It was widely shown that the cosmological parameters and dark energy can be constrained by using type-Ia supernovae (SNe Ia). Recently gamma-ray bursts (GRBs) were also argued to be promising standard candles for cosmography. In this paper, We calibrate the luminosity relations of GRBs and present constraints on cosmographic parameters by combining a recent GRB sample with SNe Ia. We find the ΛCDM model is consistent with the current data using the model independent constraints on cosmographic parameters. Furthermore, we find that the XCDM model and generalized Chaplygin gas(GCG) model could be distinguished by snap parameters. © 2008 American Institute of Physics.

Wang F. Y.;Dai Z. G.

Aip Conference Proceedings

2008

Context. Swift observations suggest that the X-ray afterglow emission of some gamma-ray bursts (GRB) may have internal origins, and the conventional external shock (ES) cannot be the exclusive source of the afterglow emission. Aims. If the central compact objects of some GRBs are millisecond magentars, the magnetar winds could play an important role in the (internal) X-ray afterglow emission, which is our focus here. Methods. The dynamics and the synchrotron radiation of the termination shock (TS) of the magmnetar winds, as well as the simultaneous GRB ES, are investigated by considering the magnetization of the winds. Results. As a result of the competition between the emission of the wind TS and the GRB ES, two basic types of X-ray afterglows are predicted, i.e., the TS-dominated and the ES-dominated types. Moreover, our results also show that both of the two types of afterglows have a shallow-decay phase and a normal-decay one, as observed by the Swift satellite. This indicates that some observed X-ray afterglows could be (internally) produced by the magnetar winds, but not necessarily GRB ESs. © 2010 ESO.

Mao Z.;Yu Y. W.;Zheng X. P.;Dai Z. G.;Pi C. M.

Astronomy and Astrophysics

2010

Hyperaccreting neutron stars or magnetar disks cooled via neutrino emission can be candidates of gamma-ray burst (GRB) central engines. The strong field ≥10¹⁵-10¹⁶ G of a magnetar can play a significant role in affecting the disk properties and even lead to the funnel accretion process. In this paper, we investigate the effects of strong fields on the disks around magnetars, and discuss implications of such accreting magnetar systems for GRBs and GRB-like events. We discuss quantum effects of the strong fields on the disk thermodynamics and microphysics due to modifications of the electron distribution and energy in the strong field environment, and use the magnetohydrodynamical conservation equations to describe the behavior of the disk flow coupled with a large-scale field, which is generated by the star-disk interaction. If the disk field is open, the disk properties mainly depend on the ratio between |Bφ/Bz| and ω/ωK with Bφ and Bz being the azimuthal and vertical components of the disk field, and ω and ωK being the accretion flow angular velocity and Keplerian velocity, respectively. On the other hand, the disk properties also depend on the magnetar spin period if the disk field is closed. In general, stronger fields give higher disk densities, pressures, temperatures, and neutrino luminosity. Moreover, strong fields will change the electron fraction and degeneracy state significantly. A magnetized disk is always viscously stable outside the Alfvén radius, but will be thermally unstable near the Alfvén radius where the magnetic field plays a more important role in transferring the angular momentum and heating the disk than the viscous stress. The funnel accretion process will be important only for an extremely strong field, which creates a magnetosphere inside the Alfvén radius and truncates the plane disk. Because of higher temperature and more concentrated neutrino emission of a ring-like belt region on the magnetar surface covered by funnel accretion, the neutrino annihilation rate from the accreting magnetar can be much higher than that from an accreting neutron star without fields. Furthermore, the neutrino annihilation mechanism, which releases the gravitational energy of the surrounding disk, and the magnetically driven pulsar wind, which extracts the stellar rotational energy from the magnetar surface, can work together to generate and feed an ultrarelativistic jet along the stellar magnetic poles. © 2010. The American Astronomical Society. All rights reserved.

Zhang Dong;Dai Z. G.

Astrophysical Journal

2010

We calculate the self Compton spectrum of synchro-curvature radiation of power-law distributed electrons. We find that the resulting spectrum can be different from the traditional radiation mechanisms if parameters are within the proper range. This result could be considered as a universal approach of moving electron's self Compton emission, and could be applied to many astrophysical phenomena, including GRBs and pulsars. We also discuss the cooling effect of electron distribution as well as Klein-Nishina cutoff in order to get a more accurate picture. © 2010 American Institute of Physics.

Bo Zhang;Dai Zi-Gao

Aip Conference Proceedings

2010

Hyperaccreting disks around neutron stars or magnetars cooled via neutrino emission can be the potential central engine of GRBs. The neutron-star disk can cool more efficiently, produce much higher neutrino luminosity and neutrino annihilation luminosity than its black hole counterpart with the same accretion rate. The neutron star surface boundary layer could increase the annihilation luminosity as well. An ultra relativistic jet via neutrino annihilation can be produced along the stellar poles. Moreover, we investigate the effects of strong fields on the disks around magnetars. In general, stronger fields give higher disk densities, pressures, temperatures and neutrino luminosity; the neutrino annihilation mechanism and the magnetically-driven pulsar wind which extracts the stellar rotational energy can work together to generate and feed an even stronger ultra-relativistic jet along the stellar magnetic poles. © 2010 American Institute of Physics.

Zhang Dong;Dai Z. G.

Aip Conference Proceedings

2010

We present broadband (radio, optical, X-ray and GeV) fits to the afterglow light curves and spectra of three long-duration gamma-ray bursts (GRBs 080916C, 090902B, and 090926A) detected by the Gamma-Ray Burst Monitor and Large Area Telescope (LAT) instruments on the Fermi satellite. Using the observed broadband data, we study the origin of the high energy emission, and suggest that the early-time GeV emission and the late-time radio, optical, and X-ray afterglows can be understood as being due to synchrotron emission from an external forward shock caused by structured ejecta propagating in a wind bubble jumping to a homogeneous density medium. If the ceasing time for a majority of the energy injection is assumed to be close to the deceleration time of the forward shock, the structured ejecta with continuous energy injection towards the forward shock can well explain the early rising feature of the GeV mission from these bursts, and the density-jump medium can account for some particular plateaus or flares in the late afterglows. From our fits, we find that, on one hand, the external shock origin of the GeV photons will make the optical depth not have a significant contribution to the early LAT rising part, which will loosen the strong constraint of lower limits of the Lorentz factor. On the other hand, these Fermi-LAT events preferentially occur in a low-density circumburst environment, in which case the Klein-Nishina cutoff will significantly suppress the Self-Synchrotron Compton radiation. Such an environment might result from superbubbles or low-metallicity progenitor stars (which have a low mass-loss rate at late times of stellar evolution) of type Ib/c supernovae. © 2011 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd.

Feng Si-Yi;Dai Zi-Gao

Research in Astronomy and Astrophysics

2011

The increasingly deep limit on the neutrino emission from gamma-ray bursts (GRBs) with IceCube observations has reached a level that could place useful constraints on the fireball properties. We first present a revised analytic calculation of the neutrino flux that predicts a flux of one order of magnitude lower than that obtained by the IceCube Collaboration. For the benchmark model parameters (e.g., the bulk Lorentz factor is Γ = 10^2.5, the observed variability time for the long GRBs is t ^obv = 0.01 s, and the ratio between the energy in the accelerated protons and in the radiation is ηp = 10 for every burst) in the standard internal shock scenario, the predicted neutrino flux from 215 bursts during the period of the 40- and 59-string configurations is a factor of 3 below the IceCube sensitivity. However, if we accept the recently found inherent relation between the bulk Lorentz factor and the burst energy, then the expected neutrino flux significantly increases and the spectral peak shifts to a lower energy. In this case, the nondetection implies that the baryon-loading ratio should be ηp ≲ 10 if the variability time of the long GRBs is fixed to t ^obv = 0.01 s. Instead, if we relax the standard internal-shock scenario but still assume ηp = 10, then the nondetection constrains the dissipation radius, R ≳ 4 × 10 ¹² cm, assuming the same dissipation radius for every burst and benchmark parameters for the fireballs. We also calculate the diffuse neutrino flux from the GRBs for different luminosity functions from the literature. The expected flux exceeds the current IceCube limit for some of the luminosity functions, and, thus, the nondetection constrains ηp ≲ 10 when the variability time of the long GRBs is fixed at t ^obv = 0.01 s. © 2012. The American Astronomical Society. All rights reserved..

He Hao-Ning;Liu Ruo-Yu;Wang Xiang-Yu;Dai Zi-Gao;Nagataki Shigehiro;Murase Kohta

Astrophysical Journal

2012

The shallow decay phase or plateau phase of early afterglows of gamma-ray bursts (GRBs), discovered by Swift, is currently understood as being due to energy injection to a relativistic blast wave. One natural scenario for energy injection invokes a millisecond magnetar as the central engine of GRBs because the conventional model of a pulsar predicts a nearly constant magnetic-dipole-radiation luminosity within the spin-down timescale. However, we note that significant brightening occurs in some early afterglows, which apparently conflicts with the above scenario. Here we propose a new model to explain this significant brightening phenomena by considering a hyperaccreting fallback disk around a newborn millisecond magnetar. We show that for typical values of the model parameters, sufficient angular momentum of the accreted matter is transferred to the magnetar and spins it up. It is this spin-up that leads to a dramatic increase of the magnetic-dipole-radiation luminosity with time and thus significant brightening of an early afterglow. Based on this model, we carry out numerical calculations and fit well early afterglows of 12 GRBs assuming sufficiently strong fallback accretion. If the accretion is very weak, our model turns out to be the conventional energy-injection scenario of a pulsar. Therefore, our model can provide a unified explanation for the shallow decay phase, plateaus, and significant brightening of early afterglows. © 2012. The American Astronomical Society. All rights reserved.

Dai Z. G.;Liu Ruo-Yu

Astrophysical Journal

2012

We present the SVOM mission that the Chinese National Space Agency and the French Space Agency have decided to jointly implement. SVOM has been designed to detect, characterise and quickly localise gamma-ray bursts (GRBs) and other types of high-energy transients. For this task the spacecraft will carry two widefield high-energy instruments: ECLAIRs, a hard X-ray imager, and the Gamma-Ray Monitor, a broadband spectrometer. Upon localising a transient, SVOM will quickly slew towards the source and start deep followup observations with two narrow-field telescopes: the Micro-channel X-ray Telescope in X-rays and the Visible Telescope in the visible. The nearly anti-solar pointing of SVOM combined with the fast transmission of GRB positions to the ground in less than 1 minute, will facilitate the observations of SVOM transients by the largest ground based telescopes. © 2012 SPIE.

Godet O.;Atteia J. L.;Barret D.;Mandrou P.;Paul J.;Claret A.;Cordier B.;G\u0308otz D.;Wei J. Y.;Deng J.;Hu J.;Qiu Y.;Wang J.;Wu C.;Yuan W.;Zhang S. N.;Dong Y.;Wu B.;Basa S.;Cuby J. G.;Dai Z.;Daigne F.;Osborne J. P.

Proceedings of SPIE the International Society for Optical Engineering

2012

Recently, the GeV radiation during the X-ray flare activity in GRB 100728A was detected by Fermi/LAT. Here, we study the dynamics and emission properties of a collision between two homogeneous shells based on the late internal shock model. The GeV photons can be produced from X-ray flare photons being upscattered by relativistic electrons that are accelerated by forward-reverse shocks, where the involved radiative processes include synchrotron self-Compton and crossing inverse-Compton scattering. Using analytical and numerical calculations, the observed spectral properties in GRB 100728A can be well explained. © 2013. The American Astronomical Society. All rights reserved.

Wang K.;Dai Z. G.

Astrophysical Journal

2013