arXiv subjects

Bing Zhang

Publications and source records attributed to Bing Zhang.

At least 577 records · Page 32Linked to original sources

arXiv2005

Pulsar shadow as the origin of double notches in radio pulse profiles

We present the model of eclipsing a rotating, spatially extended source of directional emission by a central absorber, and apply it to the pulsar magnetosphere. The model assumes the radially extended inward radio emission along the local direction of the magnetic field, and the pulsar as the absorber. The geometry of the magnetic field lines of the rotating dipole is favourable for the double eclipse events, which we identify with the double notches observed in pulse profiles of nearby pulsars. For pulsars with large dipole inclinations 70 <~ alpha <~ 110 deg the double notches are predicted to occur within a narrow phase range of 20 to 30 deg before the main radio peak. Application of the model to PSR B0950+08 establishes it as a nearly orthogonal rotator (alpha =~ 75 deg, beta =~ -10 deg) with many pulse components naturally interpreted in terms of the inward radio emission from a large range of altitudes. The inward components include the intermittently strong, leading component of the main pulse, which would traditionally have been interpeted as a conal emission in the outward direction. The model also identifies the magnetic field lines along which the radially extended inward radio emission occurs in B0950+08. These have a narrow range of the footprint parameter s close to 1.1 (closed field line region, near the last open field lines). We describe directional characteristics of inward emission from the radially extended region and compare them with characteristics of extended outward emission. Our work shows that pulse profiles of at least some pulsars may be a superposition of both inward and outward emission.

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arXiv2005

Model-Independent Multi-Variable Gamma-Ray Burst Luminosity Indicator and Its Possible Cosmological Implications

We present a multi-variable regression analysis to several observable quantities for a sample of 15 gamma-ray bursts (GRBs). The observables used in the analysis includes the isotropic gamma-ray energy (E_{gamma,iso}), the peak energy of the nu F_nu spectrum in the rest frame (E_p'), and the rest frame break time of the optical afterglow light curves(t_b'). A strong dependence of E_{gamma, iso on E_p' and t_b' is derived. We also extend the analysis to the isotropic afterglow energies in the X-ray and the optical bands, respectively, and find that they are essentially not correlated with E_p' and t_b'. Regarding the E_{gamma, iso(E_p', t_b') relationship as a luminosity indicator, we explore the possible constraints on the cosmological parameters using the GRB sample. Our results indicate 0.13<omega_M<0.49 and 0.50<Omega_Lambda<0.85 for a flat Universe. The decelerating factor and its cosmological evolution, and the transition redshift are also investigated. Through Monte Carlo simulations, we find that the GRB sample satisfying our relationship observationally tends to be a soft and bright one, and that the contraints on the cosmological parameters can be much improved either by the enlargement of the sample size or by the increase of the observational precision. Furthermore, the detections of a few high redshift GRBs satisfying the correlation could greatly tighten the constraints (abridged).

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arXiv2005

A Two-Component Explosion Model for the Giant Flare and Radio Afterglow from SGR1806-20

The brightest giant flare from the soft $γ$-ray repeater (SGR) 1806-20 was detected on 2004 December 27. The isotropic-equivalent energy release of this burst is at least one order of magnitude more energetic than those of the two other SGR giant flares. Starting from about one week after the burst, a very bright ($\sim 80$ mJy), fading radio afterglow was detected. Follow-up observations revealed the multi-frequency light curves of the afterglow and the temporal evolution of the source size. Here we show that these observations can be understood in a two-component explosion model. In this model, one component is a relativistic collimated outflow responsible for the initial giant flare and the early afterglow, and another component is a subrelativistic wider outflow responsible for the late afterglow. We also discuss triggering mechanisms of these two components within the framework of the magnetar model.

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arXiv2005

Early photon-shock interaction in stellar wind: sub-GeV photon flash and high energy neutrino emission from long GRBs

For gamma-ray bursts (GRBs) born in a stellar wind, as the reverse shock crosses the ejecta, usually the shocked regions are still precipitated by the prompt MeV γ-ray emission. Because of the tight overlapping of the MeV photon flow with the shocked regions, the optical depth for the GeV photons produced in the shocks is very large. These high energy photons are absorbed by the MeV photon flow and generate relativistic e^\pm pairs. These pairs re-scatter the soft X-ray photons from the forward shock as well as the prompt γ-ray photons and power detectable high energy emission, significant part of which is in the sub-GeV energy range. Since the total energy contained in the forward shock region and the reverse shock region are comparable, the predicted sub-GeV emission is independent on whether the GRB ejecta are magnetized (in which case the reverse shock IC and synchrotron self-Compton emission is suppressed). As a result, a sub-GeV flash is a generic signature for the GRB wind model, and it should be typically detectable by the future {\em Gamma-Ray Large Area Telescope} (GLAST). Overlapping also influence neutrino emission. Besides the 10^{15} \sim 10^{17} eV neutrino emission powered by the interaction of the shock accelerated protons with the synchrotron photons in both the forward and reverse shock regions, there comes another $10^{14}$eV neutrino emission component powered by protons interacting with the MeV photon flow. This last component has a similar spectrum to the one generated in the internal shock phase, but the typical energy is slightly lower.

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arXiv2005

Early optical-IR emission from GRB 041219a: neutron-rich internal shocks and a mildly magnetized external reverse shock

Very early optical and near infrared (IR) emission was discovered accompanying the long gamma-ray burst (GRB) 041219a. We show that the optical/IR flash tracking the gamma-ray lightcurve during the prompt emission could be understood as emission from neutron-rich internal shocks, as has been suggested by Fan & Wei. The early K_s-band afterglow lightcurve after the prompt phase could be well-modeled as the superposition of a reverse shock and a forward shock component. The radio data also support the reverse shock interpretation.

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arXiv2005

Variabilities of Gamma-Ray Burst Afterglows: Long-acting Engine, Anisotropic Jet or Many Fluctuating Regions?

We show that simple kinematic arguments can give limits on the timescale and amplitude of variabilities in gamma-ray burst (GRB) afterglows, especially when the variability timescale is shorter than the observed time since the burst Δt < t. These limits help us to identify the sources of afterglow variability. The afterglows of GRB 011211 and GRB 021004 marginally violate these limits. If such violation is confirmed by the Swift satellite, a possible explanation is that (1) the compact objects that power GRB jets continue to eject an intermittent outflow for a very long timescale (> 1 day), (2) the GRB jet from the central engine has a temporal anisotropy with a large brightness contrast > 10 and small angular structure < 10^{-2}, or (3) many (> 10^{3}) regions fluctuate simultaneously in the emitting site.

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arXiv2005

High Energy Afterglow Emission from Giant Flares of Soft Gamma-Ray Repeaters: The Case of the 2004 December 27 Event from SGR 1806-20

We discuss the high enegry afterglow emission (including high energy photons, neutrinos and cosmic rays) following the 2004 December 27 Giant Flare from SGR 1806-20. If the initial outflow is relativistic with a bulk Lorentz factor Γ_0\sim {\rm tens}, the high-energy tail of the synchrotron emission from electrons in the forward shock region gives rise to a prominent sub-GeV emission, if the electron spectrum is hard enough and if the intial Lorentz factor is high enough. This signal could serve as a diagnosis of the initial Lorentz factor of the giant flare outflow. This component is potentially detectable by GLAST if a similar giant flare occurs in the GLAST era. With the available 10 MeV data, we constrain that Γ_0 < 50 if the electron distribution is a single power law. For a broken power law distribution of electrons, a higher Γ_0 is allowed. At energies higher than 1 GeV, the flux is lower because of a high energy cut off of the synchrotron emission component. The synchrotron self-Compton emission component and the inverse Compton scattering component off the photons in the giant flare oscillation tail are also considered, but they are found not significant given a moderate Γ_0 (e.g. \leq 10). The forward shock also accelerates cosmic rays to the maximum energy 10^{17}eV, and generate neutrinos with a typical energy 10^{14}eV through photomeson interaction with the X-ray tail photons. However, they are too weak to be detectable.

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arXiv2005

Reply to Lyutikov's comments on Zhang & Kobayashi (2005)

Lyutikov (astro-ph/0503505) raised a valid point that for shock deceleration of a highly magnetized outflow, the fate of the magnetic fields after shock crossing should be considered. However, his comment that the deceleration radius should be defined by the total energy rather than by the baryonic kinetic energy is incorrect. As strictly derived from the shock jump conditions in Zhang & Kobayashi (2005), during the reverse shock crossing process the magnetic energy is not tapped. As a result, the fireball deceleration radius is defined by the baryonic energy only. The magnetic energy is expected to be transferred to the circumburst medium after the reverse shock disappears. The evolution of the system then mimicks a continuously-fed fireball. As a result, Lyutikov's naive conclusion that the forward shock dynamics is independent on the ejecta content is also incorrect. The shock deceleration dynamics and the reverse shock calculation presented in Zhang & Kobayashi (2005) are robust and correct.

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arXiv2005

Gamma-ray burst early afterglows: reverse shock emission from an arbitrarily magnetized ejecta

Evidence suggests that the gamma-ray burst (GRB) ejecta is likely magnetized, although the degree of magnetization of the ejecta is unknown. We derive a rigorous analytical solution for the relativistic 90 degree shocks under the ideal MHD condition, and use them to study the reverse shock emission properties of an arbitrarily magnetized ejecta. Contrary to the previous belief, we find that strong relativistic shocks still exist in the high-sigma limit. Assuming a constant density of the circumburst medium, we study the shell-medium interaction in detail and categorize various critical radii for shell evolution. With typical GRB parameters, a reverse shock exists when sigma is less than a few tens or a few hundreds. The shell evolution can be still categorized into the thick and thin shell regimes, but the separation between the two regime now depends on sigma and the thick shell regime greatly shrinks at high-sigma. The early optical afterglow lightcurves are calculated for GRBs with a wide range of the sigma values. We find that the reverse shock emission level increases steadily with sigma initially, but starts to decline when sigma becomes larger than unity. In the high-sigma regime the reverse shock peak is usually broadened due to the separation of the shock crossing radius and the deceleration radius in the thin shell regime. The early afterglow data and tight upper limits of known GRBs could be understood within the theoretical framework developed in this paper, with the inferred sigma value varying in a wide range.

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arXiv2005

XMM-Newton observation of the drifting pulsar B0943+10

Radio pulsar subpulse drifting has been interpreted as rotation of sub-beams (sparks) of pair plasma produced by intermittent breakdowns of an inner vacuum gap above the pulsar polar cap. This model also predicts strong thermal X-ray emission from the polar cap caused by inflowing particles created in spark discharges. We have observed the best-studied drifting pulsar B0943+10 with XMM-Newton and detected a point source coincident with the radio pulsar position. Its spectrum could be fitted with a thermal blackbody model, although a power-law model is also acceptable. The thermal fit gives a bolometric luminosity L_bol ~ 5 x 10^{28} erg/s and a surface area A ~ 10^3 (T/3MK)^{-4} m^2, much smaller than the conventional polar cap area, 6 x 10^4 m^2. Such thermal radiation can be interpreted as emitted from footprints of sparks drifting in an inner gap of a height h ~ 0.1 - 0.2 r_pc, where r_pc is the polar cap radius. However, the original vacuum gap model by Ruderman and Sutherland requires some modification to reconcile the X-ray and radio data.

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arXiv2005

Reversals of radio emission direction in PSR B1822-09

The pulse profile of pulsar B1822-09 exhibits a very peculiar kind of mode changing: a "precursor" appearing just in front of the Main-Pulse (MP) exhibits periods of nulling, during which an interpulse (IP) becomes detectable at rotation phase separated by roughly 180 deg from the precursor. We propose that this bizarre phenomenon, which requires an information transfer between the two components, occurs by means of reversal of a direction of coherent radio emission generated in the same emission region. This interpretation naturally explains the lack of weak radio emission in the off-pulse regions, as well as the problem of information transfer between emission regions associated with the MP precursor and the IP. The reversals also imply nulling. The model has profound physical implications: (i) the mechanism of coherent radio emission must allow radiation into two, opposite, intermittently changing directions; (ii) the radio waves must be able to propagate through inner regions of the neutron star magnetosphere with strong magnetic field. Most importantly, the model implies inward radio emission in pulsar magnetosphere.

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arXiv2005

The very early optical afterglow of GRBs, revealing the nature of the ejecta

We show that if the GRB ejecta itself is magnetized or neutron-rich, the very early afterglow of GRBs is very different from that powered by a pure ion fireball. In the Swift era, with the well monitored early afterglow data, we can potentially diagnose the ejecta composition and reveal the nature of the central GRB engine.

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arXiv2005

Optical afterglows of short Gamma-ray Bursts and GRB 040924

Short-duration Gamma-ray bursts (GRBs) (\leq 2{\rm s}) have remained a mystery due to the lack of afterglow detection until recently. The models to interpret short GRBs invoke distinct progenitor scenarios. Here we present a generic analysis of short GRB afterglows, and calculate the optical lightcurves of short GRBs within the framework of different progenitor models. We show that all these optical afterglows are bright enough to be detected by the Ultraviolet and Optical Telescope (UVOT) on board the {\em Swift} observatory, and that different models could be distinguished with a well-monitored lightcurve. We also model the afterglow data of the recently discovered short burst GRB 040924. We find that the limited data are consistent with a low medium density environment which is consistent with the pre-concept of the compact-star merger progenitor model, although the models with a collapsar progenitor are not ruled out.

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arXiv2005

Swift XRT and VLT Observations of the Afterglow of GRB 041223

The Swift Gamma-Ray Burst Explorer, launched on 2004 November 20, is a multiwavelength, autonomous, rapid-slewing observatory for gamma-ray burst (GRB) astronomy. On 2004 December 23, during the activation phase of the mission, the Swift X-Ray Telescope (XRT) was pointed at a burst discovered earlier that day by the Swift Burst Alert Telescope. A fading, uncataloged X-ray source was discovered by the XRT and was observed over a period of about 3 hours, beginning 4.6 hours after the burst. The X-ray detection triggered a VLT observation of the optical/NIR counterpart, located about 1.1 arcseconds from the XRT position. The X-ray counterpart faded rapidly, with a power law index of -1.72 +/- 0.20. The average unabsorbed X-ray flux 4.6-7.9 hours after the burst was 6.5 x 10^{-12} erg cm^{-2} s^{-1} in the 0.5-10 keV band, for a power-law spectrum of photon index 2.02 +/- 0.13 with Galactic absorption. The NIR counterpart was observed at three epochs between 16 and 87 hours after the burst, and faded with a power-law index of -1.14 +/- 0.08 with a reddening-corrected SED power-law slope of -0.40 +/- 0.03. We find that the X-ray and NIR data are consistent with a two-component jet in a wind medium, with an early jet break in the narrow component and an underlying electron index of 1.8-2.0.

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arXiv2004

Global Test of a Quasi-Universal Gamma-Ray Burst Jet Model Through Monte-Carlo Simulations

The possibility that long gamma-ray burst (GRB) jets are structured receives growing attention recently, and we have suggested that most GRBs and their softer, less energetic fraternity, X-ray flashes (XRFs), can be understood within a quasi-universal structured jet picture, given that the jet structure for each individual burst is a Gaussian or similar function. Here we perform a global test on such a quasi-universal Gaussian-like structured jet by comparing Monte-Carlo simulation results with a broad spectrum of observational data. Using the same set of input parameters as in the previous work (Zhang et al. 2004), we confront the model with more observational constraints. These constraints include the burst redshift distribution, jet break angle distribution, two-dimensional redshift vs. jet break angle distribution, luminosity function, and Log N - Log P distribution. The results indicate that the model is generally compatible with the data. This conclusion, together with our previous tests with the observed jet break angle vs. isotropic energy and observed peak energy vs. fluence relations, suggests that current long GRB and XRF data are generally consistent with such a quasi-standard-energy and quasi-standard-angle jet picture. With future homogeneous burst samples (such as the one to be retrieved from the Swift mission), the refined GRB jet structure can be further constrained through a global comparison between various observed and predicted burst property distributions and relations.

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arXiv2004

Gamma-ray burst internal shocks with magnetization

(Please note that the abstract has been significantly shorted) We investigate Gamma-ray Burst (GRB) internal shocks with moderate magnetization, with the magnetization parameter $σ$ ranging from 0.001 to 10. Possible magnetic dissipation in the stripped magnetized shells is also taken into account through introducing a parameter $k$ ($0<k\leq 1$), which is the ratio of the electric field strength of the downstream and the upstream. By solving the general MHD jump conditions, we show that the dynamic evolution of the shock with magnetic dissipation is different from the familiar one obtained in the ideal MHD limit. In view of the possible high degree of linear polarization of GRB 021206 and the identification of a possible highly magnetized flow in GRB 990123 and GRB 021211, we suggest that a mildly magnetized internal shock model ($0.01<σ<1$) with moderate magnetic dissipation is a good candidate to explain the GRB prompt emission data.

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arXiv2004

A model for the flaring radio emission in the double pulsar system J0737-3039

The binary J0737-3039 A & B includes two pulsars in a highly relativistic orbit. The pulsed radio flux from pulsar B brightens considerably during two portions of each orbit. This phenomenon cannot be naturally triggered by the illumination of $γ$-rays or X-rays from pulsar A or the bow shock around pulsar B. Instead, we explain these periodic flares quantitatively as episodes during which pairs from pulsar A's wind flow into the open field line region of pulsar B and emit curvature radiation at radio frequencies within an altitude of $\sim 10^8$ cm. The radio photons then travel through B's magnetosphere and eventually reach the observer on the other side of the pulsar. Our model requires that A's wind be anisotropic and that B's spin axis be somewhat misaligned relative to the orbital angular momentum. We estimate the expected $γ$-ray and X-ray emission from the system.

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arXiv2004

GeV and higher energy photon interactions in gamma-ray burst fireballs and surroundings

We have calculated the opacities and secondary production mechanisms of high energy photons arising in gamma-ray burst internal shocks, using exact cross-sections for the relevant processes. We find that for reasonable choices of parameters, photons in the range of 10's to 100's of GeV may be emitted in the prompt phase. Photons above this range are subject to electron-positron pair production with fireball photons and would be absent from the spectrum escaping the gamma-ray burst. We find that, in such cases, the fireball becomes optically thin again at ultra-high energies ($\gtrsim$ PeV). On the other hand, for sufficiently large fireball bulk Lorentz factors, the fireball is optically thin at all energies. Both for $γγ$ self-absorbed and optically thin cases, the escaping high energy photons can interact with infra-red and microwave background photons to produce delayed secondary photons in the GeV-TeV range. These may be observable with GLAST, or at low redshifts with ground-based air Cherenkov telescopes. Detection of the primary prompt spectrum constrains the bulk Lorentz factor, while detection of delayed secondary gamma-rays would provide a consistency check for the primary spectrum and the bulk Lorentz factor as well as constraints on the intergalactic magnetic field strength.

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