arXiv subjects

Bing Zhang

Publications and source records attributed to Bing Zhang.

At least 613 records · Page 34Linked to original sources

arXiv2002

Gamma-ray burst beaming: a universal configuration with a standard energy reservoir?

We consider a gamma-ray burst (GRB) model based on an anisotropic fireball with an axisymmetric power-law distribution of the energy per solid angle with index -k, and allow for the observer's viewing direction being at an arbitrary angle with respect to the jet axis. This model can reproduce the key features expected from the conventional on-axis uniform jet models, with the novelty that the achromatic break time in the broadband afterglow lightcurves corresponds to the epoch when the relativistic beaming angle is equal to the viewing angle rather than to the jet half opening angle. If all the GRB fireballs have such a similar energy distribution form with 1.5 < k < (or \sim) 2, GRBs may be modeled by a quasi-universal beaming configuration, and an approximately standard energy reservoir. The conclusion also holds for some other forms of angular energy distributions, such as the Gaussian function.

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arXiv2001

Gamma-ray bursts with continuous energy injection and their afterglow signature

(Abridged) We discuss a Gamma-ray burst (GRB) fireball with an additional energy injection, either in the form of a Poynting-flux-dominated outflow or a kinetic-energy-dominated matter shell injected after the burst. A total injection energy comparable to that of the impulsive energy in the initial fireball is required to make a detectable signature in the afterglow lightcurves. For the case of a Poynting-flux-dominated injection, this leads to a gradual achromatic bump feature. For the case when the injection is kinetic-energy-dominated, the collision between the rear (injected) and the forward shell mild if the relative velocity between the colliding shells does not exceed a critical value defined by their energy ratio. Otherwise, the injection is violent, and an additional pair of shocks will form at the discontinuity between two colliding shells, so that there are altogether three shock-heated regions from which the emission contributes to the final lightcurves. We describe the shell-merging process in detail and present broadband lightcurves with the injection signatures. A violent collision results in very complicated features, which differs from the gradual bump signature found in the Poynting-flux or mild matter injection cases. In all the cases, the energetics of the fireball as well as the absolute afterglow flux level after the injection are boosted after the injection. Identifying the different injection signatures from future early afterglow observations may provide diagnostics about the nature of the fireball and of the central engine.

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arXiv2001

On Radio Quiescence of Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters

In the hot environments of the anomalous X-ray pulsars and the soft gamma-ray repeaters, as indicated by their luminous, pulsed, quiescent X-ray emission, $γ$-rays generated from the inner gaps may have shorter attenuation lengths via two photon pair production than via magnetic photon splitting. The AXP/SGR environments may not be pairless, even if photon splitting could completely suppress one photon pair production in super-strong magnetic fields, as conjectured by Baring & Harding. Two-photon pair production more likely occurs near the threshold, which tends to generate low energy pairs that are not energetic enough to power radio emission in the observed bands. However, emission in longer wavelengths may not be prohibited in principle if these objects are indeed magnetars. The so-called ``photon splitting deathlines'' are still valid for high magnetic field pulsars which are much dimmer in X-rays, if $γ$-rays with both polarization modes split.

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arXiv2001

High Energy Spectral Components in Gamma-Ray Burst Afterglows

We investigate two high energy radiation mechanisms, the proton synchrotron and the electron inverse Compton emission, and explore their possible signatures in the broad-band spectra and in the keV to GeV light curves of gamma-ray burst afterglows. We develop a simple analytical approach, allowing also for the effects of photon-photon pair production, and explore the conditions under which one or the other of these components dominates. We identify three parameter space regions where different spectral components dominate: (I) a region where the proton synchrotron and other hadron-related emission components dominate, which is small; (II) a region where the electron inverse Compton component dominates, which is substantial; (III) a third substantial region where electron synchrotron dominates. We discuss the prospects and astrophysical implications of directly detecting the inverse Compton and the proton high energy components in various bands, in particular in the GeV band with future missions such as GLAST, and in the X-ray band with Chandra. We find that regime II parameter space is the most favorable regime for high energy emission. The inverse Compton component is detectable by GLAST within hours for bursts at typical cosmological distances, and by Chandra in days if the ambient density is high.

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arXiv2001

Gamma-ray Burst Afterglow with Continuous Energy Injection: Signature of a Highly-Magnetized Millisecond Pulsar

We investigate the consequences of a continuously injecting central engine on the gamma-ray burst afterglow emission, focusing more specifically on a highly-magnetized millisecond pulsar engine. For initial pulsar parameters within a certain region of the parameter space, the afterglow lightcurves are predicted to show a distinctive achromatic bump feature, the onset and duration of which range from minutes to months, depending on the pulsar and the fireball parameters. The detection of or upper limits on such features would provide constraints on the burst progenitor and on magnetar-like central engine models. An achromatic bump such as that in GRB 000301C afterglow may be caused by a millisecond pulsar with P0=3.4 millisecond and Bp=2.7e14 Gauss.

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arXiv2001

Inner magnetospheric accelerators in high magnetic field pulsars

Photon splitting is a QED process that can potentially suppress pair production in the inner magnetosphere of a pulsar with a super-strong magnetic field, and hence, may quench radio emission from these objects. While it is unknown how many splitting modes really operate in super-critical fields, we derive gap parameters of a high magnetic field pulsar for both the vacuum-type gap and the space-charge-limited flow accelerator under the assumption that all three splitting modes permitted by QED operate. The competition between photon splitting and pair production depends on the gap parameters, and the ``photon-splitting-dominant'' line for both cases are derived. We discuss the implications of these results and the possible connection of the high magnetic field pulsars with the anomalous X-ray pulsars and soft gamma-ray repeaters which are conjectured to be magnetars.

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arXiv2001

On the nature of soft Gamma-ray repeaters

Soft gamma-ray repeaters (SGRs) and Anomalous X-ray pulsars (AXPs) are generally accepted to be magnetars. Recently, Zhang, Xu & Qiao (2000, ApJ, 545, L127) proposed an alternative viewpoint about the nature of the SGRs (and AXPs). In this picture, SGR bursts are attributed to the impacts of some comet-like objects with the central bare strange star. Here I briefly review this model, and confront it with the detailed observations of SGRs/AXPs. A comparison to the magnetar model is also presented. Some theoretical issues concerning the nature of the SGRs (and AXPs) are discussed.

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arXiv2000

Off-Beam Gamma-Ray Pulsars and Unidentified EGRET Sources in the Gould Belt

We investigate whether gamma-ray pulsars viewed at a large angle to the neutron star magnetic pole could contribute to the new population of galactic unidentified EGRET sources associated with the Gould Belt. The faint, soft nature of these sources is distinctly different from both the properties of unidentified EGRET sources along the galactic plane and of the known gamma-ray pulsars. We explore the possibility, within the polar cap model, that some of these sources are emission from pulsars seen at lines of sight that miss both the bright gamma-ray cone beams and the radio beam. The off-beam gamma-rays come from high-altitude curvature emission of primary particles, are radiated over a large solid angle and have a much softer spectrum than that of the main beams. We estimate that the detectability of such off-beam emission is about a factor of 4-5 higher than that of the on-beam emission. At least some of the radio-quiet Gould Belt sources detected by EGRET could therefore be such off-beam gamma-ray pulsars. GLAST should be able to detect pulsations in most of these sources.

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arXiv2000

Nature and Nurture: a model for soft gamma-ray repeaters

During supernova explosions, strange stars with almost bare quark surfaces may be formed. Under certain conditions, these stars could be rapidly spun down by the torque exerted by the fossil disks formed from the fall-back materials. They may also receive large kicks and hence, have large proper motion velocities. When these strange stars pass through the spherical ``Oort'' comet cloud formed during the pre-supernova era, they will capture some small-scale comet clouds and collide with some comet-like objects occasionally. These impacts can account for the repeating bursts as observed from the soft gamma repeaters (SGRs). According to this picture, it is expected that SGR 1900+14 will become active again during 2004-2005.

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arXiv2000

What if pulsars are born as strange stars?

The possibility and the implications of the idea, that pulsars are born as strange stars, are explored. Strange stars are very likely to have atmospheres with typical mass of $\sim 5\times 10^{-15}M_\odot$ but bare polar caps almost throughout their lifetimes, if they are produced during supernova explosions. A direct consequence of the bare polar cap is that the binding energies of both positively and negatively charged particles at the bare quark surface are nearly infinity, so that the vacuum polar gap sparking scenario as proposed by Ruderman & Sutherland should operate above the cap, regardless of the sense of the magnetic pole with respect to the rotational pole. Heat can not accumulate on the polar cap region due to the large thermal conductivity on the bare quark surface. We test this ``bare polar cap strange star'' (BPCSS) idea with the present broad band emission data of pulsars, and propose several possible criteria to distinguish BPCSSs from neutron stars.

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arXiv2000

Radio Pulsar Death Line Revisited: Is PSR J2144-3933 Anomalous?

We reinvestigate the radio pulsar ``death lines'' within the framework of two different types of polar cap acceleration models, i.e., the vacuum gap model and the space-charge-limited flow model, with either curvature radiation or inverse Compton scattering photons as the source of pairs. General relativistic frame-dragging is taken into account in both models. We find that the inverse Compton scattering induced space-charge-limited flow model can sustain strong pair production in some long-period pulsars, which allows the newly detected 8.5s pulsar PSR J2144-3933 to be radio loud, without assuming a special neutron star equation-of-state or ad hoc magnetic field configurations.

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arXiv2000

High magnetic field pulsars and magnetars: a unified picture

We propose a unified picture of high magnetic field radio pulsars and magnetars by arguing that they are all rotating high-field neutron stars, but have different orientations of their magnetic axes with respective to their rotation axes. In strong magnetic fields where photon splitting suppresses pair creation near the surface, the high-field pulsars can have active inner accelerators while the anomalous X-ray pulsars cannot. This can account for the very different observed emission characteristics of the anomalous X-ray pulsar 1E 2259+586 and the high field radio pulsar PSR J1814-1744. A predicted consequence of this picture is that radio pulsars having surface magnetic field greater than about $2\times 10^{14}$ G should not exist.

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arXiv2000

The Formation of the Hubble Sequence of Disk Galaxies: The Effects of Early Viscous Evolution

We investigate a model of disk galaxies whereby viscous evolution of the gaseous disk drives material inwards to form a proto-bulge. We start from the standard picture of disk formation through the settling of gas into a dark halo potential well, with the disk initially coming into centrifugal equilibrium with detailed conservation of angular momentum. We derive generic analytic solutions for the disk-halo system after adiabatic compression of the dark halo, with free choice of the input virialized dark halo density profile and of the specific angular momentum distribution. We derive limits on the final density profile of the halo in the central regions. Subsequent viscous evolution of the disk is modelled by a variation of the specific angular momentum distribution of the disk, providing analytic solutions to the final disk structure. The assumption that the viscous evolution timescale and star formation timescale are similar leads to predictions of the properties of the stellar components. Focusing on small `exponential' bulges, ones that may be formed through a disk instability, we investigate the relationship between the assumed initial conditions, such as halo `formation', or assembly, redshift $z_f$, spin parameter $λ$, baryonic fraction $F$, and final disk properties such as global star formation timescale, gas fraction, and bulge-to-disk ratio. We find that the present properties of disks, such as the scale length, are compatible with a higher initial formation redshift if the re-distribution by viscous evolution is included than if it is ignored. We also quantify the dependence of final disk properties on the ratio $F/λ$, thus including the possibility that the baryonic fraction varies from galaxy to galaxy, as perhaps may be inferred from the observations.

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arXiv1999

Gamma-ray and X-ray luminosities from spin-powered pulsars in the full polar cap cascade model

We modify the conventional curvature radiation (inverse Compton scattering) + synchrotron radiation polar cap cascade model by including the inverse Compton scattering of the higher generation pairs. Within the framework of the space-charge-limited-flow acceleration model with frame-dragging proposed by Harding & Muslimov (1998), such a full polar cap cascade scenario can well reproduce the $L_γ\propto (L_{\rm sd})^{1/2}$ and the $L_x \sim 10^{-3} L_{\rm sd}$ dependences observed from the known spin-powered pulsars. According to this model, the ``pulsed'' soft ROSAT-band X-rays from most of the millisecond pulsars might be of thermal origin, if there are no strong multipole magnetic components near their surfaces.

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arXiv1999

Full polar cap cascade scenario: $γ$-ray and X-ray luminosities from spin-powered pulsars

We modify polar cap cascade picture to include the ICS of the higher generation pairs. In such a ``full-cascade'' scenario, not only the perpendicular portion of the energy of the pairs goes to high energy radiation via SR, but the parallel portion of the energy of the pairs can also contribute to high energy emission via ICS with the soft thermal photons from either the full neutron star surface or the hot polar cap. An important output of such a scenario is that the soft tail of the ICS spectrum can naturally result in a non-thermal X-ray component which can contribute to the luminosities observed by ROSAT and ASCA. Here we present an analytic description of such a full polar cap cascade scenario within the framework of Harding & Muslimov acceleration model. We present the theoretical predictions of the $γ$-ray luminosities, the thermal and non-thermal X-ray luminosities for the known spin-powered X-ray pulsars. Our results show that the observed different dependences of the high energy luminosities on the pulsar spin-down luminosities, i.e., $L_γ\propto (L_{\rm sd})^{1/2}$ and $L_x \sim 10^{-3} L_{\rm sd}$, are well reproduced. Our model predicts that the {\em pulsed} soft X-rays in the ROSAT band from most of the millisecond pulsars might be of thermal origin if there is no strong multipole field components near the surfaces of these pulsars.

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arXiv1999

PSR 0943+10: a bare strange star?

Recent work by Rankin & Deshpande strongly suggests that there exist strong ``micro-storms'' rotating around the magnetic axis of the 1.1s pulsar PSR 0943+10. Such a feature hints that most probably the large-voltage vacuum gap proposed by Ruderman & Sutherland (RS) does exist in the pulsar polar cap. However, there are severe arguments against the formation of the RS-type gap in pulsars, since the binding energies of both the Fe ions and the electrons in a neutron star's surface layer is too small to prevent thermionic ejection of the particles from the surface. Here we propose that PSR 0943+10 (probably also most of the other ``drifting'' pulsars) might be bare strange stars rather than normal neutron stars, in which the ``binding energy'' at the surface is merely infinity either for the case of ``pulsar'' or ``anti-pulsar''. It is further proposed that identifying a drifting pulsar as an anti-pulsar is the key criterion to distinguish strange stars from neutron stars.

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arXiv1999

Is Coherence Essential to Account for Pulsar Radio Emission?

Based on definitions, two joint-criteria, namely, the optical-thin constraint and the energy budget constraint, are proposed to judge whether the emission nature of radio pulsars is incoherent or obligatory to be coherent. We find that the widely accepted criterion, $kT_B \le ε$, is not a rational criterion to describe the optical-thin condition, even for the simplest case. The energy budget constraint could be released by introducing a certain efficient radiation mechanism (e.g. the inverse Compton scattering, QL98) with emission power of a single particle as high as a critical value $P_{sing,c} \sim 10^{-4}-10^{-3} {erg s}^{-1}$. This in principle poses the possibilities to interpret high luminosities of pulsars in terms of incoherent emission mechanisms, if the optical-thin constraint could be released by certain mechanism as well. Coherence may not be an essential condition to account for pulsar radio emission.

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arXiv1998

Two-photon annihilation in the pair formation cascades in pulsar polar caps

The importance of the photon-photon pair production process ($γ+ γ^{\prime}\to e^{+}+e^{-}$) to form pair production cascades in pulsar polar caps is investigated within the framework of the Ruderman-Sutherland vacuum gap model. It is found that this process is unimportant if the polar caps are not hot enough, but will play a non-negligible role in the pair formation cascades when the polar cap temperatures are in excess of the critical temperatures, $T_{cri}$, which are around $4\times 10^6K$ when $P=0.1$s and will slowly increase with increasing periods. Compared with the $γ-B$ process, it is found that the two-photon annihilation process may ignite a central spark near the magnetic pole, where $γ-B$ sparks can not be formed due to the local weak curvatures. This central spark is large if the gap is dominated by the ``resonant ICS mode''. The possible connection of these central sparks with the observed pulsar ``core'' emission components is discussed.

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