Fermi observations of high-energy gamma-ray emission from GRB 080916C shows that its spectrum is consistent with the band function from MeV to tens of GeV. Assuming one single emission mechanism dominates in the whole energy range, we show that this spectrum is consistent with synchrotron origin by shock-accelerated electrons. The simple electron inverse-Compton model and the hadronic model are found to be less viable. In the synchrotron scenario, the synchrotron self-Compton scattering is likely to be in the Klein-Nishina (KN) regime and therefore the resulting high-energy emission is subdominant, even though the magnetic field energy density is lower than that in relativistic electrons. The KN inverse-Compton cooling may also affect the low-energy electron number distribution and hence results in a low-energy synchrotron photon spectrum n(ν) α ν ^-1 below the peak energy. Under the framework of the electron synchrotron interpretation, we constrain the shock microphysical parameters and derive a lower limit of the upstream magnetic fields. The detection of synchrotron emission extending to about 70 GeV in the source frame in GRB 080916C favors the Bohm diffusive shock acceleration if the bulk Lorentz factor of the relativistic outflow is not significantly greater than thousands. © 2009. The American Astronomical Society. All rights reserved.

Wang Xiang-Yu;Dai Zi-Gao;Li Zhuo;M\u00e9sz\u00e1ros Peter

Astrophysical Journal

2009

Active galactic nuclei and gamma-ray bursts (GRBs) are powerful astrophysical events with relativistic jets. In this Letter, the broadband spectral properties of GRBs and well-observed blazars are compared. The distribution of GRBs is consistent with the well-known blazar sequence including the νL ν(5 GHz)-αRX and νL ν(5 GHz)-νpeak correlations, where αRX is defined as the broadband spectral slope in radio-to-X-ray bands, and νpeak is defined as the spectral peak frequency. Moreover, GRBs occupy the low radio luminosity end of these sequences. These two correlations suggest that GRBs could have a radiation process, i.e., synchrotron radiation, similar to blazars both in the prompt emission and afterglow phases. © 2014. The American Astronomical Society. All rights reserved.

Wang F. Y.;Yi S. X.;Dai Z. G.

Astrophysical Journal Letters

2014

We investigate the mass distribution of long gamma-ray burst (GRB) host galaxies and the redshift distribution of long GRBs by considering that long GRBs occur in low-metallicity environments. We calculate the upper limit on the stellar mass of a galaxy which can produce long GRBs by utilizing the mass-metallicity (M-Z) relation of galaxies. After comparing with the observed GRB host galaxies masses, we find that the observed GRB host galaxy masses can fit the predicted masses well if GRBs occur in low-metallicity 12 + log (O/H)KK04 < 8.7. GRB host galaxies have low metallicity, low mass, and high star formation rate compared with galaxies of seventh data release of the Sloan Digital Sky Survey. We also study the cumulative redshift distribution of the latest Swift long GRBs by adding dark GRBs and 10 new GRBs redshifts from the TOUGH survey. The observed discrepancy between the GRB rate and the star formation history can be reconciled by considering that GRBs tend to occur in low-metallicity galaxies with 12 + log (O/H)KK04 < 8.7. We conclude that the metallicity cutoff that can produce long GRBs is about 12 + log (O/H)KK04 < 8.7 from the host mass distribution and redshift distribution. © 2014. The American Astronomical Society. All rights reserved.

Wang F. Y.;Dai Z. G.

Astrophysical Journal Supplement Series

2014

Wang F. Y.;Dai Z. G.

Astrophysical Journal Supplement Series

2014

© 2015. The American Astronomical Society. All rights reserved. Recently, research performed by two groups has revealed that the magnetar spin-down energy injection model with full energy trapping can explain the early-time light curves of SN 2010gx, SN 2013dg, LSQ12dlf, SSS120810, and CSS121015 but fails to fit the late-time light curves of these superluminous supernovae (SLSNe). These results imply that the original magnetar-powered model is challenged in explaining these SLSNe. Our paper aims to simultaneously explain both the early- and late-time data/upper limits by considering the leakage of hard emissions. We incorporate quantitatively the leakage effect into the original magnetar-powered model and derive a new semianalytical equation. Comparing the light curves reproduced by our revised magnetar-powered model with the observed data and/or upper limits of these five SLSNe, we found that the late-time light curves reproduced by our semianalytical equation are in good agreement with the late-time observed data and/or upper limits of SN 2010gx, CSS121015, SN 2013dg, and LSQ12dlf and the late-time excess of SSS120810, indicating that the magnetar-powered model might be responsible for these SLSNe and that the gamma-ray and X-ray leakages are unavoidable when the hard photons were down-Comptonized to softer photons. To determine the details of the leakage effect and unveil the nature of SLSNe, more high-quality bolometric light curves and spectra of SLSNe are required.

Wang S. Q.;Wang L. J.;Dai Z. G.;Wu X. F.

Astrophysical Journal

2015

© 2015. The American Astronomical Society. All rights reserved. GRB 080503 is a short gamma-ray burst (GRB) detected by Swift and has been classified as a GRB originating from a compact star merger. The soft extended emission and the simultaneous late re-brightening in both the X-ray and optical afterglow light curves raise interesting questions regarding its physical origin. We show that the broadband data of GRB 080503 can be well explained within the framework of the double neutron star merger model, provided that the merger remnant is a rapidly rotating massive neutron star with an extremely high magnetic field (i.e., a millisecond magnetar). We show that the late optical re-brightening is consistent with the emission from a magnetar-powered "merger-nova." This adds one more case to the growing sample of merger-novae associated with short GRBs. The soft extended emission and the late X-ray excess emission are well connected through a magnetar dipole spin-down luminosity evolution function, suggesting that direct magnetic dissipation is the mechanism to produce these X-rays. The X-ray emission initially leaks from a hole in the merger ejecta pierced by the short GRB jet. The hole subsequently closes after the magnetar spins down and the magnetic pressure drops below ram pressure. The X-ray photons are then trapped behind the merger-nova ejecta until the ejecta becomes optically thin at a later time. This explains the essentially simultaneous re-brightening in both the optical and X-ray light curves. Within this model, future gravitational-wave sources could be associated with a bright X-ray counterpart along with the merger-nova, even if the short GRB jet beams away from Earth.

Gao He;Ding Xuan;Wu Xue-Feng;Dai Zi-Gao;Zhang Bing

Astrophysical Journal

2015

© ESO, 2015. We use two model-independent methods to standardize long gamma-ray bursts (GRBs) using the E iso - E p correlation (log E iso = a + blog E p ), where E iso is the isotropic-equivalent gamma-ray energy and E p is the spectral peak energy. We update 42 long GRBs and attempt to constrain the cosmological parameters. The full sample contains 151 long GRBs with redshifts from 0.0331 to 8.2. The first method is the simultaneous fitting method. We take the extrinsic scatter σ ext into account and assign it to the parameter E iso . The best-fitting values are a = 49.15 ± 0.26, b = 1.42 ± 0.11, σ ext = 0.34 ± 0.03 and Ω m = 0.79 in the flat ΛCDM model. The constraint on Ω m is 0.55 < Ω m < 1 at the 1σ confidence level. If reduced χ ² method is used, the best-fit results are a = 48.96 ± 0.18, b = 1.52 ± 0.08, and Ω m = 0.50 ± 0.12. The second method uses type Ia supernovae (SNe Ia) to calibrate the E iso - E p correlation. We calibrate 90 high-redshift GRBs in the redshift range from 1.44 to 8.1. The cosmological constraints from these 90 GRBs are Ω m = 0.23 ^+0.06-0.04 for flat ΛCDM and Ω m = 0.18 ± 0.11 and Ω Λ = 0.46 ± 0.51 for non-flat ΛCDM. For the combination of GRB and SNe Ia sample, we obtain Ω m = 0.271 ± 0.019 and h = 0.701 ± 0.002 for the flat ΛCDM and the non-flat ΛCDM, and the results are Ω m = 0.225 ± 0.044, Ω Λ = 0.640 ± 0.082, and h = 0.698 ± 0.004. These results from calibrated GRBs are consistent with that of SNe Ia. Meanwhile, the combined data can improve cosmological constraints significantly, compared to SNe Ia alone. Our results show that the E iso - E p correlation is promising to probe the high-redshift Universe.

Wang J. S.;Wang F. Y.;Dai Z. G.;Cheng K. S.

Astronomy and Astrophysics

2016

© 2016. The American Astronomical Society. All rights reserved. In this paper we show that the most luminous supernova discovered very recently, ASASSN-15lh, could have been powered by a newborn ultra-strongly magnetized pulsar, which initially rotates near the Kepler limit. We find that if this pulsar is a neutron star, its rotational energy could be quickly lost as a result of gravitational-radiation-driven r-mode instability; if it is a strange quark star (SQS), however, this instability is highly suppressed due to a large bulk viscosity associated with the nonleptonic weak interaction among quarks and thus most of its rotational energy could be extracted to drive ASASSN-15lh. Therefore, we conclude that such an ultra-energetic supernova provides a possible signature for the birth of an SQS.

Dai Z. G.;Wang S. Q.;Wang J. S.;Wang L. J.;Yu Y. W.

Astrophysical Journal

2016

© 2016, The American Astronomical Society. All rights reserved. Millisecond magnetars can be formed via several channels: core collapse of massive stars, accretion-induced collapse of white dwarfs (WDs), double WD mergers, double neutron star (NS) mergers, and WD-NS mergers. Because the mass of ejecta from these channels could be quite different, their light curves are also expected to be diverse. We evaluate the dynamic evolution of optical transients powered by millisecond magnetars. We find that the magnetar with a short spin-down timescale converts its rotational energy mostly into the kinetic energy of the transient, while the energy of a magnetar with a long spin-down timescale goes into radiation of the transient. This leads us to speculate that hypernovae could be powered by magnetars with short spin-down timescales. At late times the optical transients will gradually evolve into a nebular phase because of the photospheric recession. We treat the photosphere and nebula separately because their radiation mechanisms are different. In some cases the ejecta could be light enough that the magnetar can accelerate it to a relativistic speed. It is well known that the peak luminosity of a supernova (SN) occurs when the luminosity is equal to the instantaneous energy input rate, as shown by Arnett. We show that photospheric recession and relativistic motion can modify this law. The photospheric recession always leads to a delay of the peak time t pk relative to the time t x at which the SN luminosity equals the instantaneous energy input rate. Relativistic motion, however, may change this result significantly.

Wang Ling-Jun;Xu Dong;Han Yan-Hui;Wei Jian-Yan;Wang S. Q.;Dai Z. G.;Wu X. F.

Astrophysical Journal

2016

© 2016 Elsevier B.V. The neutron-rich matter ejected by compact object mergers (neutron star-neutron star merger and neutron star-black hole merger) provides one of the most important environments for the syntheses of r-process elements. In recent seventeen years, theoretical studies suggested that the energy produced during the decay of r-process elements will form optical/near infrared (NIR) radiations after thermalization. This type of optical/NIR transients are called Li-Paczyński novae, or LP-novae for short. Since the typical peak brightness of LP-novae is ~1000 times brighter than that of a typical nova, they are also called Kilonovae. Besides, both theoretical and observational studies have showed, directly or indirectly, that under certain conditions, the compact object mergers can produce the gamma-ray bursts with a rather short duration of T 90 ≤ 2s (SGRBs for short), and most SGRBs may come from compact object mergers. After the identification of SGRB afterglows, dedicated searches of the LP-novae associated with SGRBs have been taken. In this review we present the theoretical progress of LP-novae in recent seventeen years, and the observational aspect will appear in a upcoming paper.

Wang Shan-qin;Dai Zi-gao;Wu Xue-fen

Chinese Astronomy and Astrophysics

2016