Time-resolved Spectral Analysis Research Articles

CHARACTERIZING THE EVOLVING X-RAY SPECTRAL FEATURES DURING A SUPERBURST FROM 4U 1636-536

Recent studies have shown that runaway thermonuclear burning of material accreted onto neutron stars, i.e. Type I X-ray bursts, may affect the accretion disk. We investigate this by performing a detailed time-resolved spectral analysis of the superburst from 4U 1636-536 observed in 2001 with the Rossi X-ray Timing Explorer. Superbursts are attributed to the thermonuclear burning of carbon, and are approximately 1000 times more energetic than the regular short Type I bursts. This allows us to study detailed spectra for over 11 ks, compared to at most 100 s for regular bursts. A feature is present in the superburst spectra around 6.4 keV that is well fit with an emission line and an absorption edge, suggestive of reflection of the superburst off the accretion disk. The line and edge parameters evolve over time: the edge energy decreases from 9.4 keV at the peak to 8.1 keV in the tail, and both features become weaker in the tail. This is only the second superburst for which this has been detected, and shows that this behavior is present even without strong radius expansion. Furthermore, we find the persistent flux to almost double during the superburst, and return to the pre-superburst level in the tail. The combination of reflection features and increased persistent emission indicates that the superburst had a strong impact on the inner accretion disk, and it emphasizes that X-ray bursts provide a unique probe of accretion physics.

THE BROADBAND SPECTRAL VARIABILITY OF MCG–6-30-15 OBSERVED BYNUSTARANDXMM-NEWTON

MCG-6-30-15, at a distance of 37 Mpc (z=0.008), is the archetypical Seyfert 1 galaxy showing very broad Fe K$\alpha$ emission. We present results from a joint NuSTAR and XMM-Newton observational campaign that, for the first time, allows a sensitive, time-resolved spectral analysis from 0.35 keV up to 80 keV. The strong variability of the source is best explained in terms of intrinsic X-ray flux variations and in the context of the light bending model: the primary, variable emission is reprocessed by the accretion disk, which produces secondary, less variable, reflected emission. The broad Fe K$\alpha$ profile is, as usual for this source, well explained by relativistic effects occurring in the innermost regions of the accretion disk around a rapidly rotating black hole. We also discuss the alternative model in which the broadening of the Fe K$\alpha$ is due to the complex nature of the circumnuclear absorbing structure. Even if this model cannot be ruled out, it is disfavored on statistical grounds. We also detected an occultation event likely caused by BLR clouds crossing the line of sight.

Analysis of X-ray spectral variability and black hole mass determination of the NLS1 galaxy Mrk 766

We present an XMM-Newton time-resolved spectral analysis of the NLS1 galaxy Mrk 766. We analyse eight available observations of the EPIC-pn camera taken between May 2000 and June 2005 to investigate the X-ray spectral variability as produced by changes in the mass accretion rate. The 0.2-10 keV spectra are extracted in time bins longer than 3 ks to accurately trace the variations of the best fit parameters of our adopted Comptonisation spectral model. We test a bulk-motion Comptonisation (BMC) model which is in general applicable to any physical system powered by accretion onto a compact object, and assumes that soft seed photons are efficiently up-scattered via inverse Compton scattering in a hot and dense electron corona. The Comptonised spectrum has a characteristic power-law shape, whose slope was found to increase for large values of the normalisation of the seed component, that is proportional to the mass accretion rate (in Eddington units). Our baseline spectral model also includes a warm absorber lying on the line of sight and radiation reprocessing from the accretion disk or from outflowing matter in proximity of the central compact object. Our study reveals that the normalisation-slope correlation, observed in Galactic Black Hole sources (GBHs), also holds for Mrk 766: variations of the photon index in the range Gamma~1.9-2.4 are indeed likely to be related to the variations of m-dot, as observed in X-ray binary systems. We finally applied a scaling technique based on the observed correlation to estimate the BH mass in Mrk 766. This technique is commonly and successfully applied to measure masses of GBHs, and this is the first time it is applied in detail to estimate the BH mass in an AGN. We obtain a value of M_{BH}=1.26^{+1.00}_{-0.77}x10^6 M_{sun} that is in very good agreement with that estimated by the reverberation mapping

Time-resolved spectral analysis of prompt emission from long gamma-ray bursts with GeV emission

We performed detailed time-resolved spectroscopy of bright long gamma-ray bursts (GRBs) which show significant GeV emissions (GRB 080916C, GRB 090902B and GRB 090926A). In addition to the standard Band model, we also use a model consisting of a black body and a power law to fit the spectra. We find that for the latter model there are indications of an additional soft component in the spectra. While previous studies have shown that such models are required for GRB 090902B, here we find that a composite spectral model consisting of two blackbodies and a power law adequately fits the data of all the three bright GRBs. We investigate the evolution of the spectral parameters and find several interesting features that appear in all three GRBs, like (a) temperatures of the blackbodies are strongly correlated with each other, (b) fluxes in the black body components are strongly correlated with each other, (c) the temperatures of the black body trace the profile of the individual pulses of the GRBs, and (d) the characteristics of power law components like the spectral index and the delayed onset bear a close similarity to the emission characteristics in the GeV regions. We discuss the implications of these results and the possibility of identifying the radiation mechanisms during the prompt emission of GRBs.

The size of the X-ray emitting region in SWIFT J2127.4+5654 via a broad line region cloud X-ray eclipse

We present results obtained from the time-resolved X-ray spectral analysis of the Narrow-Line-Seyfert 1 galaxy SWIFT J2127.4+5654 during a ~130 ks XMM-Newton observation. We reveal large spectral variations, especially during the first ~90 ks of the XMM-Newton exposure. The spectral variability can be attributed to a partial eclipse of the X-ray source by an intervening low-ionization/cold absorbing structure (cloud) with column density N_H = 2.0^{+0.2}_{-0.3}e22 cm^-2 which gradually covers and then uncovers the X-ray emitting region with covering fraction ranging from zero to ~43 per cent. Our analysis enables us to constrain the size, number density, and location of the absorbing cloud with good accuracy. We infer a cloud size (diameter) of $D_c < 1.5e13 cm, corresponding to a density of n_c > 1.5e9 cm^-3 at a distance of R_c > 4.3e16 cm from the central black hole. All of the inferred quantities concur to identify the absorbing structure with one single cloud associated with the broad line region of SWIFT J2127.4+5654. We are also able to constrain the X-ray emitting region size (diameter) to be D_s < 2.3e13 cm which, assuming the black hole mass estimated from single-epoch optical spectroscopy (1.5e7 M_sun), translates into D_s < 10.5 gravitational radii (r_g) with larger sizes (in r_g) being associated with smaller black hole masses, and viceversa. We also confirm the presence of a relativistically distorted reflection component off the inner accretion disc giving rise to a broad relativistic Fe K emission line and small soft excess (small because of the high Galactic column density), supporting the measurement of an intermediate black hole spin in SWIFT J2127.4+5654 that was obtained from a previous Suzaku observation.

EVIDENCE FOR ACCRETION RATE CHANGE DURING TYPE I X-RAY BURSTS

The standard approach for time-resolved X-ray spectral analysis of thermonuclear bursts involves subtraction of the pre-burst emission as background. This approach implicitly assumes that the persistent flux remains constant throughout the burst. We reanalyzed 332 photospheric radius expansion bursts observed from 40 sources by the Rossi X-Ray Timing Explorer, introducing a multiplicative factor fa to the persistent emission contribution in our spectral fits. We found that for the majority of spectra the best-fit value of fa is significantly greater than 1, suggesting that the persistent emission typically increases during a burst. Elevated fa values were not found solely during the radius expansion interval of the burst, but were also measured in the cooling tail. The modified model results in a lower average value of the χ2 fit statistic, indicating superior spectral fits, but not yet to the level of formal statistical consistency for all the spectra. We interpret the elevated fa values as an increase of the mass accretion rate onto the neutron star during the burst, likely arising from the effects of Poynting–Robertson drag on the disk material. We measured an inverse correlation of fa with the persistent flux, consistent with theoretical models of the disk response. We suggest that this modified approach may provide more accurate burst spectral parameters, as well as offering a probe of the accretion disk structure.

EVIDENCE FOR A PHOTOSPHERIC COMPONENT IN THE PROMPT EMISSION OF THE SHORT GRB 120323A AND ITS EFFECTS ON THE GRB HARDNESS-LUMINOSITY RELATION

The short GRB 120323A had the highest flux ever detected with the Fermi/GBM. Here we study its remarkable spectral properties and their evolution using two spectral models: (i) a single emission component scenario, where the spectrum is modeled by the empirical Band function, and (ii) a two component scenario, where thermal (Planck-like) emission is observed simultaneously with a non-thermal component (a Band function). We find that the latter model fits the integrated burst spectrum significantly better than the former, and that their respective spectral parameters are dramatically different: when fit with a Band function only, the Epeak of the event is unusually soft for a short GRB, while adding a thermal component leads to more typical short GRB values. Our time-resolved spectral analysis produces similar results. We argue here that the two-component model is the preferred interpretation for GRB 120323A, based on: (i) the values and evolution of the Band function parameters of the two component scenario, which are more typical for a short GRB, and (ii) the appearance in the data of a significant hardness-intensity correlation, commonly found in GRBs, when we employee two-component model fits; the correlation is non-existent in the Band-only fits. GRB 110721A, a long burst with an intense photospheric emission, exhibits the exact same behavior. We conclude that GRB 120323A has a strong photospheric emission contribution, first time observed in a short GRB. Magnetic dissipation models are difficult to reconcile with these results, which instead favor photospheric thermal emission and fast cooling synchrotron radiation from internal shocks. Finally, we derive a possibly universal hardness-luminosity relation in the source frame using a larger set of GRBs L,i=(1.59+/-0.84).10^50 (Epeak,i)^(1.33+/-0.07) erg/s), which could be used as a possible redshift estimator for cosmology.

A NEW METHOD OF PULSE-WISE SPECTRAL ANALYSIS OF GAMMA-RAY BURSTS

Time-resolved spectral analysis, though a very promising method to understand the emission mechanism of gamma-ray bursts (GRBs), is difficult to implement in practice because of poor statistics. We present a new method for pulse-wise time-resolved spectral study of the individual pulses of GRBs, using the fact that many spectral parameters are either constants or smooth functions of time. We use this method for the two pulses of GRB 081221, the brightest GRB with separable pulses. We choose, from the literature, a set of possible models which includes the Band model, blackbody with a power-law (BBPL), a collection of black bodies with a smoothly varying temperature profile, along with a power-law (mBBPL), and two blackbodies with a power-law (2BBPL). First, we perform time-resolved study to confirm the spectral parameter variations, and then construct the new model to perform a joint spectral fit. We find that any photospheric emission in terms of black bodies is required mainly in the rising parts of the pulses and the falling part can be adequately explained in terms of the Band model, with the low energy photon index within the regime of synchrotron model. Interestingly, we find that 2BBPL is comparable or sometimes even better, though marginally, than the Band model, in all episodes. Consistent results are also obtained for the brightest GRB of Fermi era --- GRB 090618. We point out that the method is generic enough to test any spectral model with well defined parameter variations.

GRB 110709B in the induced gravitational collapse paradigm

Context: GRB110709B is the first source for which Swift BAT triggered twice, with a time separation of ~10 min. The first emission (Ep. 1) goes from 40s before the 1{\deg} trigger up to 60s after it. The second (Ep. 2) goes from 35s before the 2{\deg} trigger to 100s after it.[...] Within the Induced Gravitational Collapse (IGC) model, we assume the progenitor to be a close binary system composed of a core of an evolved star and a Neutron Star (NS). The evolved star explodes as a Supernova (SN) and ejects material that is partially accreted by the NS. We identify this process with Ep. 1. The accretion process brings the NS over its critical mass, thus gravitationally collapsing to a BH. This process leads to the GRB emission, Ep. 2.[...] Aims: We analyze the spectra and time variability of Ep. 1 and 2 and compute the relevant parameters of the binary progenitor[...] in the IGC paradigm. Methods: We perform a time-resolved spectral analysis of Ep. 1 with a blackbody (BB) plus a power-law (PL) spectral model. We analyze Ep. 2 within the Fireshell model, identifying the Proper-GRB (P-GRB) and simulating the light curve and spectrum. We establish the redshift to be z=0.75 [...]. Results: We find for Ep. 1 a BB temperature following a broken PL with time, with the PL slopes at early and late times \alpha=0 and \beta=-4+/-2, respectively, and a break at t=41.21s. The total energy of Ep. 1 and 2 are E_{iso}^1=1.42x10^{53}erg and E_{iso}^2=2.43x10^{52}erg, respectively. We find at transparency a Lorentz factor \Gamma~173, laboratory radius of 6.04x10^{13}cm, P-GRB observed temperature kT_{P-GRB}=12.36keV, baryon load B=0.0057 and P-GRB energy E_{P-GRB}=3.44x10^{50}erg. [...] Conclusions: We interpret GRB110709B as a member of the IGC sources, together with GRB970828, GRB090618 and GRB101023. The XRT data during Ep. 2 offers an unprecedented tool for improving the diagnostic of GRBs emission.

GRB 090227B: THE MISSING LINK BETWEEN THE GENUINE SHORT AND LONG GAMMA-RAY BURSTS

The time-resolved spectral analysis of GRB090227B, made possible by the Fermi-GBM data, allows to identify in this source the missing link between the genuine short and long GRBs. Within the Fireshell model [...] we predict genuine short GRBs: bursts with the same inner engine of the long bursts but endowed with a severely low value of the Baryon load, B<~5x10^{-5}. A first energetically predominant emission occurs at the transparency of the e+e- plasma, the Proper-GRB (P-GRB), followed by a softer emission, the extended afterglow. The typical separation between the two emissions is expected to be [...] 10^{-3}-10^{-2}s. We identify the P-GRB [...] in the first 96ms of emission, where a thermal component with [...] kT=(517+/-28)keV and a flux comparable with the non thermal part of the spectrum is observed. This non thermal component as well as the subsequent emission, where there is no evidence for a thermal spectrum, is identified with the extended afterglow. We deduce a theoretical cosmological redshift z=1.61+/-0.14. We then derive the total energy E^{tot}_{e+e-}=(2.83+/-0.15)x10^{53}erg, [...] B=(4.13+/-0.05)x10^{-5}, the Lorentz factor at transparency \Gamma_tr=(1.44+/-0.01)x10^4, and the intrinsic duration \Delta t'~0.35s. We also determine the average density of the CircumBurst Medium (CBM), <n_CBM>=(1.90+/-0.20)x10^{-5} #/cm^3. There is no evidence of beaming in the system. In view of the energetics and of the Baryon load of the source, as well as of the low interstellar medium and of the intrinsic time scale of the signal, we identify the GRB progenitor as a binary neutron star. From the recent progress in the theory of neutron stars, we obtain masses of the stars m_1=m_2=1.34M_Sun and their corresponding radii R_1=R_2=12.24km and thickness of their crusts ~0.47km, consistent with the above values of the Baryon load, of the energetics and of the time duration of the event.

EVIDENCES FOR A DOUBLE COMPONENT IN THE EMISSION OF GRB 101023

In this work we present the results of the analysis of GRB 101023 in the fireshell scenario. We infered its redshift from the Amati Relation, obtaining z=0.9. Its light curve is very similar to the already studied GRB 090618, in the sense that it presents a double emission. We called each part Episode 1 and Episode 2. We performed a time-resolved spectral analysis with RMFIT using different spectral models, and fitted the light curve with a numerical code integrating the fireshell equations of motion. We used Fermi GBM data to build the light curve, in particular the second NaI detector, in the range (8.5-1000 keV). We considered different hypotheses about which part of the light curve could be the GRB and analyzed all of them. We noticed a great variation of the temperature with time in the first episode and almost no variation of the progenitor radius. The first emission does not match the requirements for a GRB, while the second part perfectly agrees with being a canonical GRB, with a P-GRB lasting 4s.

Konus-WIND observation of the ultra-luminous GRB 110918A

The exceptionally intense long GRB 110918A was discovered by several GRB observing missions: INTEGRAL (SPI-ACS), Konus-WIND, Mars Odyssey (HEND), and MESSENGER (GRNS) on September 18, 2011. This GRB was localized by the Interplanetary Network (IPN) and its bright X-ray counterpart was found in close vicinity of the IPN box in the Swift/XRT follow-up observations starting 1.2 days after the trigger. The optical afterglow was discovered by the Isaac Newton Telescope and its spectroscopic redshift z = 0.982 was measured with the GMOS spectrograph mounted on the Gemini-N telescope. GRB 110918A is the brightest burst detected by Konus-WIND for more than 17 years of its continuous observations. The instrument’s light curves in three energy bands covering 22–1450 keV range show an extremely bright, short, hard pulse followed by three weaker, softer, partly overlapping pulses within next 25 seconds. A spectral lag between the light-curves is determined, showing a substantial increase in the course of the burst. The emission is detected up to 12 MeV. Modeling the time-integrated energy spectrum with the Band function yields a moderate value of Epeak = 340keV, while the time-resolved spectral analysis reveals strong hardness-intensity correlation and a hard-to-soft evolution of the emission: Epeak falls from ~ 4 MeV at the onset of the huge initial pulse to ~50 keV at the final stage of the burst. The total 20 keV–10 MeV energy fluence amounts to S = (7.8 ± 0.4) × 10-4erg cm-2 and a 64-ms peak flux Fmax = (9.2 ± 0.4) × 10-4erg cm-2s-1, which corresponds to a huge isotropic-equivalent energy release Eiso = (2.1 ± 0.1) × 1054erg and the record-breaking peak luminosity Liso;max = (4.7 ± 0.2) × 1054erg s-1.

A COMPREHENSIVE ANALYSIS OFFERMIGAMMA-RAY BURST DATA. II.EpEVOLUTION PATTERNS AND IMPLICATIONS FOR THE OBSERVED SPECTRUM-LUMINOSITY RELATIONS

We present a time-resolved spectral analysis of 51 long and 11 short bright GRBs observed with the {\em Femri}/GBM, paying special attention to $E_{\rm p}$ evolution within a same burst. Among 8 single-pulse long GRBs, 5 show hard-to-soft evolution, while 3 show intensity-tracking. The multi-pulse long GRBs have more complicated patterns. Among the GRBs whose time-resolved spectrum is available for the first pulse, almost half (15/32 GRBs) show clear hard-to-soft evolution, and the other half (17/32 GRBs) show clear intensity-tracking. Later pulses typically show the tracking behavior, although a hard-to-soft evolution pattern was identified in the 2nd pulse of 2 GRBs whose pulses are well separated. Statistically, the hard-to-soft evolution pulses tend to be more asymmetric than the intensity-tracking ones, with a steeper rising wing than the falling wing. Short GRBs have $E_{\rm p}$ tracking intensity exclusively with the 16ms time resolution analysis. We performed a simulation analysis, and suggest that at least for some bursts, the late intensity-tracking pulses could be a consequence of overlapping hard-to-soft pulses. However, the fact that the intensity-tracking pattern exists in the first pulse of multi-pulse long GRBs and some single-pulse GRBs suggest that intensity tracking is an independent component, which may operate in some late pulses as well. For the GRBs with measured redshifts, we present a time-resolved $E_{\rm p}-L_{\gamma, \rm iso}$ correlation analysis and show that the scatter of the correlation is comparable to that of the global Amati/Yonetoku relation. We discuss the predictions of various radiation models regarding $E_{\rm p}$ evolution, as well as the possibility of a precession jet in GRBs. It seems that the data pose great challenge to all these models, and hold the key to unveil the physics of GRB prompt emission.

Do NLS1s have a beamed outflow? An unusual X-ray perspective for Mrk 766

Recently, the detection of Gamma-ray emission from Narrow Line Seyfert 1s has suggested that these sources may emit beamed jets towards us. I report the results of a time-resolved spectral analysis of an XMM-Newton long observation of Mrk 766, showing that the X-ray source has been eclipsed several times by clouds with a cold, dense core, and a less dense, highly ionized tail. These clouds have blueshift velocities v > 104 km s−1 (as measured through He-like iron absorption lines), and are therefore part of a strong outflow. These results provide new important constraints on the geometry and structure of this source, and may be relevant to understand the peculiarity of NLS1s as a class of AGNs.

SigSpec User's Manual

SigSpec computes the spectral significance levels for the DFT∗ amplitude spectrum of a time series at arbitrarily given sampling. It is based on the analytical solution for the Probability Density Function (PDF) of an amplitude level, including dependencies on frequency and phase and referring to white noise. Using a time series dataset as input, an iterative procedure including step-by-step prewhitening of the most significant signal components and MultiSine leastsquares fitting is provided to determine a whole set of signal components, which makes the program a powerful tool for multi-frequency analysis. Instead of the step-by-step prewhitening of the most significant peaks, the program is also able to take into account several steps of the prewhitening sequence simultaneously and check for the combination associated to a minimum residual scatter. This option is designed to overcome the aliasing problem caused by periodic time gaps in the dataset. SigSpec can detect non-sinusoidal periodicities in a dataset by simultaneously taking into account a fundamental frequency plus a set of harmonics. Time-resolved spectral significance analysis using a set of intervals of the time series is supported to investigate the development of eigenfrequencies over the observation time. Furthermore, an extension is available to perform the SigSpec analysis for multiple time series input files at once. In this MultiFile mode, time series may be tagged as target and comparison data. Based on this selection, SigSpec is capable of determining differential Note from the Editor: We report with sadness that Peter Reegen, the developer of SigSpec and author of this manual, unexpectedly passed away this year. SigSpec was one of the main achievements of his scientific career. During the previous year he was able to observe the adoption of his program by a large number of astronomers. We sincerely regret the loss of our dear friend and colleague “Piet”. We thank Michael Gruberbauer, who has thoroughly looked through the paper and provided several comments and supplement information. These are represented in uncounted footnotes marked with *. Note by M. Gruberbauer: Discrete Fourier Transform (DFT) 4 SigSpec User’s Manual significance spectra for the target datasets with respect to coincidences in the comparison spectra. A built-in simulator to generate and superpose a variety of sinusoids and trends as well as different types of noise completes the software package at the present stage of development. 1. What is SigSpec? SigSpec (abbreviation of ‘SIGnificance SPECtrum’) is a program that computes a significance spectrum for a time series. It evaluates the Probability Density Function (PDF) of a given DFT amplitude level analytically, making use of the theoretical concept introduced by Reegen (2005, 2007). The FalseAlarm Probability, ΦFA (A), is the probability that an amplitude in the DFT spectrum exceeds a given limit A, and is obtained through integration of the PDF (e. g. Scargle 1982). Instead of this frequently used quantity, SigSpec calculates the spectral significance (abbreviated by ‘sig’) of an amplitude A by sig (A) := − log [ΦFA (A)] . (1) E. g., a sig equal to 5 indicates that the considered amplitude level is due to noise in one out of 10 cases.∗ This value is used as the default threshold for the termination of the prewhitening sequence. SigSpec performs an iterative process consisting of four steps: 1. computation of the significance spectrum, 2. exact determination of the peak with maximum sig, 3. a MultiSine least-squares fit of the frequencies, amplitudes and phases of all significant signal components detected so far, 4. prewhitening of the sinusoidal components. The residuals are used as input for the next iteration. If SigSpec is called without any special settings, it produces four files: 1. the DFT amplitude spectrum s000000.dat of the original time series, containing also sig and phase, 2. the DFT amplitude spectrum resspec.dat of the residual time series after prewhitening all significant signal components, containing also sig and phase, Note by M. Gruberbauer: Note that this quantitiy is defined for a single frequency rather than for the whole spectrum. See Section 15.5. for a discussion. 1The AntiAlC computation (p. 47) differs slightly from this procedure.

Variable X-ray absorption in the mini-BAL QSO PG 1126-041

X-ray studies of active galactic nuclei (AGN) with powerful nuclear winds are important for constraining the physics of the inner accretion/ejection flow around supermassive black holes (SMBHs) and for understanding the impact of such winds on the AGN environment. Our main scientific goal is to constrain the properties of the circum-nuclear matter close to the SMBH in the mini-broad absorption line quasar (mini-BAL QSO) PG 1126-041 using a multi-epoch observational campaign with XMM-Newton. We performed temporally resolved X-ray spectroscopy and simultaneous UV and X-ray photometry on the most complete set of observations and on the deepest X-ray exposure of a mini-BAL QSO ever. We found complex X-ray spectral variability on time scales of both months and hours, which is best reproduced by means of variable massive ionized absorbers along the line of sight. As a consequence, the observed optical-to-X-ray spectral index is found to be variable with time. In the highest signal-to-noise observation we detected highly ionized X-ray absorbing material outflowing much faster (v ~ 16500 km/s) than the UV absorbing one (v ~ 5000 km/s). This highly ionized absorber is found to be variable on very short (a few kiloseconds) time scales. Our findings are qualitatively consistent with line-driven accretion disk winds scenarios. Our observations have opened the time-resolved X-ray spectral analysis field for mini-BAL QSOs. Only with future deep studies will we be able to map the dynamics of the inner flow and understand the physics of AGN winds and their impact on the environment.

THE SECOND SWIFT BURST ALERT TELESCOPE GAMMA-RAY BURST CATALOG

We present the second Swift Burst Alert Telescope (BAT) catalog of gamma-ray bursts (GRBs), which contains 476 bursts detected by the BAT between 2004 December 19 and 2009 December 21. This catalog (hereafter the BAT2 catalog) presents burst trigger time, location, 90% error radius, duration, fluence, peak flux, time-averaged spectral parameters and time-resolved spectral parameters measured by the BAT. In the correlation study of various observed parameters extracted from the BAT prompt emission data, we distinguish among long-duration GRBs (L-GRBs), short-duration GRBs (S-GRBs), and short-duration GRBs with extended emission (S-GRBs with E.E.) to investigate differences in the prompt emission properties. The fraction of L-GRBs, S-GRBs and S-GRBs with E.E. in the catalog are 89%, 8% and 2% respectively. We compare the BAT prompt emission properties with the BATSE, BeppoSAX and HETE-2 GRB samples. We also correlate the observed prompt emission properties with the redshifts for the GRBs with known redshift. The BAT T90 and T50 durations peak at 70 s and 30 s, respectively. We confirm that the spectra of the BAT S-GRBs are generally harder than those of the L-GRBs. The time-averaged spectra of the BAT S-GRBs with E.E. are similar to those of the L-GRBs. Whereas, the spectra of the initial short spikes of the S-GRBs with E.E. are similar to those of the S-GRBs. We show that the BAT GRB samples are significantly softer than the BATSE bright GRBs, and that the time-averaged Epeak of the BAT GRBs peaks at 80 keV which is significantly lower energy than those of the BATSE sample which peak at 320 keV. The time-averaged spectral properties of the BAT GRB sample are similar to those of the HETE-2 GRB samples. By time-resolved spectral analysis, we find that only 10% of the BAT observed photon indices are outside the allowed region of the synchrotron shock model.

A SURVEY OF HIGH-CONTRAST STELLAR FLARES OBSERVED BYCHANDRA

The X-ray light curves of pre-main sequence stars can show variability in the form of flares altering a baseline characteristic activity level; the largest X-ray flares are characterized by a rapid rise to more than 10 times the characteristic count rate, followed by a slower quasi-exponential decay. Analysis of these high-contrast X-ray flares enables the study of the innermost magnetic fields of pre-main sequence stars. We have scanned the ANCHORS database of Chandra observations of star-forming regions to extend the study of flare events on pre-main sequence stars both in sky coverage and in volume. We developed a sample of 30 high-contrast flares out of the 14,000 stars of various ages and masses available in ANCHORS at the start of our study. Applying methods of time-resolved spectral analysis, we obtain the temperatures, confining magnetic field strengths, and loop lengths of these bright, energetic flares. The results of the flare analysis are compared to the 2MASS and Spitzer data available for the stars in our sample. We find that the longest flare loop lengths (of order several stellar radii) are only seen on stars whose IR data indicates the presence of disks. This suggests that the longest flares may stretch all the way to the disk. Such long flares tend to be more tenuous than the other large flares studied. A wide range of loop lengths are observed, indicating that different types of flares may occur on disked young stellar objects.

Spectral components in the bright, long GRB 061007: properties of the photosphere and the nature of the outflow

We present a time-resolved spectral analysis of the bright, long GRB 061007 (z=1.261) using Swift BAT and Suzaku WAM data. We find that the prompt emission of GRB 061007 can be equally well explained by a photospheric component together with a power law as by a Band function, and we explore the implications of the former model. The photospheric component, which we model with a multicolour blackbody, dominates the emission and has a very stable shape throughout the burst. This component provides a natural explanation for the hardness-intensity correlation seen within the burst and also allows us to estimate the bulk Lorentz factor and the radius of the photosphere. The power-law component dominates the fit at high energies and has a nearly constant slope of -1.5. We discuss the possibility that this component is of the same origin as the high-energy power laws recently observed in some Fermi LAT bursts.

A COMPREHENSIVE ANALYSIS OFFERMIGAMMA-RAY BURST DATA. I. SPECTRAL COMPONENTS AND THE POSSIBLE PHYSICAL ORIGINS OF LAT/GBM GRBs

We present a systematic analysis of the spectral and temporal properties of 17 GRBs co-detected by GBM and LAT on board the Fermi satellite by May 2010. We performed a time-resolved spectral analysis of all the bursts with the finest temporal resolution allowed by statistics, in order to avoid temporal smearing of different spectral components. We found that the time-resolved spectra of 14 out of 17 GRBs are best modeled with the Band function over the entire Fermi spectral range, which may suggest a common origin for emissions detected by LAT and GBM. GRB 090902B and GRB 090510 require the superposition between an MeV component and an extra power law component, with the former having a sharp cutoff above E_p. For GRB 090902B, this MeV component becomes progressively narrower as the time bin gets smaller, and can be fit with a Planck function as the time bin becomes small enough. In general, we speculate that phenomenologically there may be three elemental spectral components : (I) a Band-function component (e.g. in GRB 080916C) that extends in a wide energy range and does not narrow with reducing time bins, which may be of the non-thermal origin; (II) a quasi-thermal component (e.g. in GRB 090902B) with the spectra progressively narrowing with reducing time bins; and (III) another non-thermal power law component extending to high energies. The spectra of different bursts may be decomposed into one or more of these elemental components. We compare this sample with the BATSE sample and investigate some correlations among spectral parameters. We discuss the physical implications of the data analysis results for GRB prompt emission, including jet compositions (matter-dominated vs. Poynting-flux-dominated outflow), emission sites (internal shock, external shock or photosphere), as well as radiation mechanisms (synchrotron, synchrotron self-Compton, or thermal Compton upscattering).