same energy range.
Preliminary result shows that the time-averaged spectrum from
T0-0.5 s to T0+24.5 s is well fitted by a power-law with exponential
cutoff model:
dN/dE ~ E^{-alpha} * exp(-(2-alpha)*E/Epeak) with
alpha: 0.91 +/- 0.10, and
Epeak: 885 (-38, +39) keV (chi^2/d.o.f. = 25.5/24).
Due to this GRB brightness, a 3% systematcic error was
added for low energy channels. In addition, there might be
some calibration uncertainties in fluence because the GRB
photons came into the WAM through the base plate of the satellite.
All the quoted errors are at 90% confidence level.
The light curves for this burst are available at:
http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html
- GCN Circular #9916
J. Haislip, D. Reichart, K. Ivarsen, A. LaCluyze, A. Foster, J. Moore, A.
Oza, M. Schubel, J. Styblova, A. Trotter, J. A. Crain, and M. Nysewander
report:
Skynet observed the Fermi/LAT localization of GRB 090902B (de Palma et al.,
GCN 9867) with four of the 16" PROMPT telescopes at CTIO beginning 13.3
hours after the trigger in BVRI through variable cloud cover.
We do not detect the afterglow (Kennea et al., GCN 9868; Swenson & Siegel,
GCN 9869). Stacking only images that increase the limiting magnitude
yields:
mean 1-sig. 1-sig.
time 3-sig. sys. stat.
since lim. cal. cal. cal.
trig. tel. exp. fil. mag. stars unc. unc.
(h) (# x s) (mag) (mag)
14.4 PROMPT-4 18 x 80 R 19.8 240 USNO B1 0.185 0.001
14.4 PROMPT-3 18 x 80 B 19.0 100 USNO B1 0.296 0.002
14.4 PROMPT-5 18 x 80 I 19.0 278 USNO B1 0.234 0.001
14.4 PROMPT-2 17 x 80 V 19.0 217 NOMAD 0.255 0.001
- GCN Report 249.1
GCN_Report 249.1 has been posted:
http://gcn.gsfc.nasa.gov/reports/report_249_1.pdf
by G. Stratta
at ASDC
titled: "Swift Observations on Fermi-LAT GRB 090902B"
- 1003.3885 from 23 Mar 10
S. McBreen et al.: Optical and near-infrared follow-up observations of four Fermi/LAT GRBs : Redshifts, afterglows, energetics and host galaxies
Fermi can measure the spectral properties of gamma-ray bursts over a very large energy range and is opening a new window on the prompt emission
of these energetic events. Localizations by the instruments on Fermi in combination with follow-up by Swift provide accurate positions for
observations at longer wavelengths leading to the determination of redshifts, the true energy budget, host galaxy properties and facilitate
comparison with pre-Fermi bursts. Multi-wavelength follow-up observations were performed on the afterglows of four bursts with high energy
emission detected by Fermi/LAT : GRB090323, GRB090328, GRB090510 and GRB090902B. They were obtained in the optical/near-infrared bands with
GROND mounted at the MPG/ESO 2.2m telescope and additionally of GRB090323 in the optical with the 2 m telescope in Tautenburg, Germany. Three
of the events are classified as long bursts while GRB090510 is a well localized short GRB with GeV emission. In addition, host galaxies were
detected for three of the four bursts. Spectroscopic follow-up was initiated with the VLT for GRB090328 and GRB090510. The afterglow
observations in 7 bands are presented for all bursts and their host galaxies are investigated. Knowledge of the distance and the local dust
extinction enables comparison of the afterglows of LAT-detected GRBs with the general sample. The spectroscopic redshifts of GRB090328 and
GRB090510 were determined to be z=0.7354+/-0.0003 and z=0.903 +/- 0.001 and dust corrected star-formation rates of 4.8 Mdot yr^-1 and 0.60
M_dot yr^-1 were derived for their host galaxies, respectively. The afterglows of long bursts exhibit power-law decay indices alpha from less
than 1 to ~2.3 and spectral indices (beta) values from 0.65 to ~1.2 which are fairly standard for GRB afterglows. Constraints are placed on the
jet half opening angles of less than 2.1 deg to greater than 6.4 deg which allows limits to be placed on the beaming corrected energies. These
range from less than 5x10^50 erg to the one of the highest values ever recorded, greater than 2.2x10^52 erg for GRB090902B, and are not
consistent with a standard candle. The extremely energetic long Fermi bursts have optical afterglows which lie in the top half of the
brightness distribution of all optical afterglows detected in the Swift era or even in the top 5% if incompleteness is considered. The
properties of the host galaxies of these LAT detected bursts in terms of extinction, star formation rates and masses do not appear to differ
from previous samples.
- 1003.4250 from 23 Mar 10
S. B. Pandey et al.: GRB 090902B: afterglow observations and implications
The optical-infrared afterglow of the LAT-detected long duration burst,
GRB 090902B, has been observed by several instruments. The earliest
detection by ROTSE-IIIa occurred 80 minutes after detection by the GBM
instrument onboard the Fermi Gamma-Ray Space Telescope, revealing a
bright afterglow and a decay slope suggestive of a reverse shock origin.
Subsequent optical-IR observations followed the light curve for 6.5
days. The temporal and spectral behavior at optical-infrared frequencies is
consistent with synchrotron fireball model predictions; the cooling
break lies between optical and XRT frequencies ~ 1.9 days after the burst. The
inferred electron energy index is $p = 1.8 \pm 0.2$, which would
however imply an X-ray decay slope flatter than observed. The XRT and LAT data
have similar spectral indices and the observed steeper value of
the LAT temporal index is marginally consistent with the predicted temporal
decay in the radiative regime of the forward shock model. Absence
of a jet break during the first 6 days implies a collimation-corrected
$\gamma$-ray energy $E_{\gamma} > 2.2\times10^{52}\rm$ ergs, one of the
highest ever seen in a long-duration GRBs. More events combining GeV photon
emission with multi-wavelength observations will be required to
constrain the nature of the central engine powering these energetic explosions
and to explore the correlations between energetic quanta and
afterglow emission.
- 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.
- 1007.2228 from 15 Jul 10
Asaf Pe'er et al.: The Connection Between Thermal and Non-Thermal Emission in
Gamma-ray Bursts: General Considerations and GRB090902B as a Case Study
Photospheric (thermal) emission is inherent to the gamma-ray burst (GRB)
"fireball" model. We show here, that inclusion of this component in
the analysis of the GRB prompt emission phase naturally explains some of the
prompt GRB spectra seen by the Fermi satellite over its entire
energy band. The sub-MeV peak is explained as multi-color black body emission,
and the high energy tail, extending up to the GeV band, results
from roughly similar contributions of synchrotron emission, synchrotron self
Compton (SSC) and Comptonization of the thermal photons by
energetic electrons originating after dissipation of the kinetic energy above
the photosphere. We show how this analysis method results in a
complete, self consistent picture of the physical conditions at both emission
sites of the thermal and non-thermal radiation. We study the
connection between the thermal and non-thermal parts of the spectrum, and show
how the values of the free model parameters are deduced from the
data. We demonstrate our analysis method on GRB090902B: We deduce a Lorentz
factor in the range 780 <= \eta <= 1000, photospheric radius r_{ph}
~ 6.1 - 7.8*10^{11} cm and dissipation radius r_\gamma >= 3.8 - 5.0*10^{15} cm.
By comparison to afterglow data, we deduce that a large
fraction, \epsilon_d ~ 85% - 95% of the kinetic energy is dissipated, and that
large fraction, ~equipartition of this energy is carried by the
electrons and the magnetic field. This high value of \epsilon_d questions the
"internal shock" scenario as the main energy dissipation
mechanism for this GRB.
- 1009.1289 from 8 Sep 10
Ruo-Yu Liu et al.: Modeling the broadband emission of Fermi/LAT GRB 090902B
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)
GRB 090902B, detected by Fermi Large Array Telescope (Fermi/LAT), shows extend high-energy emission (>100 MeV) up to 10^3 s after the burst,
which decays with time in a power-law as t^{-1.5}. It has been also observed by several follow-up low-energy instruments, including an early
optical detection around 5000 s after the burst. The optical emission at early time decays faster than t^{-1.6}, which has been suspected to
originate from the reverse shock. We here explore the models that can possibly explain the the broadband afterglow emission of GRB 090902B. We
find that the reverse shock model for the early optical emission would overpredict the radio afterglow flux that is inconsistent with
observations. A partially radiative blast wave model, which though is able to produce a sufficiently steep decay slope, can not explain the
broadband data of GRB 090902B. The two-component jet model, which consists of a narrow and bright jet component in the core and a surrounding
wider and less energetic jet component, is shown to be able to explain the broadband afterglow data, including the LAT high-energy data after
~50 s and low-energy (radio, optical and X-ray) afterglow data. The early-time high-energy emission detected by LAT before ~50 s is likely due
to internal origin as that of the sub-MeV emission. The highest energy (33 GeV) photon of GRB090902B detected at 80 s can be marginally
accommodated within the forward shock emission under the optimistic condition that electrons are accelerated by the Bohm diffusive shock.
- 1009.5178 from 28 Sep 10
Katsuaki Asano et al.: Prompt X-ray and Optical Excess Emission due to Hadronic Cascades in Gamma-Ray Bursts
A fraction of gamma-ray bursts exhibit distinct spectral features in their prompt emission below few 10s of keV that exceed simple
extrapolations of the low-energy power-law portion of the Band spectral model. This is also true for the prompt optical emission observed in
several bursts. Through Monte Carlo simulations, we model such low-energy spectral excess components as hadronic cascade emission initiated by
photomeson interactions of ultra-high-energy protons accelerated within GRB outflows. Synchrotron radiation from the cascading, secondary
electron-positron pairs can naturally reproduce the observed soft spectra in the X-ray band, and in some cases the optical spectra as well.
These components can be directly related to the higher energy radiation at GeV energies due to the hadronic cascades. Depending on the spectral
shape, the total energy in protons is required to be comparable to or appreciably larger than the observed total photon energy. In particular,
we apply our model to the excess X-ray and GeV emission of GRB 090902B, and the bright optical emission of the "naked-eye" GRB 080916B. Besides
the hard GeV components detected by {\it Fermi}, such X-ray or optical spectral excesses are further potential signatures of ultra-high-energy
cosmic ray production in gamma-ray bursts.
- 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.
- 1103.0708 from 4 Mar 11
Felix Ryde et al.: Observational evidence of dissipative photospheres in gamma-ray bursts
Chiang, Miranda S. Jackson, Stefan Larsson, Francesco Longo, Sinead McGlynn, Nicola Omodei
The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a
Planck function or it can be broadened and have the shape of a Band function. This fact is best illustrated by studying GRB090902B: The main
gamma-ray spectral component is initially close to a Planck function, which can only be explained by emission from the jet photosphere. Later,
the same component evolves into a broader Band function. This burst thus provides observational evidence that the photosphere can give rise to
a non-thermal spectrum. We show that such a broadening is most naturally explained by subphotospheric dissipation in the jet. The broadening
mainly depends on the strength and location of the dissipation, on the magnetic field strength, and on the relation between the energy
densities of thermal photons and of the electrons. We suggest that the evolution in spectral shape observed in GRB090902B is due to a decrease
of the bulk Lorentz factor of the flow, leading to the main dissipation becoming subphotospheric. Such a change in the flow parameters can also
explain the correlation observed between the peak energy of the spectrum and low-energy power law slope, alpha, a correlation commonly observed
in GRBs. We conclude that photospheric emission could indeed be a ubiquitous feature during the prompt phase in GRBs and play a decisive role
in creating the diverse spectral shapes and spectral evolutions that are observed.
- 1107.1236 from 8 Jul 11
Rodolfo Barniol Duran et al.: Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B
Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs),
called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar
medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of
the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the
external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy
distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other
models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of
shocked plasma in magnetic field to be ~10^{-6} using late time afterglow data, which is consistent with the value obtained using early
gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modeling and numerical simulations of
particle acceleration in collisionless shocks. We also provide detailed calculations for the early (< 10^3 s) high energy (> 100 MeV) emission
and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ~10 keV excess observed
in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.
- 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.
- 1111.0308 from 3 Nov 11
T. Nymark et al.: Subphotospheric heating in GRBs: analysis and modeling of GRB090902B as observed by Fermi
We analyze the spectral evolution of GRB 090902B and show that subphotospheric dissipation can explain both the spectra and the spectral
evolution. The emission from a GRB photosphere can give rise to a variety of spectral shapes. The spectrum can have a shape close to that of a
Planck function (as is observed during the first half of GRB090902B) or be broadened, resembling a typical Band function (as is observed during
the second half of GRB090902B). The shape mainly depends on the strength and location of the dissipation in the jet, the ratio of the energy
densities of thermal photons and of the electrons at the dissipation site, as well as on the strength of the magnetic field. We further discuss
numerical models of the dissipation and relate these to the observed spectra.
- 1212.4418 from 19 Dec 12
N. Fraija et al.: Long and short high energy components presented in GRBs
We present a leptonic model on the external shock framework to describe the long- and short- lasting GeV component of some GRBs. This model was
already applied successfully to GRB 090926A, and we extend it to describe the high-energy emission of GRB 090902B and GRB 090510. We argue that
the high-energy emission consists of two components, one at MeV energies with a duration of a few seconds during the prompt phase, and a second
GeV component lasting hundred of seconds after the prompt phase. The short high-energy component can be described as SSC emission from a
reverse shock and the longer component arises from SSC emission of the forward shock. The main assumption of our model is that the jet is
magnetized and evolves in the thick-shell case. The calculated fluxes and break energies are all consistent with the observed values.
- 1302.0235 from 4 Feb 13
R. Hascoët et al.: Prompt thermal emission in gamma-ray bursts
GRB spectra appear non-thermal, but recent observations of a few bursts with Fermi GBM have confirmed previous indications from BATSE of the
presence of an underlying thermal component. Photospheric emission is indeed expected when the relativistic outflow emerging from the central
engine becomes transparent to its own radiation, with a quasi-blackbody spectrum in absence of additional sub-photospheric dissipation.
However, its intensity strongly depends on the acceleration mechanism - thermal or magnetic - of the flow. We aim to compute the thermal and
non-thermal emissions produced by an outflow with a variable Lorentz factor, where the power injected at the origin is partially thermal
(fraction epsilon_th) and partially magnetic (fraction 1-epsilon_th). The thermal emission is produced at the photosphere, and the non-thermal
emission in the optically thin regime. Apart from the value of epsilon_th, we want to test how the other model parameters affect the observed
ratio of the thermal to non-thermal emission. If the non-thermal emission is made by internal shocks, we self-consistently obtained the light
curves and spectra of the thermal and non-thermal components for any distribution of the Lorentz factor in the flow. If the non-thermal
emission results from magnetic reconnection we were unable to produce a light curve and could only compare the respective non-thermal and
thermal spectra. In the different considered cases, we varied the model parameters to see when the thermal component in the light curve and/or
spectrum is likely to show up or, on the contrary, to be hidden. We finally compared our results to the proposed evidence for the presence of a
thermal component in GRB spectra. Focussing on GRB 090902B and GRB 10072B, we showed how these observations can be used to constrain the nature
and acceleration mechanism of GRB outflows.
- 1303.1564 from 8 Mar 13
D. Zaborov et al.: The HAWC observatory as a GRB detector
The High Altitude Water Cherenkov Observatory (HAWC) is an air shower array currently under construction in Mexico at an altitude of 4100 m.
HAWC will consist of 300 large water tanks covering an area of about 22000 square meters and instrumented with 4 photomultipliers each. The
experimental design allows for highly efficient detection of photon-induced air showers in the TeV and sub-TeV range and gamma-hadron
separation. We show that HAWC has a reasonable chance to observe the high-energy power law components of GRBs that extend to 50 GeV. In
particular, HAWC will be capable of observing events similar to GRB 090510 and GRB 090902B. The observations (or non-observations) of GRBs by
HAWC will provide information on the high-energy spectra of GRBs. An engineering array consisting of 6 water tanks was operated at the HAWC
site since September 2011, collecting 3 months of data. An upper limit on high energy emission from GRB 111016B is derived from these data.
- 1305.3689 from 17 May 13
Martin Lemoine et al.: On the magnetisation of gamma-ray burst blast waves
The origin of magnetic fields that permeate the blast waves of gamma-ray bursts is a long-standing problem. The present paper argues that in
four GRBs revealing extended emission at >100 MeV, with follow-up in the radio, optical and X-ray domains at later times, this magnetisation
can be described as the partial decay of the microturbulence that is generated in the shock precursor. Assuming that the extended high energy
emission can be interpreted as synchrotron emission of shock accelerated electrons, we model the multi-wavelength light curves of GRB 090902B,
GRB 090323, GRB 090328 and GRB 110731A, using a simplified then a full synchrotron calculation with power law decaying microturbulence
\epsilon_B \propto t^{\alpha_t} (t denotes the time since injection through the shock, in the comoving blast frame). We find that these models
point to a consistent value of the decay exponent -0.5 < \alpha_t < -0.4.
- 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.
- 1308.2819 from 14 Aug 13
Rupal Basak et al.: A lingering non-thermal component in the GRB prompt emission: predicting GeV emission from the MeV spectrum
The high energy GeV emission of gamma-ray bursts (GRBs), detected by \emph{Fermi}/LAT, has a significantly different morphology compared to the
lower energy MeV emission, detected by \emph{Fermi}/GBM. Though the late time GeV emission is believed to be synchrotron radiation produced via
an external shock, this emission as early as the prompt phase is puzzling. Meaningful connection between these two emissions can be drawn only
by an accurate description of the prompt MeV spectrum. We perform a time-resolved spectroscopy of the GBM data of long GRBs having significant
GeV emission, using a model consisting of 2 blackbodies and a power-law. We examine in detail the evolution of the spectral components and
found that GRBs having high GeV emission (GRB 090902B and GRB 090926A) have a delayed onset of the power-law component, in the GBM spectrum,
which lingers at the later part of the prompt emission. This behaviour mimics the flux evolution in LAT. In contrast, bright GBM GRBs with an
order of magnitude lower GeV emission (GRB 100724B and GRB 091003) show a coupled variability of the total and the power-law flux. Further, by
analyzing the data for a set of 17 GRBs, we find a strong correlation between the power-law fluence in the MeV and the LAT fluence (Pearson
correlation: r=0.88 and Spearman correlation: $\rho=0.81$). We demonstrate that this correlation is not influenced by the correlation between
the total and the power-law fluences at a confidence level of 2.3$\sigma$. We speculate the possible radiation mechanisms responsible for the
correlation.
- 1402.7022 from 28 Feb 14
Bing Zhang: Gamma-Ray Burst Prompt Emission
The origin of gamma-ray burst (GRB) prompt emission, bursts of gamma-rays lasting from shorter than one second to thousands of seconds, remains
not fully understood after more than 40 years of observations. The uncertainties lie in several open questions in the GRB physics, including
jet composition, energy dissipation mechanism, particle acceleration mechanism, and radiation mechanism. Recent broad-band observations of
prompt emission with Fermi sharpen the debates in these areas, which stimulated intense theoretical investigations invoking very different
ideas. I will review these debates, and argue that the current data suggest the following picture: A quasi-thermal spectral component
originating from the photosphere of the relativistic ejecta has been detected in some GRBs. Even though in some cases (e.g. GRB 090902B) this
component dominates the spectrum, in most GRBs, this component either forms a sub-dominant "shoulder" spectral component in the low energy
spectral regime of the more dominant "Band" component, or is not detectable at all. The main "Band" spectral component likely originates from
the optically thin region due to synchrotron radiation. The diverse magnetization in the GRB central engine is likely the origin of the
observed diverse prompt emission properties among bursts.
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