Brandt, Søren

Foley, S.

Hanlon, L.

Martin-Carrillo, A.

McBreen, B.

Meehan, S.

Tierney, D.

Topinka, M.

Since its launch in 2002, INTEGRAL has triggered onmore than 78 g –ray bursts in the 20-200 keV energy range with the IBIS/ISGRI instrument. Almost 30% of these bursts occurred within the fully coded field of view of the JEM-X detector (5) which operates in the 3-35 keV energy range. A detailed study of the spectral and temporal evolution of a subset of 7 INTEGRAL g – ray bursts across a wide energy range from 3 keV to 8MeV has been carried out. This GRBsample is characterised by long multi-peaked bursts that are bright in the JEM-X energy range and encompass X-ray rich bursts, X-ray flashes and classical GRBs. We report the detection of X-ray prompt and afterglow emission from GRB 041219A and GRB081003A with JEM-X for the first time. At least two temporal breaks have been identified in the X-ray afterglow light curve of GRB 081003A. These results demonstrate INTEGRAL’s broadband capabilities for the studyof the transition from X-ray prompt to afterglow emission in g –ray bursts

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Spectral and temporal properties of long GRBs detected by INTEGRAL from 3 keV to 8MeVMartin-Carrillo, A.; Topinka, M.; Hanlon, L.; Meehan, S.; Foley, S.; McBreen, B.; Brandt, Søren; Tierney,D.Published in:P o S - Proceedings of SciencePublication date:2010Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Martin-Carrillo, A., Topinka, M., Hanlon, L., Meehan, S., Foley, S., McBreen, B., ... Tierney, D. (2010). Spectraland temporal properties of long GRBs detected by INTEGRAL from 3 keV to 8 MeV. P o S - Proceedings ofScience.PoS(INTEGRAL 2010)043Spectral and temporal properties of long GRBsdetected by INTEGRAL from 3 keV to 8 MeVA. Martin-Carrillo∗1, M. Topinka1, L. Hanlon1, S. Meehan1, S. Foley1,2, B. McBreen1,S. Brandt3, D. Tierney11UCD School of Physics, Dublin, Ireland2Max Planck Institut Garching (MPE)3DTU, DenmarkE-mail: antonio.martin-carrillo@ucd.ieSince its launch in 2002, INTEGRAL has triggered on more than 78 γ–ray bursts in the 20-200 keVenergy range with the IBIS/ISGRI instrument. Almost 30% of these bursts occurred within thefully coded field of view of the JEM-X detector (5◦) which operates in the 3-35 keV energyrange. A detailed study of the spectral and temporal evolution of a subset of 7 INTEGRAL γ–ray bursts across a wide energy range from 3 keV to 8 MeV has been carried out. This GRBsample is characterised by long multi-peaked bursts that are bright in the JEM-X energy rangeand encompass X-ray rich bursts, X-ray flashes and classical GRBs. We report the detection ofX-ray prompt and afterglow emission from GRB 041219A and GRB 081003A with JEM-X forthe first time. At least two temporal breaks have been identified in the X-ray afterglow light curveof GRB 081003A. These results demonstrate INTEGRAL’s broadband capabilities for the studyof the transition from X-ray prompt to afterglow emission in γ–ray bursts.8th INTEGRAL Workshop "The Restless Gamma-ray Universe"- Integral2010,September 27-30, 2010Dublin Ireland∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(INTEGRAL 2010)043Properties of long INTEGRAL GRBs from 3 keV A. Martin-Carrillo1. IntroductionSince its launch in October 2002 INTEGRAL [1] has triggered on more than 78 γ–ray bursts(GRBs) within the IBIS field of view (FOV) [2] (20-200 keV energy range), a rate of∼0.8 GRBs/month.The triggering is done by the INTEGRAL Burst Alert System (IBAS)1 [3], an automatic ground-based system providing a localisation accuracy of ∼ 3 arcminutes. Due to the high sensitivity ofthe IBIS/ISGRI detector layer, INTEGRAL is one of the best observatories for the detection of faintGRBs (peak flux∼0.1 ph cm−2 s−1) [4]. In addition, INTEGRAL has broadband spectral capabilityafforded by IBIS (9◦ × 9◦ fully coded field of view), JEM-X (3–35 keV), with its 5◦ × 5◦ fullycoded field of view [5] and the SPI Germanium detector (20 keV to 8 MeV, 16◦ × 16◦ fully codedfield of view [6].INTEGRAL’s broad energy range provides a window to explore the emission processes whichgive rise to the GRB spectral characteristics. Despite the significant progress that has been made inthe past decade in our understanding of the progenitors, afterglows, host galaxies and distances ofGRBs, fundamental characteristics of the bursts themselves still remain unexplained. For example,it is not yet established how the observed non-thermal spectrum is produced despite major theoreti-cal efforts [7]. The spectral output peaks at a few hundred keV and continues as a∼ -2.2 power-law,consistent with the index expected from Fermi acceleration, extending up to GeV energies in somecases.In the classical optically thin synchrotron shock model the γ–ray emission comes from anisotropic population of shock-accelerated electrons that have a power-law energy distribution abovea characteristic cut-off energy. The spectra predicted by this model are asymptotically brokenpower-laws which are very reminiscent of the Band model (Band et al. 1993). However, in manycases the low-energy photon number index violates the -2/3 limit predicted in the simplest reali-sation of synchrotron emission [8]. Modified synchrotron models, incorporating features such assynchrotron cooling and/or an anisotropic distribution of electrons with small mean pitch anglescan reproduce the global distributions of spectral parameters, but not all aspects of the observations.Taking these characteristics into account, along with new observations by Fermi/LAT of additionalpower-law components extending well into the GeV range [15], it is clear that the emission processis much more complex than ’straightforward’ synchrotron radiation.Further motivation for broadband GRB spectroscopy comes from observations by BATSE of aspectral excess between 5 and 10 keV in∼ 14% of GRBs [9, 10], indicating the presence of an addi-tional, possibly thermal, component to the synchrotron model. There is also evidence for a thermalcomponent in the first stages of GRB emission which accompanies the underlying non-thermalemission [13, 12, 14] and which provides a natural explanation for the known correlation betweenpeak energy and luminosity, since, for a thermal emitter, the luminosity and temperature are corre-lated. The thermal component may be attributable to photospheric emission, while the non-thermalcomponent (spectral index varying from −1.5 to −2.1) at late times in the burst is consistent withthe high-energy power-law of the synchrotron cooling spectrum. The thermal emission componenthas been shown to be a powerful new diagnostic tool for determining the initial size and Lorentzfactor of GRB fireballs in a more model-independent way than other techniques [11] and requiresbroadband observations. The instruments on-board the INTEGRAL and Fermi observatories are1http://ibas.iasf-milano.inaf.it/IBAS_Results.html2PoS(INTEGRAL 2010)043Properties of long INTEGRAL GRBs from 3 keV A. Martin-Carrillocapable of studying simultaneous prompt emission in both the γ–ray and low X-ray (< 10 keV)energy bands.The launch of Swift in 2004 represented a new era for the study of early time X-ray afterglows.For GRBs with prompt emission lasting ∼ 100 s, Swift has been able to observe the transition phasebetween the final stages of the prompt emission and the beginning of the X-ray afterglow [16]. Adip feature has been observed in the X-ray hardness ratios of GRBs at the end of the steep decayphase [17], which can be interpreted as the turn on of the X-ray afterglow in accordance withprevious predictions [18]. The JEM-X data can be used to independently verify the presence of thedip in the hardness ratio seen in some Swift bursts.2. Data Sample and AnalysisTwenty one GRBs detected by IBIS/ISGRI have occurred within the 5◦ FOV of JEM-X todate. Most of these bursts were faint, with peak fluxes ≤ 0.7 ph cm−2 s−1 in the 20-200 keV energyrange. All 21 bursts were detected by JEM-X with peak fluxes (3-35 keV) ranging between 1 and39 ph cm−2 s−1. A limit on the fluence (3-35 keV) of ≥ 2.5× 10−7 erg cm−2 was applied in thisanalysis to restrict the sample to bursts with sufficient statistics for spectroscopy. The final sampleof 7 GRBs covers all three classes of GRBs based on their fluence ratios [19] i.e. classical GRBs,X-ray rich (XRR) bursts and X-ray flashes (XRF). The properties of the bursts in the sample arepresented in Table 1.The data from the three high energy instruments on board INTEGRAL were reduced using anautomatic tool for GRB analysis (grb_analyser) built over the Off-line Science Analysis, OSAv92.This tool also provides all the temporal properties of the bursts and, by an iterative procedure,the best response matrix for each burst. The data from each instrument were then combined forbroadband spectral analysis using XSPECv12.5. The normalisations of the fits to the JEM-X andSPI data were allowed to float as free parameters in the joint spectral analysis. The calibratednormalisation factors provided by the ISDC team3 (with IBIS/ISGRI as 1.0) are 0.95 for JEM-Xand 1.19 for SPI. Due to the low statistics in the SPI detector for these events, the normalisationfactor obtained was 1.5±0.4, slightly greater than the calibrated value. Good agreement was foundbetween the ISDC normalisation factor and the observed value for JEM-X (0.9±0.4).3. ResultsThe broadband spectrum of each GRB is fit with four spectral models: power-law, Band model,blackbody plus power-law and power-law with exponential cut-off. A summary of the GRB prop-erties and best fit spectral results is given in Table 1 and some of the highlight results are presentedin more detail below.2http://www.isdc.unige.ch/integral/analysis#Documentation3http://isdcul3.unige.ch/Soft/download/osa/osa_doc/osa_doc-9.0/osa_cross_cal-1.0.pdf3PoS(INTEGRAL 2010)043Properties of long INTEGRAL GRBs from 3 keV A. Martin-CarrilloTable 1: Summary of properties and best fit spectral results for the 7 GRBs considered in the combined(JEM-X, IBIS and SPI) spectral analysis.‘PF’ is the peak flux in the 20-200 keV energy range; ‘PL’ is thepower-law model, ’Band’ is the Band model and ‘BB+PL’ is the blackbody plus power-law model. α and βrepresent the power-law indexes below and above the break energy.GRBs Class T90 PF (20-200 keV) Best fit α β kT Epeak Red. χ2/dof(sec) ph cm−2 s−1 (keV) (keV)040812 XRF 21 0.5 PL — -2.33±0.85 — — 0.6/66041219A GRB 365 30 Band -1.44±0.01 -2.06±0.05 — 146±11 1.54/212050520 GRB 54 0.98 BB+PL -1.49±0.09 — 22±4 — 0.68/76051105B XRR 13 0.31 PL -1.91±0.06 — — — 0.63/49080613 GRB 20 1.3 PL -1.23±0.6 — — — 0.34/37081003A XRR 22 0.36 BB+PL -1.66±0.11 — 6.4±0.8 — 0.77/44081016 GRB 26 3.87 BB+PL -1.66±0.09 — 18.6±1.7 — 0.88/563.1 GRB 041219ADue to the extreme brightness of this GRB [12], there are substantial gaps in the IBIS/ISGRI datafor most of the burst duration (Fig. 1, left panel). The 3-35 keV GRB lightcurve as seen by JEM-Xis shown in Fig. 1 (right panel). The hardness ratio (Fig. 1, lower left) between the IBIS/ISGRIsoft band (20-50 keV) and its hard band (50-200 keV) shows the typical hard to soft evolutionwith additional hardening at the onset of each pulse. Hardness-intensity tracking is also seen inthe main pulses at T0+300 s and T0+385 s. A soft flare after the precursor, beginning at T0+100 s(Fig. 1, right panel) is easily seen in the JEM-X lightcurve and is also observed below 50 keV byIBIS/ISGRI. The overall decline in the hardness ratio between the precursor and the flare indicatesa possible connection between them. The increased hardness at the beginning of the major pulse atT0+250 s, suggests that this is a separate flaring episode after a 100 s quiescent phase.Due to the IBIS/ISGRI data gaps, only JEM-X and SPI are used for the broadband spectralanalysis of the whole burst. As shown in Table 1, the reduced χ2 for the best fit is significantlygreater than 1, most probably due to the significant spectral variability during the burst (Fig. 1).However, the burst is sufficiently bright that the spectrum may also have been affected by instru-mental systematics. Although the Band model is statistically a slightly better fit than the BB+PLmodel, the high energy tail seen in the multi-instrument time integrated spectrum is better fit by theBB+PL model.3.2 GRB 081016GRB 081016 was triggered by IBIS/ISGRI [20] as two bright pulses separated by less than20 s (Fig. 2, left panel). The two pulses have almost equal amplitude between 50 and 200 keV,with the second pulse becoming more pronounced towards lower energies, indicative of significantsoftening. The first pulse is not detected below 10 keV.The broadband multi-instrument spectrum over the T90 interval is shown in Fig. 2 (right panel).The best fit model (Table 1) shows a black-body component (kT=18.6±1.7 keV) with a high energypower-law tail.3.3 X-ray afterglow emissionA search for X-ray (3-35 keV) afterglow emission (where burst duration is defined by the T904PoS(INTEGRAL 2010)043Properties of long INTEGRAL GRBs from 3 keV A. Martin-CarrilloFigure 1: Left: GRB 041219A IBIS/ISGRI light curve in the 20-200 keV energy range. Data gaps in thebrightest part of the GRB are caused by saturation of the detector. The standard analysis cuts the data atT0+400 s although the pointing direction did not change until T0+550 s. The dashed vertical lines show the‘precursor’ and the ‘soft flare’ regions. Right: JEM-X lightcurve of GRB 041219A in the 3-35 keV energyrange. The last 150 s of JEM-X data were recovered from the housekeeping data. The total and T90 durationsmeasured in the γ-ray band are represented by the solid and dashed lines respectively.Figure 2: Left: Energy resolved light curve of GRB 081016 from JEM-X (top), IBIS/ISGRI (middle) andSPI (bottom). Right: The broadband spectrum of GRB 081016 from 3 keV to 4 MeV using data from JEM-X(diamonds), IBIS/ISGRI (squares) and SPI (circles). The solid line represents the best fit model (BB+PL) tothe data. Dashed lines shows the black-body (curved) and power-law models separately.duration in the γ–ray band) has been carried out for several of the 21 GRBs detected by JEM-X. Arapid decline in the X-ray light curve is expected after the prompt emission, based on the canonicalX-ray afterglow behaviour [21].In the case of GRB 041219A, X-ray emission is detected at the 3.5σ level, beginning at 609 safter the trigger time, for an integration time of 250 s. A power-law model gives the best fit to thespectrum in this interval, with α = -2.2±0.7 and a flux of 2.9× 10−10 ergs cm−2 s−1 in the 3-35 keVenergy range. Most of the detected emission comes from the soft band (3-10 keV), with 20% of themeasured flux detected in the 10-35 keV band.Post-burst X-ray emission is also detected in the X-ray rich burst GRB 081003A which has aT90= 22 s. From T0+22 s to T0+70 s the X-ray emission decays as a power-law of slope -0.98±0.05(Fig. 3). Thereafter the decay slope steepens to a slope of -3.88±0.11 until T0+250 s. These results5PoS(INTEGRAL 2010)043Properties of long INTEGRAL GRBs from 3 keV A. Martin-Carrilloshow the first detection of an X-ray afterglow by INTEGRAL.Later observations (T0+104 s) by Swift indicate that the X-ray light curve must flatten outsometime after T0+250 s. A lower limit on the slope of -0.80±0.14 is derived, assuming the flat-tening starts at T0+250 s. Further analysis of the JEM-X data is ongoing.Figure 3: The JEM-X light curve of GRB 081003A in the 3-10 keV band. The vertical dashed line marksthe end of the T90 measured in the 20-200 keV energy range. The later Swift/XRT data at ∼ 104 s are alsoshown (grey diamonds).AcknowledgementsThis study is based on observations with INTEGRAL, an ESA project with instruments and sci-ence data centre funded by ESA member states (especially the PI countries: Denmark, France, Ger-many, Italy, Switzerland, Spain), Poland and with the participation of Russia and the USA. AMCand LH acknowledge financial support from Science Foundation Ireland grant 09/RFP/AST/2400.References[1] Winkler, C.; Courvoisier, G.; Di Cocco, G. et al. 2003, A&A, 411, L1[2] Ubertini, P.; Lebrun F.; Di Cocco, G. et al. 2003, A&A, 411, L131[3] Mereghetti, S.; Götz, D.; Borkowski, J. et al. 2003, A&A, 411, L291[4] Foley, S.; McGlynn, S.; Hanlon, L. et al. 2008 A&A 484, 143[5] Lund, N.; Budtz-Jorgensen, C.; Westergaard, N. 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Spectral and temporal properties of long GRBs detected by INTEGRAL from 3 keV to 8 MeV

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Spectral and temporal properties of long GRBs detected by INTEGRAL from 3 keV to 8 MeV

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