In the first two years since the launch of the Fermi Observatory, the
Gamma-ray Burst Monitor (GBM) has detected over 500 Gamma-Ray Bursts (GRBs), of
which 18 were confidently detected by the Large Area Telescope (LAT) above 100
MeV. Besides GRBs, GBM has triggered on other transient sources, such as Soft
Gamma Repeaters (SGRs), Terrestrial Gamma-ray Flashes (TGFs) and solar flares.
Here we present the science highlights of the GBM observations.Comment: 4 pages, 1 figure, Proceedings of the 8th Workshop on Science with
  the New Generation of High Energy Gamma-ray Experiments (SciNeGHE 2010),
  Nuovo Cimento C, in pres

BISSALDI E.

on behalf of the Fermi GBM Collaboration

English

arXiv

In the first two years since the launch of the Fermi Observatory, the Gamma-ray Burst Monitor (GBM) has detected over 500 Gamma-Ray Bursts (GRBs), of which 18 were confidently detected by the Large Area Telescope (LAT) above 100 MeV. Besides GRBs, GBM has triggered on other transient sources, such as Soft Gamma Repeaters (SGRs), Terrestrial Gamma-ray Flashes (TGFs) and solar flares. Here we present the science highlights of the GBM observations

Bissaldi, E.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10846-5Colloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011The Fermi Gamma-ray Burst Monitor:Results from the first two yearsE. Bissaldi on behalf of the Fermi/GBM CollaborationMax-Planck-Institut fu¨r extraterrestrische Physik - Giessenbachstr. 185748 Garching, GermanyInstitute of Astro and Particle Physics - Technikerstr. 25, 6020 Innsbruck, Austria(ricevuto il 25 Febbraio 2011; pubblicato online il 7 Aprile 2011)Summary. — In the first two years since the launch of the Fermi Observatory,the Gamma-ray Burst Monitor (GBM) has detected over 500 Gamma-Ray Bursts(GRBs), of which 18 were confidently detected by the Large Area Telescope (LAT)above 100MeV. Besides GRBs, GBM has triggered on other transient sources, suchas Soft Gamma Repeaters (SGRs), Terrestrial Gamma-ray Flashes (TGFs) and solarflares. Here we present the science highlights of the GBM observations.PACS 95.55.Ka – X- and γ-ray telescopes and instrumentation.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.1. – IntroductionThe Fermi Gamma-ray Space Telescope, which was successfully launched on June 11,2008, is an international and multi-agency space observatory. The payload comprises twoscience instruments, the Large Area Telescope [1], a pair conversion telescope operating inthe energy range between 20MeV and 300GeV and the Gamma-Ray Burst Monitor [2],which extends the Fermi energy range to lower energies (from 8 keV to 40MeV). Theprimary role of the GBM is to augment the science return from Fermi in the study ofGRBs by discovering transient events within a larger field of view (FoV) and performingtime-resolved spectroscopy of the measured burst emission.2. – The GBM detectorsThe GBM flight hardware comprises 12 thallium activated sodium iodide (NaI(Tl),hereafter NaI) scintillation detectors and two bismuth germanate (BGO) scintillationdetectors. The individual NaI detectors (∼ 8–1000 keV) are mounted around the space-craft and their axes are oriented such that the positions of GRBs can be derived from themeasured relative counting rates. With their energy range extending between ∼ 0.2 and∼ 40MeV, two BGO detectors provide the overlap in energy with the LAT instrumentc© Societa` Italiana di Fisica 910 E. BISSALDI on behalf of the FERMI/GBM COLLABORATIONand are crucial for in-flight inter-instrument calibration [3]. They are mounted on oppo-site sides of the Fermi spacecraft covering a net ∼ 8 sr FoV.A burst trigger occurs when the flight software detects an increase in the count ratesof two or more detectors above an adjustable threshold specified in units of the standarddeviation of the background rate. 28 different trigger algorithms operate simultaneously,each with a distinct threshold. The trigger algorithms currently implemented includefour energy ranges: 50–300 keV, which is the standard GRB trigger range, 25–50 keV toincrease sensitivity for SGRs and GRBs with soft spectra, > 100 keV, and > 300 keVto increase sensitivity for hard GRBs and TGFs. When a burst trigger occurs, onboardsoftware determines a direction to the source using the relative rates in the 12 NaIdetectors. These rates are compared to a table of calculated relative rates. The locationwith the best chi-squared fit is converted into right ascension and declination usingspacecraft attitude information and transmitted to the ground. Improved locations areautomatically computed on the ground in near real-time by the Burst Alert Processor(BAP) and later interactively by the GBM Burst Advocate.The Fermi Observatory incorporates the capability to autonomously alter the observ-ing plan to slew to and maintain pointing at a GRB for a set period of time, nominally5 hours, subject to Earth limb constraints. This allows the LAT to observe delayedhigh-energy emission, as has been previously observed by instruments on the ComptonGamma-Ray Observatory [4]. Either the GBM or the LAT can generate an AutonomousRepoint Request (ARR) to point at a GRB. A request originating from GBM is trans-mitted to the LAT. The LAT either revises the recommendation or forwards the requestto the spacecraft. The LAT software may, for example, provide a better location to thespacecraft, or cancel the request due to operational constraints. The GBM flight softwarespecifies different repoint criteria depending on whether or not the burst is already withinthe LAT FoV, defined as within 70o of the +Z axis. An ARR is generated by GBM ifthe trigger exceeds a specified threshold for peak flux or fluence. These thresholds arereduced if the burst spectrum exceeds a specified hardness ratio.3. – Two years scientific results3.1. GRB observations. – During its two years of operations since trigger enabling(July 13th, 2009-September 6th, 2010), GBM detected ∼ 540 GRBs, with ∼ 270 burstsplaced inside the LAT FoV. In 45 cases, an ARR was positively issued by GBM torepoint the LAT and follow particularly bright events. Morover, 18 GRBs were de-tected at energies above 100MeV. The most energetic events include GRB 080916C [5],GRB 090510 [6], GRB 090902B [7], and GRB 090926A [8]. In all four cases, the LAT de-tected photons with energies > 10GeV, with GRB 090902B being the record holder with33GeV, the highest photon energy reported so far for a GRB. Moreover, GRB 090510was the first short GRB showing emission up to 31GeV. For this burst, no evidencewas found for the violation of Lorentz invariance, thus disfavoring quantum-gravity the-ories in which the quantum nature of space-time on a very small scale alters the speedof light in a way that depends linearly on photon energy. From the spectral point ofview, joint GBM-LAT observations uncovered the presence of two spectral componentsin GRB 090902B. For this burst, a power law component dominates the Band componentat both low (< 50 keV) and high (> 100MeV) energies and is significant within just theGBM data. Furthermore, GRB 090926A showed evidence for a high energy cutoff.Recently, [9] presented an extremely detailed time-resolved spectroscopy at timescalesas short as 2ms for the three brightest short GRBs observed with GBM (GRB 090227B,THE FERMI GAMMA-RAY BURST MONITOR: RESULTS FROM THE FIRST TWO YEARS 11Fig. 1. – Distribution of spectral hardness vs. duration for a portion of the GBM bursts.GRB 090228 and GRB 090510). The time-integrated spectra of the events deviate fromthe Band function, indicating the existence of an additional spectral component. Thetime-integrated Epeak values exceed 2MeV for two of the bursts, and are well above thevalues observed in the brightest long GRBs. Their Epeak values and their low-energypower law indices confirm that short GRBs are harder than long ones. We find thatshort GRBs are very similar to long ones, but with light curves contracted in time andwith harder spectra stretched towards higher energies.Regarding common Fermi-Swift [10] GRB observations, the overlapping FoVs of GBMand of the Burst Alert Telescope (BAT) onboard Swift contributed to ∼ 40 commondetections/y and resulted in 1 common GBM/LAT/BAT detection so far, namely in thecase of the long-lived emission (from UV to GeV) of GRB 090510 [11]. The finer LATlocalizations errors (< 0.4o) resulted in Swift follow-up of several Fermi bursts, providing8 redshifts measurements (by ground-based observatories) so far.3.2. GRB catalog . – Catalogs covering the first two years of GBM GRBs are cur-rently being produced. The main catalog [12] summarizes basic parameters for alltriggered GRBs, including sky location, fluence in two energy ranges (50–300 keV and10–1000 keV), peak flux for the same two energy ranges and three timscales (64ms, 256msand 1024ms) and duration (T50 and T90 in the 50–300 keV range). The spectroscopycatalog [13] includes spectral fits using several standard functions for all sufficiently brightGRBs. The standard functions include power law, power law with exponential cut-off,Band function, and smoothly broken power law. For each burst two sets of spectral fitsare performed: a 3.5 σ signal-to-background selection for the duration of the burst (flu-ence spectra), and a peak count rate selection (peak flux spectra). The peak count rateinterval is 1 s for long bursts and 64ms for short bursts, based on the burst T90 duration.Figure 1 shows the distribution of spectral hardness versus duration for a portion of theGBM bursts. Although the results are preliminary, the well-known tendency for shortbursts to have harder spectra is clearly shown.3.3. Non-GRB science. – Apart from GRBs, GBM triggered 170 times on SGR out-bursts, mostly on soft, short trigger algorithms. Those triggers originated from fourdifferent active SGRs. Of those four, one is an already known source (SGR J1806–20),two are new sources discovered with Swift (SGR J0501+4516 and SGR J1550–5418) andone was discovered with GBM (SGR J0418+5729) [14]. An interesting finding by [15] was12 E. BISSALDI on behalf of the FERMI/GBM COLLABORATIONthe identification of a ∼ 150-s-long enhanced persistent emission during the January 2009outburst of SGR J1550–5418, which exhibited intriguing timing and spectral properties.The area of the blackbody-emitting region which could be estimated (≈ 0.046 (D/5 kpc)2km2, i.e. roughly a few ×105 of the neutron star area) is the smallest “hot spot” evermeasured for a magnetar and most likely corresponds to the size of magnetically confinedplasma near the neutron star surface.GBM also triggered 93 times from TGF signals, all on hard, short trigger algorithms(see also [16]). First observations of a smaller sample of TGFs and their properties arereported in [17]. The temporal properties of a considerably larger sample of TGFs ob-served with GBM can be found in [18], who discuss several distinct categories of TGFsmainly identified by their time profiles. Moreover, [19] recently presented a search forcorrelations between TGFs detected by GBM and lightning strokes measured using theWorld Wide Lightning Location Network (WWLLN) [20]. In addition to gamma-rayTGFs, GBM has observed several TGFs by the propagation of charged particles alonggeomagnetic field lines. Strong 511 keV annihilation lines have been observed, demon-strating that both electrons and positrons are present in the particle beams [21].Finally, GBM triggered 33 times on solar flares. Several other triggers were causedby well-known gamma-ray emitters, such as Cyg X-1, or by accidental particle events.Besides triggered transient sources, which are detected by the on-board trigger algorithm,GBM can be used to study hard X-ray pulsars with periods greater than a few seconds.These are monitored using Fourier transforms and epoch folding [22]. Moreover, GBMbackground data is extremely useful for a number of other studies, enabling a wide rangeof guest investigations. These data are currently used to monitor variable X-ray sourcesusing the Earth occultation technique [23].REFERENCES[1] Atwood W. B. et al., Astrophys. J., 697 (2009) 1071.[2] Meegan C. A. et al., Astrophys. J., 702 (2009) 791.[3] Bissaldi E. et al., Exp. Astron., 24 (2009) 47.[4] Hurley K. et al., Nature, 372 (1994) 652.[5] Abdo A. A. et al., Science, 323 (2009) 1688.[6] Abdo A. A. et al., Nature, 462 (2009) 331.[7] Abdo A. A. et al., Astrophys. J., 707 (2009c) 580.[8] Abdo A. A. et al., Astrophys. J., in preparation.[9] Guiriec S. et al., Astrophys. J., 725 (2010) 225.[10] Gehrels N. et al., Astrophys. J., 611 (2004) 1005.[11] De Pasquale M. et al., Astrophys. J., 709 (2010) 146.[12] Paciesas W. S. et al., Astrophys. J., in preparation.[13] Goldstein A. et al., Astrophys. J., in preparation.[14] Van der Horst A. et al., Astrophys. J. Lett., 711 (2010) L1.[15] Kaneko Y. et al., Astrophys. J., 710 (2010) 1335.[16] Wilson-Hodge C. et al., these proceedings.[17] Briggs M. S. et al., J. Geophys. Res., 115 (2010) A07323.[18] Fishman G. J. et al., submitted to J. Geophys. Res.[19] Connaughton V. et al., J. Geophys. Res., 115 (2010) A12307.[20] Rodger C. J. et al., AIP Conf. Proc., 1118 (2009) 15.[21] Briggs M. S. et al., to be published in Geophys. Res. Lett.[22] Finger M. H. et al., Fermi Symposium, eConf Proc., arXiv:0912.3847 (2010).[23] Wilson-Hodge C. A. et al., Fermi Symposium, eConf Proc., arXiv:0912.3831 (2010).

The Fermi Gamma-ray Burst Monitor: Results from the first two years

In the first two years since the launch of the Fermi Observatory, the Gamma-ray Burst Monitor (GBM) has detected over 500 Gamma-Ray Bursts (GRBs), of which 18 were confidently detected by the Large Area Telescope (LAT) above 100 MeV. Besides GRBs, GBM has triggered on other transient sources, such as Soft Gamma Repeaters (SGRs), Terrestrial Gamma-ray Flashes (TGFs) and solar flares. Here we present the science highlights of the GBM observations. © Società Italiana di Fisica

The Fermi Gamma-ray Burst Monitor: Results from the first two years

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