Da-Ming Wei

A possible macronova in the late afterglow of the long-short burst GRB 060614

Long-duration (>2 s) γ-ray bursts that are believed to originate from the death of massive stars are expected to be accompanied by supernovae. GRB 060614, that lasted 102 s, lacks a supernova-like emission down to very stringent limits and its physical origin is still debated. Here we report the discovery of near-infrared bump that is significantly above the regular decaying afterglow. This red bump is inconsistent with even the weakest known supernova. However, it can arise from a Li-Paczyński macronova—the radioactive decay of debris following a compact binary merger. If this interpretation is correct, GRB 060614 arose from a compact binary merger rather than from the death of a massive star and it was a site of a significant production of heavy r-process elements. The significant ejected mass favours a black hole–neutron star merger but a double neutron star merger cannot be ruled out.

High-energy afterglow emission from gamma-ray bursts

We calculate the very high-energy (sub-GeV to TeV) inverse Compton emission of GRB afterglows. We argue that this emission provides a powerful test of the currently accepted afterglow model. We focus on two processes: synchrotron self-Compton emission within the afterglow blast wave, and external inverse Compton emission which occurs when flare photons (produced by an internal process) pass through the blast wave. We show that if our current interpretations of the Swift X-ray telescope (XRT) data are correct, there should be a canonical high-energy afterglow emission light curve. Our predictions can be tested with high-energy observatories such as GLAST, Whipple, HESS and MAGIC. Under favourable conditions we expect afterglow detections in all these detectors.

High Energy Afterglow from Gamma-ray Bursts

We calculate the very high-energy (sub-GeV to TeV) inverse Compton emission of GRB afterglows. We argue that this emission provides a powerful test of the currently accepted afterglow model. We focus on two processes: synchrotron self-Compton emission within the afterglow blast wave, and external inverse Compton emission which occurs when flare photons (produced by an internal process) pass through the blast wave. We show that if our current interpretations of the Swift X-ray telescope (XRT) data are correct, there should be a canonical high-energy afterglow emission light curve. Our predictions can be tested with high-energy observatories such as GLAST, Whipple, HESS and MAGIC. Under favourable conditions we expect afterglow detections in all these detectors.

REVISITING THE LONG/SOFT-SHORT/HARD CLASSIFICATION OF GAMMA-RAY BURSTS IN THE FERMI ERA

We perform a statistical analysis of the temporal and spectral properties of the latest Fermi gamma-ray bursts (GRBs) to revisit the classification of GRBs. We find that the bimodalities of duration and the energy ratio (E-peak/Fluence) and the anti-correlation between spectral hardness (hardness ratio (HR), peak energy, and spectral index) and duration (T-90) support the long/soft-short/hard classification scheme for Fermi GRBs. The HR-T-90 anti-correlation strongly depends on the spectral shape of GRBs and energy bands, and the bursts with the curved spectra in the typical BATSE energy bands show a tighter anti-correlation than those with the power-law spectra in the typical BAT energy bands. This might explain why the HR-T-90 correlation is not evident for those GRB samples detected by instruments like Swift with a narrower/softer energy bandpass. We also analyze the intrinsic energy correlation for the GRBs with measured redshifts and well-defined peak energies. The current sample suggests E-p,E-rest = 2455x(E-iso/10(52))(0.59) for short GRBs, significantly different from that for long GRBs. However, both the long and short GRBs comply with the same E-p,(rest)-L-iso correlation.

The Macronova in GRB 050709 and the GRB-macronova connection

GRB 050709 was the first short Gamma-ray Burst (sGRB) with an identified optical counterpart. Here we report a reanalysis of the publicly available data of this event and the discovery of a Li-Paczynski macronova/kilonova that dominates the optical/infrared signal at t>2.5 days. Such a signal would arise from 0.05 r-process material launched by a compact binary merger. The implied mass ejection supports the suggestion that compact binary mergers are significant and possibly main sites of heavy r-process nucleosynthesis. Furthermore, we have reanalysed all afterglow data from nearby short and hybrid GRBs (shGRBs). A statistical study of shGRB/macronova connection reveals that macronova may have taken place in all these GRBs, although the fraction as low as 0.18 cannot be ruled out. The identification of two of the three macronova candidates in the I-band implies a more promising detection prospect for ground-based surveys.

Are there cosmological evolution trends on gamma-ray burst features?

The variability of a gamma-ray burst (GRB) is thought to be correlated with its absolute peak luminosity, and this relation had been used to derive an estimate of the redshifts of GRBs. Recently, Amati et al. presented the results of spectral and energetic properties of several GRBs with known redshifts. Here, we analyse the properties of two groups of GRBs: one group with known redshift from afterglow observation and another group with redshift derived from the luminosity-variability relation. We study the redshift dependence of various GRBs features in their cosmological rest frames, including the burst duration, the isotropic luminosity and radiated energy, and the peak energy E-p of nuF(nu) spectra. We find that, for these two groups of GRBs, their properties are all redshift-dependent, i.e. their intrinsic duration, luminosity, radiated energy and peak energy E-p are all correlated with the redshift, which means that there are cosmological evolution effects on gamma-ray burst features, and this can provide an interesting clue to the nature of GRBs. If this is true, then the results also imply that the redshift derived from the luminosity-variability relation may be reliable.

The very early afterglow powered by ultra-relativistic mildly magnetized outflows

In the Poynting Flux-dominated outflow (the initial ratio of the electromagnetic energy flux to the particle energy flux sigma(0) much greater than 1) model for gamma-ray bursts, particularly the gamma-ray emission phase, nearly half of the internally dissipated magnetic energy is converted into the gamma-ray energy emission and the rest is converted into the kinetic energy of the outflow. Consequently, at the end of the gamma-ray burst, sigma decreases significantly (sigma similar to 1 or even smaller). We numerically investigate the very early reverse shock emission powered by such mildly magnetized outflows interacting with medium-uniform interstellar medium (ISM) or stellar wind (WIND). We show that for sigma similar to 0.05-1 and typical parameters of gamma-ray bursts, both the ISM-ejecta interaction and the WIND-ejecta interaction can power very strong optical emission (m(R) similar to 10-12th mag or even brighter). Similar to the very early afterglow powered by the non-magnetized ejecta interacting with the external medium, the main difference between the ISM-ejecta interaction case and the WIND-ejecta interaction case is that, before the reverse shock crosses the ejecta, the R-band emission flux increases rapidly for the former, but for the latter it increases only slightly.

The Light Curve of the Macronova Associated with the Long-Short Burst GRB 060614

The Swift-detected GRB 060614 was a unique burst that straddles an imaginary divide between long- and short-duration gamma-ray bursts (GRBs), and its physical origin has been heavily debated over the years. Recently, a distinct, very soft F814W-band excess at t similar to 13.6 days after the burst was identified in a joint-analysis of VLT and Hubble Space Telescope optical afterglow data of GRB 060614, which has been interpreted as evidence for an accompanying macronova (also called a kilonova). Under the assumption that the afterglow data in the time interval of 1.7-3.0 days after the burst are due to external FS emission, when this assumption is extrapolated to later times it is found that there is an excess of flux in several multi-band photometric observations. This component emerges at similar to 4 days after the burst, and it may represent the first time that a multi-epoch/band light curve of a macronova has been obtained. The macronova associated with GRB 060614 peaked at t less than or similar to 4 days after the burst, which is significantly earlier than that observed for a supernova associated with a long-duration GRB. Due to the limited data, no strong evidence for a temperature evolution is found. We derive a conservative estimate of the macronova rate of similar to 16.3(8.2)(+16.3) Gpc(-3) yr(-1), implying a promising prospect for detecting the gravitational wave radiation from compact-object mergers by upcoming Advanced LIGO/VIRGO/KAGRA detectors (i.e., the rate is R-GW similar to 0.5(-0.25)(+0.5)(D/200 Mpc)3 yr(-1)).

γ-ray burst ultraviolet/optical afterglow polarimetry as a probe of quantum gravity

A possible birefringence effect that arises in quantum gravity leads to a frequency-dependent rotation of the polarization angle of linearly polarized emission from distant sources. Here we use the ultraviolet/optical polarization data of the afterglows of GRB 020813 and GRB 021004 to constrain this effect. We find an upper limit on the Gambini & Pulin birefringence parameter vertical bar eta vertical bar < 2 x 10(-7). This limit is of three orders better than the previous limits from observations of active galactic nuclei and of the Crab pulsar. Much stronger limits may be obtained by the future observation of polarization of the prompt gamma-rays.

HIGH-ENERGY EMISSION OF GRB 130427A: EVIDENCE FOR INVERSE COMPTON RADIATION

A nearby superluminous burst GRB 130427A was simultaneously detected by six gamma-ray space telescopes (Swift, the Fermi GLAST Burst Monitor (GBM)/Large Area Telescope, Konus-Wind, SPI-ACS/INTEGRAL, AGILE, and RHESSI) and by three RAPTOR full-sky persistent monitors. The isotropic gamma-ray energy release is similar to 10(54) erg, rendering it the most powerful explosion among gamma-ray bursts (GRBs) with a redshift z 1 TeV neutrinos from GRB 130427A by IceCube are discussed.