Shu-Jin Hou

A COMPREHENSIVE ANALYSIS OF SWIFT/X-RAY TELESCOPE DATA. IV. SINGLE POWER-LAW DECAYING LIGHT CURVES VERSUS CANONICAL LIGHT CURVES AND IMPLICATIONS FOR A UNIFIED ORIGIN OF X-RAYS

By systematically analyzing the Swift/XRT light curves detected before 2009 July, we find 19 light curves that monotonously decay as a single power law (SPL) with an index of 1 ~ 1.7 from tens (or hundreds) of seconds to ~105 s post the gamma-ray burst (GRB) trigger. They are apparently different from the canonical light curves characterized by a shallow-to-normal decay transition. We compare the observations of the prompt gamma rays and the X-rays for these two samples of GRBs (SPL vs. canonical). No statistical difference is found in the prompt gamma-ray properties for the two samples. The X-ray properties of the two samples are also similar, although the SPL sample tends to have a slightly lower neutral hydrogen absorption column for the host galaxies and a slightly larger energy release compared with the canonical sample. The SPL X-ray Telescope (XRT) light curves in the burst frame gradually merge into a conflux, and their luminosities at 105 s are normally distributed at log L/ergs s?1 = 45.6 ? 0.5. The normal decay segment of the canonical XRT light curves has the same feature. Similar to the normal decay segment, the SPL light curves satisfy the closure relations and therefore can be roughly explained with external shock models. In the scenario that the X-rays are the afterglows of the GRB fireball, our results indicate that the shallow decay would be due to energy injection into the fireball and the total energy budget after injection for both samples of GRBs is comparable. More intriguing, we find that a prior X-ray emission model proposed by Yamazaki is more straightforward to interpret the observed XRT data. We show that the zero times (T 0) of the X-rays are 102-105 s prior to the GRB trigger for the canonical sample, and satisfy a log-normal distribution. The negligible T 0s of the SPL sample are consistent with being the tail of T 0 distributions at low end, suggesting that the SPL sample and the canonical sample may be from a same parent sample. Referenced to T 0, the canonical XRT light curves well trace the SPL light curves. The T 0s of the canonical light curves in our analysis are usually much larger than the offsets of the known precursors from the main GRBs. If the prior emission hypothesis is real, the X-ray emission is better interpreted within the external shock models based on the spectral and temporal indices of the X-rays. The lack of detection of a jet-like break in most XRT light curves implies that the opening angle of the prior emission jet would be usually large.

Giant X-ray Bump in GRB 121027A: Evidence for Fall-back Disk Accretion

A particularly interesting discovery in observations of GRB 121027A is that of a giant X-ray bump detected by the Swift/X-Ray Telescope. The X-ray afterglow re-brightens sharply at similar to 10(3) s after the trigger by more than two orders of magnitude in less than 200 s. This X-ray bump lasts for more than 10(4) s. It is quite different from typical X-ray flares. In this Letter we propose a fall-back accretion model to interpret this X-ray bump within the context of the collapse of a massive star for a long-duration gamma-ray burst. The required fall-back radius of similar to 3.5x10(10) cm and mass of similar to 0.9-2.6M(circle dot) imply that a significant part of the helium envelope should survive through the mass loss during the last stage of the massive progenitor of GRB 121027A.

RADIAL ANGULAR MOMENTUM TRANSFER AND MAGNETIC BARRIER FOR SHORT-TYPE GAMMA-RAY-BURST CENTRAL ENGINE ACTIVITY

Soft extended emission (EE) following initial hard spikes up to 100 s was observed with Swift/BAT for about half of known short-type gamma-ray bursts (SGRBs). This challenges the conversional central engine models of SGRBs, i.e., compact star merger models. In the framework of black-hole-neutron-star merger models, we study the roles of radial angular momentum transfer in the disk and the magnetic barrier around the black hole in the activity of SGRB central engines. We show that radial angular momentum transfer may significantly prolong the lifetime of the accretion process, which may be divided into multiple episodes by the magnetic barrier. Our numerical calculations based on models of neutrino-dominated accretion flows suggest that disk mass is critical for producing the observed EE. In the case of the mass being similar to 0.8 M-circle dot, our model can reproduce the observed timescale and luminosity of both the main and the EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE within about one order of magnitude and the timescale is shorter than 20 s if the disk mass is similar to 0.2 M-circle dot. Swift/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component could be a probe for the merger process and disk formation for compact star mergers.

Photosphere emission in the X-ray flares of swift gamma-ray bursts and implications for the fireball properties

X-ray flares of gamma-ray bursts (GRBs) are usually observed in the soft X-ray range and the spectral coverage is limited. In this paper, we present an analysis of 32 GRB X-ray flares that are simultaneously observed by both Burst Alert Telescope and X-Ray Telescope on board the Swift mission, so that a joint spectral analysis with a wider spectral coverage is possible. Our results show that the joint spectra of 19 flares are fitted with the absorbed single power law or the Band function models. More interestingly, the joint spectra of the other 13 X-ray flares are fitted with the absorbed single power-law model plus a blackbody component. Phenomenally, the observed spectra of these 13 flares are analogous to several GRBs with a thermal component, but only with a much lower temperature of kT = 1 similar to 3 keV. Assuming that the thermal emission is the photosphere emission of the GRB fireball, we derive the fireball properties of the 13 flares that have redshift measurements, such as the bulk Lorentz factor Gamma(ph) of the outflow. The derived Gamma(ph) range from 50 to 150 and a relation of Gamma(ph) to the thermal emission luminosity is found. It is consistent with the Gamma(0) - L-iso relations that are derived for the prompt gamma-ray emission. We discuss the physical implications of these results within the content of jet composition and the radiation mechanism of GRBs and X-ray flares.

JET LUMINOSITY OF GAMMA-RAY BURSTS: THE BLANDFORD–ZNAJEK MECHANISM VERSUS THE NEUTRINO ANNIHILATION PROCESS

A neutrino-dominated accretion flow (NDAF) around a rotating stellar-mass black hole (BH) is one of the plausible candidates for the central engine of gamma-ray bursts (GRBs). Two mechanisms, i.e., the Blandford-Znajek (BZ) mechanism and the neutrino annihilation process, are generally considered to power GRBs. Using the analytic solutions from Xue et al. and ignoring the effects of the magnetic field configuration, we estimate the BZ and neutrino annihilation luminosities as functions of the disk masses and BH spin parameters to contrast the observational jet luminosities of GRBs. Our results show that although the neutrino annihilation processes could account for most GRBs, the BZ mechanism is more effective, especially for long-duration GRBs. Actually, if the energy of the afterglows and flares of GRBs is included, then the distinction between these two mechanisms is more significant. Furthermore, massive disk mass and high BH spin are beneficial for powering the high luminosities of GRBs. Finally, we discuss possible physical mechanisms that could enhance the disk mass or neutrino emission rate of NDAFs and the relevant difference between these two mechanisms.

CAN BLACK HOLE NEUTRINO-COOLED DISKS POWER SHORT GAMMA-RAY BURSTS?

Stellar-mass black holes (BHs) surrounded by neutrino-dominated accretion flows (NDAFs) are plausible sources of power. for. gamma-ray bursts (GRBs) via neutrino. emission and their annihilation. The progenitors of short-duration GRBs (SGRBs) are generally considered to be compact binary. mergers. According to the simulation results, the disk mass of the NDAF is limited after merger events. We can estimate such disk masses. using the current SGRB observational data and fireball model. The results show that the disk mass of a certain SGRB mainly depends on its output energy, jet opening angle, and central BH characteristics. Even for the extreme BH parameters, some SGRBs require massive disks, which approach or exceed the limits in simulations. We suggest that there may exist alternative MHD processes or mechanisms that increase the neutrino emission to produce SGRBs with reasonable BH parameters and disk masses.

Time Evolution of Flares in GRB 130925A: Jet Precession in a Black Hole Accretion System

GRB 130925A, composed of three gamma-ray emission episodes and a series of orderly flares, has been detected by Swift, Fermi, Konus-Wind, and INTEGRAL. If the third weakest gamma-ray episode can be considered a giant flare, we find that after the second gamma-ray episode observed by INTEGRAL located at about 2000 s, a positive relation exists between the time intervals of the adjacent flares and the time since the episode. We suggest that the second gamma-ray episode and its flares originate from the resumption of the accretion process due to the fragments from the collapsar falling back; such a relation may be related to a hyperaccretion disk around a precessed black hole (BH). We propose that the origin and time evolution of the flares, and the approximately symmetrical temporal structure and spectral evolution of the single flare can be explained well by a jet precession model. In addition, the mass and spin of the BH can be constrained, which indicates a stellar-mass, fast-rotating BH located in the center of GRB 130925A.

Variability of the giant X-ray bump in GRB 121027A and its possible origin

The giant X-ray bump of GRB 121027A observed by Swift is different from the typical X-ray flares in gamma-ray bursts. The observed structural variability in the rise and decay phases of the bump has four components. Of these four components, only the data in the bump from about 5300 to about 6100 s is of good enough quality to be analysed using the stepwise filter correlation method. A 86(-9.4)(+5.9)s periodic oscillation is postulated, which is confirmed by the Lomb-Scargle method. A jet precession model is proposed to account for this variability.

Central Engine of Late-Time X-ray Flares with Internal Origin

This work focuses on a sample of seven extremely late-time X-ray flares with peak time

EVOLUTIONS OF STELLAR-MASS BLACK HOLE HYPERACCRETION SYSTEMS IN THE CENTER OF GAMMA-RAY BURSTS

t_{\rm p} > 10^4 {\rm s}