arXiv subjects

Shu-Jin Hou

Publications and source records attributed to Shu-Jin Hou.

26 records · Page 2Linked to original sources

arXiv2014

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

The particular giant X-ray bump of GRB 121027A triggered by \emph{Swift} is quite different from the typical X-ray flares in gamma-ray bursts. There exhibit four parts of the observed structural variabilities in the rise and decay phase of the bump. Considering the quality of four parts of the data, we can only analyze the data from about 5300 s to about 6100 s in the bump using the stepwise filter correlation method (Gao et al. 2012), and find that the $86^{+5.9}_{-9.4}~\rm s$ periodic oscillation may exist, which is confirmed by the Lomb-Scargle method (Scargle 1982). Furthermore, a jet precession model (Liu et al. 2010) is proposed to account for such a variability.

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arXiv2014

Time evolution of flares in GRB 130925A: jet precession in 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 as a giant flare, we find that after the second gamma-ray episode observed by $INTEGRAL$ located at about 2000 s, there exists a positive relation 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 the jet precession model \citep{Liu2010}. 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.

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arXiv2013

Revisiting the Light Curves of Gamma-ray Bursts in the Relativistic Turbulence Model

Rapid temporal variability has been widely observed in the light curves of gamma-ray bursts (GRBs). One possible mechanism for such variability is related to the relativistic eddies in the jet. In this paper, we include the contribution of the inter-eddy medium together with the eddies to the gamma-ray emission. We show that the gamma-ray emission can either lead or lag behind the observed synchrotron emission, where the latter originates in the inter-eddy medium and provides most of seed photons for producing gamma-ray emission through the inverse-Compton scattering. As a consequence, we argue that the lead/lag found in non-stationary short-lived light curves may not reveal the intrinsic lead/lag of different emission components. In addition, our results may explain the lead of gamma-ray emission with respect to optical emission observed in GRB 080319B.

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arXiv2013

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 about 1000 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 ks. 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 about 3.5e10 cm and mass of about 0.9-2.6 solar masses 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.

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arXiv2012

Is There a Relation between Duration and E_iso in Gamma-Ray Bursts?

The system of accretion disk and black hole is usually considered as the central engine of Gamma-ray Bursts (GRBs). It is usually thought that the disk in the central engine of GRBs is the advection-dominated accretion disk, which is developed from a massive (mass M_disk) torus at radius r_disk. We find a positive correlation between the isotropic gamma-ray energy E_iso and duration (so-called T_90) for GRBs. We interpret this correlation within the advection-dominated accretion disk model, associating E_iso and T_90 with M_ disk and viscous timescale respectively.

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arXiv2012

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 seconds was observed with {\em Swift}/BAT for about half of 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 the black hole-neutron star merger models, we study the roles of the radial angular momentum transfer in the disk and the magnetic barrier around the black hole for the activity of SGRB central engines. We show that the radial angular momentum transfer may significantly prolong the lifetime of the accretion process and multiple episodes may be switched by the magnetic barrier. Our numerical calculations based on the models of the neutrino-dominated accretion flows suggest that the disk mass is critical for producing the observed EE. In case of the mass being $\sim 0.8M_{\odot}$, our model can reproduce the observed timescale and luminosity of both the main and EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE with about one order of magnitude and the timescale is shorter than 20 seconds if the disk mass being $\sim 0.2M_{\odot}$. {\em Swift}/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component would be a probe for merger process and disk formation for compact star mergers.

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arXiv2012

Physical origin of multi-wavelength emission of GRB 100418A and implications for its progenitor

GRB 100418A is a long burst at z=0.624 without detection of any associated supernova (SN). Its lightcurves in both the prompt and afterglow phases are similar to GRB 060614, a nearby long GRB without an associated SN. We analyze the observational data of this event and discuss the possible origins of its multi-wavelength emission. We show that its joint lightcurve at 1 keV derived from Swift BAT and XRT observations is composed of two distinguished components. The first component, whose spectrum is extremely soft (Γ= 4.32), ends with a steep decay segment, indicating the internal origin of this component. The second component is a slowly-rising, broad bump which peaks at ~10^5 seconds post the BAT trigger. Assuming that the late bump is due to onset of the afterglow, we derive the initial Lorentz factor (Gamma_0) of the GRB fireball and find that it significantly deviates from the relation between the Gamma_0 and Eiso of typical GRBs. We also check whether it follows the same anti-correlation between X-ray luminosity and the break time observed in the shallow decay phase of many typical GRBs, which is usually regarded as a signal of late energy injection from the GRB central engine. However, we find that it does not obey this correlation. We propose that the late bump could be contributed by a two-component jet. We fit the second component with an off-axis jet model for a constant medium density and find the late bump can be represented by the model. The derived jet half-opening angle is 0.30 rad and the viewing angle is 0.315 rad. The medium density is 0.05 cm^-3, possibly suggesting that it may be from a merger of compact stars. The similarity between GRBs 060614 and 100418A may indicate that the two GRBs are from the same population and the late bump observed in the two GRBs may be a signal of a two-component jet powered by the GRB central engine.

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arXiv2009

A Comprehensive Analysis of Swift/XRT Data: IV. Single Power-Law Decaying Lightcurves vs. Canonical Lightcurves and Implications for a Unified Origin of X-rays

By analyzing the Swift/XRT lightcurves detected before 2009 July, we find 19 cases that monotonously decay as a single power law (SPL) with an index of 1 ~ 1.7 from tens (or hundreds) to ~ 10^5 seconds post the GRB trigger, apparently different from the canonical lightcurves characterized by a shallow-to-normal decay transition. No statistical difference is found in their prompt gamma-rays, and the X-ray properties of the two samples are also similar, although the SPL sample tend to have a slightly lower NH value of the host galaxies and larger energy release compared with the canonical sample. The SPL XRT lightcurves in the burst frame gradually merge into a conflux.The normal decay segment for the canonical sample has the same feature. Similar to the normal decay segment, the SPL lightcurves satisfy the closure relations of external shock models. If the X-rays are the afterglow of the GRB fireball, our results indicate that the shallow decay would be due to energy injection 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 data. We show that the zero times (T0) of the X-rays atisfy a log-normal distribution, and the negligible T0's of the SPL sample are consistent with being the tail of T0 distributions at low end. Referenced to T0, the canonical XRT lightcurves well trace the SPL lightcurves. The T0's of the canonical lightcurves in our analysis are usually much larger than the offsets of the known precursors from the main GRBs, indicating the X-rays would be an emission component from external shocks prior to GRB trigger. The lack of detection of a jet-like break in most XRT lightcurves implies that the opening angle of its jet would be usually large.

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