The Gamma-ray Burst Monitor consists of 12 sodium iodide and 2 bismuth germanate scintillation detectors designed to enhance the scientific returns from
Fermi
in the study of
γ
-ray bursts. It has a ...fixed non-paralyzing dead time of 2.604
μ
s for each detected event except for those in the overflow channel of the 12 bit analog to digital converter of each detector, which have a longer dead time of 10.42
μ
s. At very high counting rates the pulse pile-up effect in the detectors leads to spectral distortions and also to additional event losses. These effects are relevant for the spectral analysis of high-flux astrophysical events such as gamma-ray bursts, soft gamma repeaters and solar flares, as well as terrestrial gamma-ray flashes. In this paper we present the results of post-launch tests using engineering detectors and equipment on the ground, along with detailed simulations of these effects at very high counting rates. The simulations enable us to assess the qualitative and quantitative changes in spectral shapes and event losses at different count rates. The observed spectra at various count rates using high intensity radioactive sources such as Cs
137
and Co
60
agree well with the simulated spectra. We also re-test the analytical model developed previously to correct for the pulse pile-up effects in GBM detectors. We also study the expected spectral distortions of specific spectral models commonly used in GRB analysis, including the Band model and power-law models with and without a high-energy cutoff.
The NaI and BGO detectors on the Gamma-ray Burst Monitor (GBM) on Fermi are now being used for long-term monitoring of the hard X-ray/low-energy gamma-ray sky. Using the Earth occultation technique ...as demonstrated previously by the BATSE instrument on the Compton Gamma-Ray Observatory, GBM can be used to produce multiband light curves and spectra for known sources and transient outbursts in the 8 keV to 1 MeV energy range with its NaI detectors and up to 40 MeV with its BGO detectors. Over 85% of the sky is viewed every orbit, and the precession of the Fermi orbit allows the entire sky to be viewed every ~26 days with sensitivity exceeding that of BATSE at energies below ~25 keV and above ~1.5 MeV. We briefly describe the technique and present preliminary results using the NaI detectors after the first two years of observations at energies above 100 keV. Eight sources are detected with a significance greater than 7 Delta *s: the Crab, Cyg X-1, SWIFT J1753.5--0127, 1E 1740-29, Cen A, GRS 1915+105, and the transient sources XTE J1752--223 and GX 339-4. Two of the sources, the Crab and Cyg X-1, have also been detected above 300 keV.
The observations of the exceptionally bright gamma-ray burst (GRB) 130427A by the Large Area Telescope aboard the Fermi Gamma-ray Space Telescope provide constraints on the nature of these unique ...astrophysical sources. GRB 130427A had the largest fluence, highest-energy photon (95 GeV), longest γ-ray duration (20 hours), and one of the largest isotropie energy releases ever observed from a GRB. Temporal and spectral analyses of GRB 130427A challenge the widely accepted model that the nonthermal high-energy emission in the afterglow phase of GRBs is synchrotron emission radiated by electrons accelerated at an external shock.
We present detailed observations of the bright short-hard gamma-ray burst GRB 090510 made with the Gammaray Burst Monitor (GBM) and Large Area Telescope (LAT) on board the Fermi observatory. GRB ...090510 is the first burst detected by the LAT that shows strong evidence for a deviation from a Band spectral fitting function during the prompt emission phase. The time-integrated spectrum is fit by the sum of a Band function with E-peak = 3.9 +/- 0.3 MeV, which is the highest yet measured, and a hard power-law component with photon index -1.62 +/- 0.03 that dominates the emission below approximate to 20 keV and above approximate to 100 MeV. The onset of the high-energy spectral component appears to be delayed by similar to 0.1 s with respect to the onset of a component well fit with a single Band function. A faint GBM pulse and a LAT photon are detected 0.5 s before the main pulse. During the prompt phase, the LAT detected a photon with energy 30.5(-2.6)(+5.8) GeV, the highest ever measured from a short GRB. Observation of this photon sets a minimum bulk outflow Lorentz factor, Gamma greater than or similar to 1200, using simple.. opacity arguments for this GRB at redshift z = 0.903 and a variability timescale on the order of tens of ms for the approximate to 100 keV-few MeV flux. Stricter high confidence estimates imply Gamma greater than or similar to 1000 and still require that the outflows powering short GRBs are at least as highly relativistic as those of long-duration GRBs. Implications of the temporal behavior and power-law shape of the additional component on synchrotron/synchrotron self-Compton, external-shock synchrotron, and hadronic models are considered.
We have performed detailed temporal and time-integrated spectral analysis of 286 bursts from SGR J1550-5418 detected with the Fermi Gamma-ray Burst Monitor (GBM) in 2009 January, resulting in the ...largest uniform sample of temporal and spectral properties of SGR J1550-5418 bursts. We have used the combination of broadband and high time-resolution data provided with GBM to perform statistical studies for the source properties. We determine the durations, emission times, duty cycles, and rise times for all bursts, and find that they are typical of SGR bursts. We explore various models in our spectral analysis, and conclude that the spectra of SGR J1550-5418 bursts in the 8-200 keV band are equally well described by optically thin thermal bremsstrahlung (OTTB), a power law (PL) with an exponential cutoff (Comptonized model), and two blackbody (BB) functions (BB+BB). In the spectral fits with the Comptonized model, we find a mean PL index of -0.92, close to the OTTB index of -1. We show that there is an anti-correlation between the Comptonized E sub(peak) and the burst fluence and average flux. For the BB+BB fits, we find that the fluences and emission areas of the two BB functions are correlated. The low-temperature BB has an emission area comparable to the neutron star surface area, independent of the temperature, while the high-temperature BB has a much smaller area and shows an anti-correlation between emission area and temperature. We compare the properties of these bursts with bursts observed from other SGR sources during extreme activations, and discuss the implications of our results in the context of magnetar burst models.
Here, we report on the observation of the bright, long gamma-ray burst (GRB), GRB 090902B, by the Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on-board the Fermi ...observatory. This was one of the brightest GRBs to have been observed by the LAT, which detected several hundred photons during the prompt phase. With a redshift of z = 1.822, this burst is among the most luminous detected by Fermi. Time-resolved spectral analysis reveals a significant power-law component in the LAT data that is distinct from the usual Band model emission that is seen in the sub-MeV energy range. This power-law component appears to extrapolate from the GeV range to the lowest energies and is more intense than the Band component, both below ~50 keV and above 100 MeV. The Band component undergoes substantial spectral evolution over the entire course of the burst, while the photon index of the power-law component remains constant for most of the prompt phase, then hardens significantly toward the end. After the prompt phase, power-law emission persists in the LAT data as late as 1 ks post-trigger, with its flux declining as t–1.5. The LAT detected a photon with the highest energy so far measured from a GRB, 33.4+2.7 –3.5 GeV. This event arrived 82 s after the GBM trigger and ~50 s after the prompt phase emission had ended in the GBM band. In conclusion, we discuss the implications of these results for models of GRB emission and for constraints on models of the extragalactic background light.
Context. From late September 2017 to January 2018, the Be X-ray binary (BeXB) Swift J0243.6+6124 underwent a giant outburst that was unprecedently bright. The reported X-ray luminosities were so high ...that the system was classified as an ultraluminous X-ray source (ULX). It was also the first BeXB pulsar to show radio jet emission. The source was not only bright in X-rays and radio, but also in the optical and ultraviolet (UV) wavelenghts as well. Aims. In this work, we aim to understand the origin of the observed optical/UV fluxes simultaneous to the X-ray emission. Methods. We studied the optical/UV light curves in comparison with the X-ray fluxes along the outburst. We considered the main mechanisms that can explain the optical/UV emission in X-ray binaries. Due to the tight correlation observed between the optical/UV and X-ray light curves, reprocessing of X-rays seems to be the most plausible explanation. We calculated the timescales of the light curve decays and studied the correlation indexes between the optical and X-ray emission. Finally, we built a physical model that considers the X-ray heating of the surface of the donor star, irradiation of the accretion disk, and emission from a viscously heated accretion disk, so that we could reproduce the observed optical/UV spectral energy distributions (SEDs) along the outburst. In our model, we considered the Be circumstellar disk to be co-planar to the orbit and then we neglected its irradiation in the current model. As an input for the model, we used as incident X-ray luminosities those calculated from the bolometric X-ray fluxes obtained from the spectral fit of the Swift /XRT and BAT observations. Results. The timescales of the exponential decay of the outburst are between two and four times longer for the UV and optical light curves than for the X-ray light curve. The correlation index between the optical/UV and X-ray fluxes varies between optical/UV filters and when different X-ray bands are considered and is larger for the rise than for the decay phase of the outburst for the fluxes at redder wavelengths. The modelling of the SED shows that X-ray heating of the companion star surface is the main mechanism contributing to the UV emission and contributes significantly to the optical emission during the whole outburst. The X-ray irradiation of the accretion disk is necessary to reproduce the optical observed fluxes from MJD 58047 to 58120 and contributes significantly to the UV fluxes close to the peak of the outburst. As a first attempt, the fits yield an increasing value of the outer radius of the accretion disk along the outburst. An alternative interpretation points to variations in the geometry of the inner flow and the fraction of reprocessed X-ray emission during the outburst. On the other hand, variations in the geometry of the Be circumstellar disk could also play a role, but they have not been considered in the current model. Conclusions. Reprocessing of X-rays via the X-ray heating of the Be star surface and as irradiation of the accretion disk is the main mechanism that can reproduce the observed optical/UV emission during the 2017−2018 giant outburst of Swift J0243.6+6124.
One of the scientific objectives of NASA’s Fermi Gamma-ray Space Telescope is the study of Gamma-Ray Bursts (GRBs). The Fermi Gamma-Ray Burst Monitor (GBM) was designed to detect and localize bursts ...for the Fermi mission. By means of an array of 12 NaI(Tl) (8 keV to 1 MeV) and two BGO (0.2 to 40 MeV) scintillation detectors, GBM extends the energy range (20 MeV to > 300 GeV) of Fermi’s main instrument, the Large Area Telescope, into the traditional range of current GRB databases. The physical detector response of the GBM instrument to GRBs is determined with the help of Monte Carlo simulations, which are supported and verified by on-ground individual detector calibration measurements. We present the principal instrument properties, which have been determined as a function of energy and angle, including the channel-energy relation, the energy resolution, the effective area and the spatial homogeneity.
We report on the observation of the bright, long gamma-ray burst, GRB 090926A, by the Gamma-ray Burst Monitor and Large Area Telescope (LAT) instruments on board the Fermi Gamma-ray Space Telescope. ...GRB 090926A shares several features with other bright LAT bursts. In particular, it clearly shows a short spike in the light curve that is present in all detectors that see the burst, and this in turn suggests that there is a common region of emission across the entire Fermi energy range. In addition, while a separate high-energy power-law component has already been observed in other gamma-ray bursts, here we report for the first time the detection with good significance of a high-energy spectral break (or cutoff) in this power-law component around 1.4 GeV in the time-integrated spectrum. If the spectral break is caused by opacity to electron-positron pair production within the source, then this observation allows us to compute the bulk Lorentz factor for the outflow, rather than a lower limit.
GRB110721A was observed by the Fermi Gamma-ray Space Telescope using its two instruments, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). The burst consisted of one major ...emission episode which lasted for ~24.5 s (in the GBM) and had a peak flux of (5.7 + or - 0.2) x 10 super(?5) erg s super(?1) cm super(?2). The time-resolved emission spectrum is best modeled with a combination of a Band function and a blackbody spectrum. The peak energy of the Band component was initially 15 + or - 2 MeV, which is the highest value ever detected in a GRB. This measurement was made possible by combining GBM/BGO data with LAT Low Energy events to achieve continuous 10-100 MeV coverage. The peak energy later decreased as a power law in time with an index of -1.89 + or - 0.10. The temperature of the blackbody component also decreased, starting from ~80 keV, and the decay showed a significant break after ~2 s. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies.
PDF
