Gamma-ray Burst 090831

(All information courtesy of the instrument teams.)

Previous IAU Circulars

none

Results of Observations

GCN Circular #9850
Arne Rau (MPE) reports on behalf of the Fermi GBM Team:
"At 07:36:36.58 UT on 31 August 2009, the Fermi Gamma-Ray Burst Monitor triggered and located GRB 090831 (trigger 273396998 / 090831317).
The on-ground calculated location, using the GBM trigger data, is RA = 145.1, DEC = 51.0 (J2000 degrees, equivalent to 09 h 40m m, +51 d 00 '), with an uncertainty of 1.9 degrees (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 107 degrees.
This burst was also independently detected by INTEGRAL SPI-ACS.
The GBM light curve consists of two structured main pulses with a duration (T90) of about 69.1 s (8-1000 keV). The time-averaged spectrum from T+0.0 s to T0+47.7 s is best fit by a power law function with an exponential high energy cutoff. The power law index is -1.57 +/- 0.03 and the cutoff energy, parameterized as Epeak, is 399.6 +/- 86.2 keV (chi squared 443 for 415 d.o.f.).
The event fluence (8-1000 keV) in this time interval is (1.66 +/- 0.05)E-05 erg/cm^2. The 1.024-sec peak photon flux measured starting from T0+8.784 s in the 8-1000 keV band is 9.4 +/- 0.4 ph/s/cm^2.
A Band function fits the spectrum equally well (chi squared 441 for 414 d.o.f.) with Epeak= 243.8 +/- 85.3 keV, alpha = -1.52 +/- 0.06 and beta = -1.96 +/- 0.14.
The spectral analysis results presented above are preliminary; final results will be published in the GBM GRB Catalog."
GCN Circular #9852
M. Matsuoka (JAXA), S. Miyoshi, M. Nakajima, H. Negoro, R. Ishiwata (Nihon Univ.), K. Kawasaki, S. Ueno, H. Tomida, M. Suzuki, M. Ishikawa (JAXA), T. Mihara, M. Kohama, Y.E. Nakagawa, M. Sugizaki, T. Yamamoto (RIKEN), N. Kawai, M. Morii, K. Sugimori (Tokyo Tech), A. Yoshida, K. Yamaoka, S. Nakahira (Aoyama G. Univ.), H. Tsunemi, M. Kimura (Osaka Univ.), Y. Ueda, N. Isobe, S. Eguchi and K. Hiroi (Kyoto Univ.) report on behalf of the MAXI team:
The Gas Slit Camera (GSC) of MAXI, the all-sky X-ray monitor on the ISS, detected a bright X-ray source at the position consistent with GRB 090831 (Rau et al, GCN 9850) in the energy range of 2-30 keV. The nominal location of the source as measured by GSC is
(R.A., Dec.) = (145.4 deg, +51.4 deg) = (09:41:36, +51:24:00) (J2000)
with an estimated systematic uncertainty of about 1 degree.
The source was detected for 60 seconds during the transit of GSC over the source starting approximately at 07:36:35 UT on 31 August 2009, very close to the Fermi GBM trigger time. The X-ray flux at the peak of the transit was about 3 Crab. There are significant time structure within the transit light curve, which would be simply triangular for a steady source.
The all-sky image and the transit light curve is shown at the MAXI web site http://maxi.riken.jp/ in the "News" section.
MAXI is currently in the commissioning phase that runs for three months. The systematic uncertainty in localization will be significantly improved with the progress of the alignment calibration.
GCN Circular #9861
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:
GRB 090831A (Rau, GCN 9850; Matsuoka et al., GCN 9852) triggered Konus-Wind at T0=27393.820 s UT (07:36:33.820)
The burst started with a short (~250 ms) spike which resembles a classical short/hard GRB. The spike is followed by a structured profile with a total duration of ~50 s. The Konus-Wind light curves of this GRB are available at http://www.ioffe.ru/LEA/GRBs/GRB090831_T27393/
As observed by Konus-Wind, the burst had a fluence of 1.33(-0.2,+0.25)x10-5 erg/cm2, and a 16-ms peak flux, measured from T0, of 9.3(-2.2,+3.2)x10-6 erg/cm2/s (both in the 20 keV - 2 MeV energy range).
The time-integrated spectrum of the burst (from T0 to T0+49.408 s) is well fitted (in the 20 keV - 2 MeV range) by a power law with exponential cutoff model: dN/dE ~ (E^alpha)*exp(-E*(2+alpha)/Ep), with alpha = -1.53(-0.14,+0.16), and Ep = 253(-69,+162)keV (chi2 = 69/61 dof). Fitting by the GRB (Band) model yields to the same alpha and Ep values with the high energy photon index beta of -9 (< -2.1)
The spectrum at the maximum count rate, measured from T0 to T0 + 0.256s (initial short pulse), is well fitted (in the 20 keV - 2 MeV range) by the GRB (Band) model with:
```
alpha = -1.28(-0.3,+0.7),
beta  = -2.5( < -1.7), and
Ep    = 273(-212,+331)keV (chi2 = 12/22 dof).
```
All the quoted values are preliminary. All the quoted errors are at the 90% confidence level.
GCN Circular #9864
S. Golenetskii, R. Aptekar, E. Mazets, V. Pal'shin, D. Frederiks, and T. Cline on behalf of the Konus-Wind team,
V. Connaughton, M. Briggs, and C. Meegan, on behalf of the Fermi GBM team,
K. Yamaoka, M. Ohno, Y. Fukazawa, T. Takahashi, M. Tashiro, Y. Terada, T. Murakami, K. Makishima, and Y. Hanabata on behalf of the Suzaku-WAM team,
M. Suzuki, A. Yoshida, N. Kawai, S. Nakahira, Y.E. Nakagawa, M. Kohama and M. Matsuoka on behalf of the MAXI team
and A. von Kienlin, G. Lichti, A. Rau, and K. Hurley on behalf of the INTEGRAL SPI-ACS GRB team, report:
GRB 090831A, reported in GCN 9850 (Rau) as Fermi/GBM trigger 273396998 / 090831317, and as a MAXI event in GCN 9852 (Matsuoka et al.= ), and as a Konus event in GCN 9861 (Golenetskii et al.), was also detected = by Suzaku-WAM and INTEGRAL (SPI-ACS).
We have triangulated this burst to a Konus-GBM annulus centered at RA(2000)=3D155.584 (10h 22m 20s) Dec(2000)=3D+3.772 (+3d 46' 21"), with a radius of 49.244 =B1 0.177 deg (3 sigma).
The Konus-WAM annulus is centered at RA(2000)=3D155.807 (10h 23m 14s) Dec=3D+3.771 (+3d 46' 16"), with a radius of 49.210 =B1 0.249 deg (3 sigma).
The Konus-SPI-ACS annulus is centered at RA(2000)=3D156.539 (10h 26m 09s) Dec(2000)=3D-3.361 (-3d 21' 41"), with radius of 56.520 =B1 0.412 deg (3 sigma).
A map is posted at http://www.ioffe.ru/LEA/GRBs/GRB090831_T27393/IPN/ showing the GBM best-fit position (blue star), the MAXI localization circ= le and best-fit position (pink star), and the IPN annuli (solid lines with centers dot-dashed). The shaded area shows the intersection of the Konus-GBM annulus with the 1-deg MAXI localization circle. The maximum distance between points on this overlapping region is 1.47 de= g, at coordinates RA, Dec =3D 143.985 (09h 35m 56s) +51.878 (+51d 52' 40") and 146.304 (09h 45m 13s) +52.229 (+52d 13' 46"). The area of this region is 0.160 sq. deg.
This map will also be posted at http://ssl.berkeley.edu/ipn3/090831A.
The IPN localization may be improved.
GCN Circular #9900
N. Ohmori, K. Noda, E. Sonoda, K. Kono, H. Hayashi, A. Daikyuji, Y. Nishioka, M. Yamauchi (Univ. of Miyazaki), M. Ohno, M. Suzuki, M. Kokubun, T. Takahashi (ISAS/JAXA), K. Yamaoka, S. Sugita (Aoyama Gakuin U.), Y. E. Nakagawa, T. Tamagawa (RIKEN), S. Hong (Nihon U.), N. Vasquez (Tokyo Tech.), Y. Hanabata, T. Uehara, T. Takahashi, Y. Fukazawa (Hiroshima U.), W. Iwakiri, M. Tashiro, Y. Terada, A. Endo, K. Onda, T. Sugasahara (Saitama U.), Y. Urata (NCU), T. Enoto, K. Nakazawa, K. Makishima (Univ. of Tokyo), on behalf of the Suzaku WAM team, report:
The long GRB 090831A (Fermi-GBM trigger 273396998 / 090831317; Arne et al., GCN 9850, MAXI-GSC; Matsuoka et al., GCN 9852) triggered the Suzaku Wide-band All-sky Monitor (WAM) which covers an energy range of 50 keV - 5 MeV at 07:36:36.472 UT (=T0).
The observed light curve shows main three peaks starting at T0s, ending at T0+10s, followed by a long weak tail seen up to T0+48s. The total T90 duration is about 53 seconds.
The fluence in 100 - 1000 keV was 6.8 (-1.8, +1.0)x10-6 erg/cm2. The 1-s peak flux measured from T0+4 s was 1.8 (-0.2, +0.3) photons/cm2/s in the same energy range.
Preliminary result shows that the time-averaged spectrum from T0 s to T0+48 s is well fitted by a power-law with exponential cutoff model:
```
  dN/dE ~  E^{-alpha} * exp(-(2-alpha)*E/Epeak) with
  alpha        0.89(-0.91, +0.65), and
  Epeak        286(-57, +88) keV (chi2/d.o.f. = 93/80).
```
All the quoted errors are at statistical 90% confidence level, in which the systematic uncertainties are not included.
The light curves for this burst is available at: http://www.astro.isas.jaxa.jp/suzaku/HXD-WAM/WAM-GRB/grb/trig/grb_table.html

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Jochen Greiner, last update: 05-Sep-2009 [Disclaimer]