Some recent studies exposed rather strong statistical evidence of in-vacuo-dispersion-like spectral lags for gamma-ray bursts (GRBs), a linear correlation between time of observation and energy of GRB particles. Those results focused on testing in-vacuo dispersion for the most energetic GRB particles, and in particular only included photons with energy at emission greater than 40 GeV. We here extend the window of the statistical analysis down to 5 GeV and find results that are consistent with what had been previously noticed at higher energies.
Over the last 15 years there has been considerable interest in the possibility of quantum-gravity-induced in-vacuo dispersion, the possibility that spacetime itself might behave essentially like a dispersive medium for particle propagation. Two very recent studies have exposed what might be in-vacuo dispersion features for GRB (gamma-ray-burst) neutrinos of energy in the range of 100 TeV and for GRB photons with energy in the range of 10 GeV. We here show that these two features are roughly compatible with a description such that the same effects apply over 4 orders of magnitude in energy. We …
It was well known that most of gamma-ray bursts (GRBs) are dominated by positive spectral lags, while a small fraction of GRBs show negative lags. However, Wei et al. firstly identified a well-defined transition from positive lags to negative lags in GRB 160625B, and then got robust limits on possible violation of Lorentz Invariance (LIV) based on the observation. Recently, such a transition has been found in three different emission episodes in \thisgrb by Gunapati et al., which provides us a great opportunity to investigate whether the transition results from LIV-induced observed spectral la…
Lorentz invariance violation (LIV) can manifest itself by an energy-dependent vacuum dispersion of light, which leads to arrival-time differences of photons with different energies originating from the same astronomical source. The spectral lags of gamma-ray bursts (GRBs) have been widely used to investigate the possible LIV effect. However, all current investigations used lags extracted in the observer frame only. In this work, we present, for the first time, an analysis of the LIV effect and its redshift dependence in the cosmological rest frame. Using a sample of 56 GRBs with known redshift…
A sample of 427 gamma-ray bursts from a database (February 2002 - April 2008) of the RHESSI satellite is analyzed statistically. The spectral lags and peak-count rates, which have been calculated for the first time in this paper, are studied completing an earlier analysis of durations and hardness ratios. The analysis of the RHESSI database has already inferred the existence of a third group with intermediate duration, apart from the so-called short and long groups. First aim of this article is to discuss the properties of these intermediate-duration bursts in terms of peak-count rates and spe…
Primordial Black Holes (PBHs), which may have been created in the early Universe, are predicted to be detectable by their Hawking radiation. PBHs with an initial mass of 5.0 * 10^14 g should be expiring today with a burst of high energy particles. Evaporating PBHs in the solar neighborhood are candidate Gamma-Ray Bursts (GRBs) progenitors. We propose spectral lag, which is the temporal delay between the high energy photon pulse and the low energy photon pulse, as a possible method to detect PBH evaporation events with the Fermi Gamma-ray Space Telescope Observatory.
We present a new technique to calculate the spectral lags of gamma-ray bursts (GRBs). Unlike previous processing methods, we first smooth the light curves of gamma-ray bursts in high and low energy bands using the "Loess" filter, then, we directly define the spectral lags as such to maximize the cross-correlation function (CCF) between two smoothed light curves. This method is suitable for various shapes of CCF; it effectively avoids the errors caused by manual selections for the fitting function and fitting interval. Using the method, we have carefully measured the spectral lags of individual…
Time lags as proxy of spectral evolution in gamma-ray bursts
Università di Palermo（巴勒莫大学）；INAF - Osservatorio Astronomica di Brera（意大利国家天体物理研究所布里拉天文台）；Sorbonne Université（索邦大学）；Institut d’Astrophysique de Paris（巴黎天体物理学研究所）；Institut Universitaire de France（法国高等研究院）；INFN, Sezione di Trieste（意大利国家核物理研究所的里雅斯特分部）；INFN – Sezione di Milano–Bicocca（意大利国家核物理研究所米兰-比可卡分部）
Assuming an intrinsic `Band' shape spectrum and an intrinsic energy-independent emission profile we have investigated the connection between the evolution of the rest-frame spectral parameters and the spectral lags measured in gamma-ray burst (GRB) pulses by using a pulse model. We first focus our attention on the evolution of the peak energy, $E_{0,p}$, and neglect the effect of the curvature effect. It is found that the evolution of $E_{0,p}$ alone can produce the observed lags. When $E_{0,p}$ varies from hard to soft only the positive lags can be observed. The negative lags would occur in t…
Spectral lag, the time difference between the arrival of high-energy and low-energy photons, is a common feature in Gamma-ray Bursts (GRBs). Norris et al. 2000 reported a correlation between the spectral lag and the isotropic peak luminosity of GRBs based on a limited sample. More recently, a number of authors have provided further support for this correlation using arbitrary energy bands of various instruments. In this paper we report on a systematic extraction of spectral lags based on the largest Swift sample to date of 31 GRBs with measured redshifts. We extracted the spectral lags for all…
The Fermi satellite has been reporting the detailed temporal properties of gamma-ray bursts (GRBs) in an extremely broad spectral range, 8 keV - 300 GeV, in particular, the unexpected delays of the GeV emission onsets behind the MeV emission of some GRBs. We focus on GRB 080916C, one of the Fermi-LAT GRBs for which the data of the delayed high-energy emission are quite extensive, and we show that the behavior of the high-energy emission of this burst can be explained by a model in which the prompt emission consists of two components: one is the MeV component due to the synchrotron-self-Compton…
Violations of Lorentz invariance can lead to an energy-dependent vacuum dispersion of light, which results in arrival-time differences of photons arising with different energies from a given transient source. In this work, direction-dependent dispersion constraints are obtained on nonbirefringent Lorentz-violating effects, using the observed spectral lags of the gamma-ray burst GRB 160625B. This burst has unusually large high-energy photon statistics, so we can obtain constraints from the true spectral time lags of bunches of high-energy photons rather than from the rough time lag of a single …
We investigated the rest frame spectral lags of two complete samples of bright long (50) and short (6) gamma-ray bursts (GRB) detected by Swift. We analysed the Swift/BAT data through a discrete cross-correlation function (CCF) fitted with an asymmetric Gaussian function to estimate the lag and the associated uncertainty. We find that half of the long GRBs have a positive lag and half a lag consistent with zero. All short GRBs have lags consistent with zero. The distributions of the spectral lags for short and long GRBs have different average values. Limited by the small number of short GRBs, …
We process the Fermi LAT data on GRB130427A using the Fermi Science Tools, and we summarize some of the key facts that render this observation truly remarkable, especially concerning the quality of information on high-energy emission by GRBs. We then exploit this richness for a search of spectral lags, of the type that has been recently of interest for its relevance in quantum-spacetime research. We do find some evidence of systematic soft spectral lags: when confining the analysis to photons of energies greater than 5 GeV there is an early hard development of minibursts within this long burst…
Pulse-resolved Classification and Characteristics of Long-duration GRBs with \emph{Swift}-BAT Data.I. Precursors versus Main Bursts
Institute of Fundamental Physics and Quantum Technology, Ningbo University（宁波大学基础物理与量子技术研究院）；School of Physical Science and Technology, Ningbo University（宁波大学物理科学与技术学院）；Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）；School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）
The Fermi Gamma-ray Space Telescope recently detected the most energetic gamma-ray burst so far, GRB 080916C, and reported its detailed temporal properties in an extremely broad spectral range: (i) the time-resolved spectra are well described by broken power-law forms over the energy range of $10 {\rm keV}-10$ GeV, (ii) the high-energy emission (at $ε> 100$ MeV) is delayed by $\approx 5$s with respect to the $ε\lesssim 1$ MeV emission, and (iii) the emission onset times shift towards later times in the higher energy bands. We show that this behavior of the high-energy emission can be explained…
Relations linking the temporal or/and spectral properties of the prompt emission of gamma-ray bursts (hereafter GRBs) to the absolute luminosity are of great importance as they both constrain the radiation mechanisms and represent potential distance indicators. Here we discuss two such relations: the lag-luminosity relation and the newly discovered duration-luminosity relation of GRB pulses. We aim to extend our previous work on the origin of spectral lags, using the duration-luminosity relation recently discovered by Hakkila et al. to connect lags and luminosity. We also present a way to test…
It is postulated in Einstein's relativity that the speed of light in vacuum is a constant for all observers. However, the effect of quantum gravity could bring an energy dependence of light speed. Even a tiny speed variation, when amplified by the cosmological distance, may be revealed by the observed time lags between photons with different energies from astrophysical sources. From the newly detected long gamma ray burst GRB~160509A, we find evidence to support the prediction for a linear form modification of light speed in cosmological space.
The intermediate-duration gamma-ray bursts (GRBs) identified in the data of the RHESSI satellite are investigated with respect to their spectral lags, peak count rates, redshifts, supernova observations, and star formation rates of their host galaxies. Standard statistical tests like Kolmogorov-Smirnov and Student t-test are used. It is discussed whether these bursts belong to the group of so-called short or long GRBs, or if they significantly differ from both groups.
Possible violations of Lorentz invariance (LIV) have been investigated for a long time using the observed spectral lags of gamma-ray bursts (GRBs). However, these generally have relied on using a single photon in the highest energy range. Furthermore, the search for LIV lags has been hindered by our ignorance concerning the intrinsic time lag in different energy bands. GRB 160625B, the only burst so far with a well-defined transition from $positive$ lags to $negative$ lags provides a unique opportunity to put new constraints on LIV. Using multi-photon energy bands we consider the contributions…
Despite decades of investigations, the physical mechanism that powers the bright prompt $γ$-ray emission from gamma-ray bursts (GRBs) is still not identified. One important observational clue that remains not properly interpreted so far is the existence of time lags of broad light curve pulses in different energy bands, named "spectral lags". Here we show that the traditional view invoking the high-latitude emission "curvature effect" of a relativistic jet cannot account for spectral lags. Rather, the observed spectral lags demand the sweep of a spectral peak across the observing energy band i…
The prompt emission phase of gamma-ray bursts (GRBs) exhibits two distinct patterns of the peak-energy ($E_p$) evolution, i.e., time-resolved spectral analyses of $νF_ν$ spectra of broad pulses reveal (1) "hard-to-soft" and (2) "flux-tracking" patterns of $E_p$ evolution in time, the physical origin of which still remains not well understood. We show here that these two patterns can be successfully reproduced within a simple physical model invoking synchrotron radiation in a bulk-accelerating emission region. We show further that the evolution patterns of the peak energy have, in fact, direct …
We have investigated the empirical lag-luminosity relation in the Gamma-ray Burst (GRB) source-frame. We selected two energy bands (100-200 keV and 300-400 keV) in the GRB source-frame, which after redshift correction, lie in the observer-frame energy range of the Swift Burst Alert Telescope (BAT). The spectral lags between these energy channels are then presented as a function of the isotropic peak luminosity of the GRBs in the sample.
Models of quantum gravity suggest that the vacuum should be regarded as a medium with quantum structure that may have non-trivial effects on photon propagation, including the violation of Lorentz invariance. Fermi Large Area Telescope (LAT) observations of gamma-ray bursts (GRBs) are sensitive probes of Lorentz invariance, via studies of energy-dependent timing shifts in their rapidly-varying photon emissions. In this paper we analyze the Fermi-LAT measurements of high-energy gamma rays from GRBs with known redshifts, allowing for the possibility of energy-dependent variations in emission time…
Gamma-ray bursts are still not fully understood events. However, their exploration could provide a useful tool for a better understanding of the early Universe because they belong to the most distant and violent objects that astronomers know. This thesis tries to bring more information about a so-called group of intermediate-duration bursts claimed by different authors employing different data samples. Firstly, duration and spectral hardness properties of bursts from the Reuven Ramaty High-Energy Solar Spectroscopic Imager are statistically analysed. The obtained results bring a suspicion that…
In this catalog, we present the results of a systematic study of 295 short gamma-ray bursts (GRBs) detected by Konus-Wind (KW) from 1994 to 2010. From the temporal and spectral analyses of the sample, we provide the burst durations, the spectral lags, the results of spectral fits with three model functions, the total energy fluences and the peak energy fluxes of the bursts. We discuss evidence found for an additional power-law spectral component and the presence of extended emission in a fraction of the KW short GRBs. Finally, we consider the results obtained in the context of the Type I (merg…
The effect of quantum gravity can bring a tiny light speed variation which is detectable through energetic photons propagating from gamma ray bursts (GRBs) to an observer such as the space observatory. Through an analysis of the energetic photon data of the GRBs observed by the Fermi Gamma-ray Space Telescope (FGST), we reveal a surprising regularity of the observed time lags between photons of different energies with respect to the Lorentz violation factor due to the light speed energy dependence. Such regularity suggests a linear form correction of the light speed $v(E)=c(1-E/E_{\rm LV})$, w…
We have investigated temporal and spectral properties of a large sample of thermonuclear bursts with oscillations from eight different sources with spin frequencies varying from 270 to 620 Hz. For our sample we chose those bursts, for which the oscillation is sufficiently strong and of relatively long duration. The emission from the hot-spot that is formed during a thermonuclear burst is modulated by several physical processes and the burst oscillation profiles unavoidably carry signatures of these. In order to probe these mechanisms, we examined the amplitude and phase lags of the burst oscil…
Some Quantum Gravity models predict a violation of Lorentz invariance. Namely, the velocity of photons in vacuum could depend on their energies. One possibility for Lorentz Invariance Violation (LIV) searches is to look for energy-dependent delays in the arrival time of very high energy (VHE) photons coming from distant sources such as TeV emitting Active Galactic Nuclei (AGN), mainly blazars. Up to now, observations of flaring AGN have only provided upper limits on LIV energy scale since no significant time-delay was found and confirmed. However, AGN are not perfect sources for such LIV searc…
Cygnus X-1 is a Galactic black-hole binary and a microquasar. Its X-ray emission originates most likely in the accretion flow, while the radio emission is from a magnetized jet. We study long-term cross-correlations between its X-ray and radio fluxes, searching for lags of the peak of the correlation. We use soft X-ray data from the All Sky Monitor on board of Rossi X-ray Timing Explorer, hard X-ray data from the Burst Alert Telescope on board of Neil Gehrels Swift, and 15 GHz radio monitoring data from the Ryle and AMI telescopes, which light curves cover all of the spectral states of Cyg X-1…
A number of works have claimed detections of a turn-over in the spectral lag data for individual Gamma-Ray Bursts (GRBs), caused by an energy-dependent speed of light, which could be a possible manifestation of Lorentz invariance violation (LIV). Here, we stack the spectral lag data from a total of 37 GRBs (with a total of 91 measurements), to verify if the combined data is consistent with a unified model consisting of intrinsic astrophysical emission, along with another contribution due to LIV. We then carry out Bayesian model comparison to ascertain if this combined spectral lag data shows a…
Spectral lag, which is defined as the difference in time of arrival of high and low energy photons, is a common feature in Gamma-ray Bursts (GRBs). Previous investigations have shown a correlation between this lag and the isotropic peak luminosity for long duration bursts. However, most of the previous investigations used lags extracted in the observer-frame only. In this work (based on a sample of 43 Swift long GRBs with known redshifts), we present an analysis of the lag-luminosity relation in the GRB source-frame. Our analysis indicates a higher degree of correlation -0.82 +/- 0.05 (chance …
We analyze the MeV/GeV emission from four bright Gamma-Ray Bursts (GRBs) observed by the Fermi-Large Area Telescope to produce robust, stringent constraints on a dependence of the speed of light in vacuo on the photon energy (vacuum dispersion), a form of Lorentz invariance violation (LIV) allowed by some Quantum Gravity (QG) theories. First, we use three different and complementary techniques to constrain the total degree of dispersion observed in the data. Additionally, using a maximally conservative set of assumptions on possible source-intrinsic spectral-evolution effects, we constrain any…
The spectral evolution and spectral lag behavior of 92 bright pulses from 84 gamma-ray bursts (GRBs) observed by the Fermi GBM telescope are studied. These pulses can be classified into hard-to-soft pulses (H2S, 64/92), H2S-dominated-tracking pulses (21/92), and other tracking pulses (7/92). We focus on the relationship between spectral evolution and spectral lags of H2S and H2S-dominated-tracking pulses. %in hard-to-soft pulses (H2S, 64/92) and H2S-dominating-tracking (21/92) pulses. The main trend of spectral evolution (lag behavior) is estimated with $\log E_p\propto k_E\log(t+t_0)$ (${\hat…
Ultrarelativistic gamma-ray burst (GRB) jets are strong gravitational wave (GW) sources with memory-type signals. The plateau (or shallow decay) phases driven by the energy injection might appear in the early X-ray afterglows of GRBs. In this paper, we investigate the GW signal as well as X-ray afterglow emission in the framework of GRB jets with energy injection, and both short- and long-duration GRBs are considered. We find that, regardless of the case, because of the antibeaming and time delay effects, a rising slope emerging in the waveform of GW signal due to the energy injection lags far…
It has been suggested that short gamma ray bursts (GRBs) have shorter or undetectable spectral lags than long GRB because in the former, the observer's line of sight makes a larger angle with the GRB jet axis than for the latter. It is proposed that simultaneous gravitational wave - short GRB events could provide a simple test of this hypothesis. Multimessenger astronomy eventually may test whether event horizons are a necessary ingredient for GRB.
In this catalog, we present the results of a systematic study of gamma-ray bursts (GRBs) with reliable redshift estimates detected in the triggered mode of the Konus-Wind (KW) experiment during the period from 1997 February to 2016 June. The sample consists of 150 GRBs (including twelve short/hard bursts) and represents the largest set of cosmological GRBs studied to date over a broad energy band. From the temporal and spectral analyses of the sample, we provide the burst durations, the spectral lags, the results of spectral fits with two model functions, the total energy fluences, and the pea…
Jet precession has previously been proposed to explain the apparently repeating features in the light curves of a few gamma-ray bursts (GRBs). In this {\it Letter}, we further apply the precession model to a bright GRB 220408B by examining both its temporal and spectral consistency with the predictions of the model. As one of the recently confirmed GRBs observed by our GRID CubeSat mission, GRB 220408B is noteworthy as it exhibits three apparently similar emission episodes. Furthermore, the similarities are reinforced by their strong temporal correlations and similar features in terms of spect…
The observed spectral lags of gamma-ray bursts (GRBs) have been widely used to explore possible violations of Lorentz invariance. However, these studies were generally performed by concentrating on the rough time lag of a single highest-energy photon and ignoring the intrinsic time lag at the source. A new way to test nonbirefringent Lorentz-violating effects has been proposed by analyzing the multi-photon spectral-lag behavior of a GRB that displays a positive-to-negative transition. This method gives both a plausible description of the intrinsic energy-dependent time lag and comparatively ro…
Spectral lag of the low-energy photons with respect to the high-energy ones is a common astrophysical phenomenon (such as Gamma-ray bursts and the Crab pulsar) and may serve as a key probe to the underlying radiation mechanism. However, spectral lag in keV range of the magnetar bursts has not been systematically studied yet. In this work, we perform a detailed spectral lag analysis with the Li-CCF method for SGR J1935+2154 bursts observed by {\it Insight}-HXMT, GECAM and Fermi/GBM from July 2014 to Jan 2022. We discover that the spectral lags of about 61\% (non-zero significance >1$σ$) bursts …
We demonstrate that the `smoke' of limited instrumental sensitivity smears out structure in gamma-ray burst (GRB) pulse light curves, giving each a triple-peaked appearance at moderate signal-to-noise and a simple monotonic appearance at low signal-to-noise. We minimize this effect by studying six very bright GRB pulses (signal-to-noise generally $> 100$), discovering surprisingly that each exhibits $\textit{complex time-reversible}$ wavelike residual structures. These `mirrored' wavelike structures can have large amplitudes, occur on short timescales, begin/end long before/after the onset of …
We analyzed data obtained by the SPI telescope onboard the INTEGRAL observatory to search for short transient events with a duration from 1 ms to a few tens of seconds. An algorithm for identifying gamma-ray events against the background of a large number of charged particle interactions with the detector has been developed. The classification of events was made. Apart from the events associated with cosmic gamma-ray bursts (GRBs) confirmed by other space experiments and the activity of known soft gamma repeaters (for example, SGR 1806-20), previously unreported GRBs have been found. GRB candi…
We present the relation between the ($z-$ and $k-$corrected) spectral lags, $τ$, for the standard Swift energy bands 50-100 keV and 100-200 keV and the peak isotropic luminosity, $L_{\mathrm{iso}}$ (a relation reported first by Norris et al.), for a subset of 12 long Swift GRBs taken from a recent study of this relation by Ukwatta et al. The chosen GRBs are also a subset of the Dainotti et al. sample, a set of Swift GRBs of known redshift, employed in establishing a relation between the (GRB frame) luminosity, $L_X$, of the shallow (or constant) flux portion of the typical XRT GRB-afterglow li…
We continue our earlier studies of quasi-periodic oscillations (QPOs) in the power spectra of accreting, rapidly-rotating black holes that originate from the geometric "light echoes" of X-ray flares occurring within the black hole ergosphere. Our present work extends our previous treatment to three-dimensional photon emission and orbits to allow for arbitrary latitudes in the positions of the distant observers and the X-ray sources in place of the mainly equatorial positions and photon orbits of the earlier consideration. Following the trajectories of a large number of photons we calculate the…
Several trends have been identified in the prompt gamma-ray burst (GRB) emission: e.g. hard-to-soft evolution, pulse width evolution with energy, time lags, hardness-intensity/-fluence correlations. Recently Fermi has significantly extended the spectral coverage of GRB observations and improved the characterization of this spectral evolution. We study how internal shocks can reproduce these observations. In this model the emission comes from the synchrotron radiation of shock accelerated electrons, and the spectral evolution is governed by the evolution of the physical conditions in the shocke…
The delay in the arrival times between high and low energy photons from cosmic sources can be used to test the violation of the Lorentz invariance (LIV), predicted by some quantum gravity theories, and to constrain its characteristic energy scale ${\rm E_{QG}}$ that is of the order of the Planck energy. Gamma-ray bursts (GRBs) and blazars are ideal for this purpose thanks to their broad spectral energy distribution and cosmological distances: at first order approximation, the constraints on ${\rm E_{QG}}$ are proportional to the photon energy separation and the distance of the source. However,…
The spectral lags of gamma-ray bursts (GRBs) have been viewed as the most promising probes of the possible violations of Lorentz invariance (LIV). However, these constraints usually depend on the assumption of the unknown intrinsic time lag in different energy bands and the use of a single highest-energy photon. A new approach to test the LIV effects has been proposed by directly fitting the spectral lag behavior of a GRB with a well-defined transition from positive lags to negative lags. This method simultaneously provides a reasonable formulation of the intrinsic time lag and robust lower li…
The microquasar GRS 1915+105, exhibits a large variety of characteristic states, according to its luminosity, spectral state, and variability. The most interesting one is the so-called rho-state, whose light curve shows recurrent bursts. This paper presents a model based on Fitzhugh-Nagumo equations containing two variables: x, linked to the source photon luminosity L detected by the MECS, and y related to the mean photon energy. We aim at providing a simple mathematical framework composed by non-linear differential equations useful to predict the observed light curve and the energy lags for t…
In this paper we examine gamma-ray and optical data of GRB 091024, a gamma-ray burst (GRB) with an extremely long duration of T90~1020 s, as observed with the Fermi Gamma-Ray Burst Monitor (GBM). We present spectral analysis of all three distinct emission episodes using data from Fermi/GBM. Because of the long nature of this event, many ground-based optical telescopes slewed to its location within a few minutes and thus were able to observe the GRB during its active period. We compare the optical and gamma-ray light curves. Furthermore, we estimate a lower limit on the bulk Lorentz factor from…
We discuss the prospects for bounding and perhaps even measuring quantum gravity effects on the dispersion of light using the highest energy photons produced in gamma ray bursts measured by the Fermi telescope. These prospects are brigher than might have been expected as in the first 10 months of operation Fermi has reported so far eight events with photons over 100 MeV seen by its Large Area Telescope (LAT). We review features of these events which may bear on Planck scale phenomenology and we discuss the possible implications for the alternative scenarios for in-vacua dispersion coming from …
We analyze the spectral and timing properties of IGR J17498-2921 and the characteristics of X-ray bursts to constrain the physical processes responsible for the X-ray production in this class of sources. The broad-band average spectrum is well-described by thermal Comptonization with an electron temperature of kT_e ~ 50 keV, soft seed photons of kT_bb ~ 1 keV, and Thomson optical depth \taut ~ 1 in a slab geometry. The slab area corresponds to a black body radius of R_bb ~9 km. During the outburst, the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scatterin…
IGR J17511-3057 is the second X-ray transient accreting millisecond pulsar discovered by INTEGRAL. It was in outburst for about a month from September 13, 2009. The broad-band average spectrum is well described by thermal Comptonization with an electron temperature of kT_e ~ 25 keV, soft seed photons of kT_bb ~ 0.6 keV, and Thomson optical depth τ_T ~ 2 in a slab geometry. During the outburst the spectrum stays remarkably stable with plasma and soft seed photon temperatures and scattering optical depth being constant within errors. We fitted the outburst profile with the exponential model, and…
