Jin-Jun Geng

Anti-glitch Induced by Collision of a Solid Body with the Magnetar 1E 2259+586

Glitches have been frequently observed in neutron stars. Previously these glitches unexceptionally manifest as sudden spin-ups that can be explained as due to impulsive transfer of angular momentum from the interior superfluid component to the outer solid crust. Alternatively, such spin-up glitches may also be due to large-scale crust-cracking events. However, an unprecedented anti-glitch was recently reported for the magnetar 1E 2259+586. In this case, the magnetar clearly exhibited a sudden spin-down, strongly challenging previous glitch theories. Here we show that the anti-glitch can be well explained by the collision of a small solid body with the magnetar. The intruder has a mass of about

COALESCENCE OF STRANGE-QUARK PLANETS WITH STRANGE STARS: A NEW KIND OF SOURCE FOR GRAVITATIONAL WAVE BURSTS

1.1 \times 10^{21}

REVISITING THE EMISSION FROM RELATIVISTIC BLAST WAVES IN A DENSITY-JUMP MEDIUM

g. Its orbital angular momentum is assumed to be antiparallel to that of the spinning magnetar, so that the sudden spin-down can be naturally accounted for. The observed hard X-ray burst and decaying softer X-ray emission associated with the anti-glitch can also be reasonably explained. Our study indicates that a completely different type of glitches as due to collisions between small bodies and neutron stars should exist and may have already been observed previously. It also hints a new way for studying the capture events by neutron stars: through accurate timing observations of pulsars.

A CORRELATED STUDY OF OPTICAL AND X-RAY AFTERGLOWS OF GRBs

Strange-quark matter (SQM) may be the true ground state of hadronic matter, indicating that the observed pulsars may actually be strange stars (SSs), but not neutron stars. According to the SQM hypothesis, the existence of a hydrostatically stable sequence of SQM stars has been predicted, ranging from 1 to 2 solar mass SSs, to smaller strange dwarfs and even strange planets. While gravitational wave (GW) astronomy is expected to open a new window to the universe, it will shed light on the search for SQM stars. Here we show that due to their extreme compactness, strange planets can spiral very close to their host SSs without being tidally disrupted. Like inspiraling neutron stars or black holes, these systems would serve as new sources of GW bursts, producing strong GWs at the final stage. The events occurring in our local universe can be detected by upcoming GW detectors, such as Advanced LIGO and the Einstein Telescope. This effect provides a unique probe to SQM objects and is hopefully a powerful tool for testing the SQM hypothesis.

A Model of White Dwarf Pulsar AR Scorpii

Re-brightening bumps are frequently observed in gamma-ray burst (GRB) afterglows. Many scenarios have been proposed to interpret the origin of these bumps, of which a blast wave encountering a density-jump in the circumburst environment has been questioned by recent works. We develop a set of differential equations to calculate the relativistic outflow encountering the density-jump by extending the work of Huang et al. (1999). This approach is a semi-analytic method and is very convenient. Our results show that late high-amplitude bumps can not be produced under common conditions, only short plateau may emerge even when the encounter occurs at early time (

The Origin of the Prompt Emission for Short GRB 170817A: Photosphere Emission or Synchrotron Emission?

< 10^4

Brightening X-Ray/Optical/Radio Emission of GW170817/SGRB 170817A: Evidence for an Electron–Positron Wind from the Central Engine?

s). In general, our results disfavor the density-jump origin for those observed bumps, which is consistent with the conclusion drawn from full hydrodynamics studies. The bumps thus should be due to other scenarios.

PROPAGATION OF RELATIVISTIC, HYDRODYNAMIC, INTERMITTENT JETS IN A ROTATING, COLLAPSING GRB PROGENITOR STAR

We study an extensive sample of 87 gamma-ray bursts (GRBs) for which there are well-sampled and simultaneous optical and X-ray light curves. We extract the cleanest possible signal of the afterglow component. and compare the temporal behaviors of the X-ray light. curve, observed by Swift XRT, and optical data, observed by UVOT and ground-based telescopes for each individual burst. Overall we find that 62% of the GRBs. are consistent with the standard afterglow model. When more advanced modeling is invoked, up to 91% of the bursts in our sample may be consistent with the external-shock model. A large fraction of these bursts are consistent with occurring in a constant interstellar density medium (61%) while only 39% of them occur in a wind-like medium. Only nine cases have afterglow light curves that exactly match the standard fireball model prediction, having a single power-law decay in both energy bands that are observed during their entire duration. In particular, for the bursts with chromatic behavior, additional model assumptions must be made over limited segments of the light curves in order for these bursts to fully agree with the external-shock model. Interestingly, for 54% of the X-ray and 40% of the optical band observations, the end of the shallow decay (t(similar to-0.5)) period coincides with the jet-break (t(similar to-p)) time, causing an abrupt change in decay slope. The fraction of the burst that is consistent with the external-shock model is independent of the observational epochs in the rest frame of GRBs. Moreover, no cases can be explained by the cooling frequency crossing the X-ray or optical band.

Continued Brightening of the Afterglow of GW170817/GRB 170817A as Being Due to a Delayed Energy Injection

A 3.56-hour white dwarf (WD) - M dwarf (MD) close binary system, AR Scorpii, was recently reported to show pulsating emission in radio, IR, optical, and UV, with a 1.97-minute period, which suggests the existence of a WD with a rotation period of 1.95 minutes. We propose a model to explain the temporal and spectral characteristics of the system. The WD is a nearly perpendicular rotator, with both open field line beams sweeping the MD stellar wind periodically. A bow shock propagating into the stellar wind accelerates electrons in the wind. Synchrotron radiation of these shocked electrons can naturally account for the broad-band (from radio to X-rays) spectral energy distribution of the system.

Probing Magnetic Fields of GRB X-Ray Flares with Polarization Observations

The first gravitational-wave event from the merger of a binary neutron star system (GW170817) was detected recently. The associated short gamma-ray burst (GRB 170817A) has a low isotropic luminosity (