arXiv subjects

Yi-Zhong Fan

Publications and source records attributed to Yi-Zhong Fan.

At least 235 records · Page 13Linked to original sources

arXiv2013

A supra-massive magnetar central engine for short GRB 130603B

We show that the peculiar early optical and in particular X-ray afterglow emission of the short duration burst GRB 130603B can be explained by continuous energy injection into the blastwave from a supra-massive magnetar central engine. The observed energetics and temporal/spectral properties of the late infrared bump (i.e., the "kilonova") are also found consistent with emission from the ejecta launched during an NS-NS merger and powered by a magnetar central engine. The isotropic-equivalent kinetic energies of both the GRB blastwave and the kilonova are about $E_{\rm k}\sim 10^{51}$ erg, consistent with being powered by a near-isotropic magnetar wind. However, this relatively small value demands that most of the initial rotational energy of the magnetar $(\sim {\rm a~ few \times 10^{52}~ erg})$ is carried away by gravitational wave radiation. Our results suggest that (i) the progenitor of GRB 130603B would be a NS-NS binary system, whose merger product would be a supra-massive neutron star that lasted for about $\sim 1000$ seconds; (ii) the equation-of-state of nuclear matter would be stiff enough to allow survival of a long-lived supra-massive neutron star, so that it is promising to detect bright electromagnetic counterparts of gravitational wave triggers without short GRB associations in the upcoming Advanced LIGO/Virgo era.

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arXiv2013

Signature of gravitational radiation in afterglow of short Gamma-Ray Bursts?

Short Gamma-Ray Bursts (GRBs), brief intense emission of $γ-$rays characterized by a duration shorter than 2 seconds that are plausibly powered by the coalescence of binary neutron stars, are believed to be strong gravitational wave radiation (GWR) sources. The test of such a speculation has been thought to be impossible until the performance of the detectors like advanced LIGO. Recently there has been growing evidence for the formation of highly-magnetized neutron star (i.e., magnetar) in the double neutron star mergers. In this work we re-examine the interpretation of the X-ray plateau followed by an abrupt decline detected in some short GRB afterglows within the supramassive magnetar model and find that the maximum gravitational mass of the non-rotating neutron stars is $\sim 2.3M_{\odot}$ and the observed duration of some X-ray plateaus are significantly shorter than that expected in the magnetic dipole radiation scenario, suggesting that the collapse of the supramassive magnetars has been considerably enhanced by the energy loss via GWR. Such a result demonstrates that the signature of GWR may have already existed in current electromagnetic data of short GRBs.

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arXiv2013

Cosmological Time Dilation in Durations of Swift Long Gamma-Ray Bursts

Cosmological time dilation is a fundamental phenomenon in an expanding universe, which stresses that both the duration and wavelength of the emitted light from a distant object at the redshift $z$ will be dilated by a factor of $1+z$ at the observer. By using a sample of 139 \emph{Swift} long GRBs with known redshift ($z\leq8.2$), we measure the observed duration ($T_{90}$) in the observed energy range between $140/(1+z)$ keV and $350/(1+z)$ keV, corresponding to a fixed energy range of 140-350 keV in the rest frame. We obtain a significant correlation between the duration and the factor $1+z$, i.e., $T_{\rm{90}}=10.5(1+z)^{0.94\pm0.26}$, which is well consistent with that expected from cosmological time dilation effect.

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arXiv2013

Is the late near-infrared bump in short-hard GRB 130603B due to the Li-Paczynski kilonova?

Short-hard gamma-ray bursts (GRBs) are widely believed to be produced by the merger of two binary compact objects, specifically by two neutron stars or by a neutron star orbiting a black hole. According to the Li-Paczynski kilonova model, the merger would launch sub-relativistic ejecta and a near-infrared/optical transient would then occur, lasting up to days, which is powered by the radioactive decay of heavy elements synthesized in the ejecta. The detection of a late bump using the {\em Hubble Space Telescope} ({\em HST}) in the near-infrared afterglow light curve of the short-hard GRB 130603B is indeed consistent with such a model. However, as shown in this Letter, the limited {\em HST} near-infrared lightcurve behavior can also be interpreted as the synchrotron radiation of the external shock driven by a wide mildly relativistic outflow. In such a scenario, the radio emission is expected to peak with a flux of $\sim 100 μ$Jy, which is detectable for current radio arrays. Hence, the radio afterglow data can provide complementary evidence on the nature of the bump in GRB 130603B. It is worth noting that good spectroscopy during the bump phase in short-hard bursts can test validity of either model above, analogous to spectroscopy of broad-lined Type Ic supernova in long-soft GRBs.

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arXiv2013

High energy emission of GRB 130427A: evidence for inverse Compton radiation

A nearby super-luminous burst GRB 130427A was simultaneously detected by six $γ$-ray space telescopes ({\it Swift}, Fermi-GBM/LAT, Konus-Wind, SPI-ACS/INTEGRAL, AGILE and RHESSI) and by three RAPTOR full-sky persistent monitors. The isotropic $γ-$ray energy release is of $\sim 10^{54}$ erg, rendering it the most powerful explosion among the GRBs with a redshift $z\leq 0.5$. The emission above 100 MeV lasted about one day and four photons are at energies greater than 40 GeV. We show that the count rate of 100 MeV-100 GeV emission may be mainly accounted for by the forward shock synchrotron radiation and the inverse Compton radiation likely dominates at GeV-TeV energies. In particular, an inverse Compton radiation origin is established for the $\sim (95.3,~47.3,~41.4,~38.5,~32)$ GeV photons arriving at $t\sim (243,~256.3,~610.6,~3409.8,~34366.2)$ s after the trigger of Fermi-GBM. Interestingly, the external-inverse-Compton-scattering of the prompt emission (the second episode, i.e., $t\sim 120-260$ s) by the forward-shock-accelerated electrons is expected to produce a few $γ-$rays at energies above 10 GeV, while five were detected in the same time interval. A possible unified model for the prompt soft $γ-$ray, optical and GeV emission of GRB 130427A, GRB 080319B and GRB 090902B is outlined. Implication of the null detection of $>1$ TeV neutrinos from GRB 130427A by IceCube is discussed.

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arXiv2013

TeV-PeV neutrinos over the atmospheric background: originating from two groups of sources?

In addition to the two ~1 PeV neutrinos, the IceCube Collaboration recently reported a detection of 26 neutrino candidates at energies from 30 TeV to 250 TeV, implying a confidence level of 4.3σover the atmospheric background. We suggest that these TeV-PeV non-atmospheric neutrinos may originate from two groups of sources, motivated by the non-detection of neutrinos in the energy range 250 TeV- 1 PeV in current data. If intrinsic, the non-detection of 250 TeV-1 PeV neutrinos disfavors the single power-law spectrum model for the TeV-PeV non-atmospheric neutrinos at a confidence level of ~ 2σ. We then interpret the current neutrino data with a two-component spectrum model. One has a flat spectrum with a cutoff at the energy ~ 250 TeV and the other has a sharp peak at ~1 PeV. The former is likely via pp collision while the latter may be generated by the photomeson interaction.

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arXiv2013

AMS-02 positron excess: new bounds on dark matter models and hint for primary electron spectrum hardening

The data collected by ATIC, CREAM and PAMELA all display remarkable cosmic-ray-nuclei spectrum hardening above the magnetic rigidity $\sim$ 240 GV. One natural speculation is that the primary electron spectrum also gets hardened (possibly at $\sim 80$ GV) and the hardening partly accounts for the electron/positron total spectrum excess discovered by ATIC, HESS and Fermi-LAT. If it is the case, the increasing behavior of the subsequent positron-to-electron ratio will get flattened and the spectrum hardening should be taken into account in the joint fit of the electron/psoitron data otherwise the inferred parameters will be biased. Our joint fits to the latest AMS-02 positron fraction data together with the PAMELA/Fermi-LAT electron/positron spectrum data suggest that the primary electron spectrum hardening is needed in most though not all modelings. The bounds on dark matter models have also been investigated. In the presence of spectrum hardening of primary electrons, the amount of dark-matter-originated electron/positron pairs needed in the modeling is smaller. Even with such a modification, the annihilation channel $χχ\rightarrow μ^{+}μ^{-}$ has been tightly constrained by the Fermi-LAT Galactic diffuse emission data. The decay channel $χ\rightarrow μ^{+}μ^{-}$ is found to be viable.

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arXiv2013

Constraining the distribution of dark matter in inner galaxy with indirect detection signal: The case of tentative 130 gev γ-ray line

The dark matter distribution in the very inner region of our Galaxy is still in debate. In the N-body simulations a cuspy dark matter halo density profile is favored. Several dissipative baryonic processes however are found to be able to significantly flatten dark matter distribution and a cored dark matter halo density profile is possible. The baryons dominate the gravitational potential in the inner Galaxy, hence a direct constrain on the abundance of the dark matter particles is rather challenging. Recently, a few groups have identified a tentative 130 GeV line signal in the Galactic center, which could be interpreted as the signal of the dark matter annihilation. With current 130 GeV line data and adopting the generalized Navarro-Frenk-White profile of the dark matter halo, for local dark matter density ρ_0=0.4 GeV cm^{-3} and r_s=20 kpc we obtain a 95% confidence level lower (upper) limit on the inner slope of dark matter density distribution α= 1.06 (the cross section of dark matter annihilation into gamma-rays <σv >_{χχ-> γγ}= 1.3\times 10^{-27} cm^3 s^{-1}). Such a slope is consistent with the results of some N-body simulations, and if the signal is due to dark matter, suggests that baryonic processes may be unimportant.

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arXiv2013

Diffuse PeV neutrino emission from Ultra-Luminous Infrared Galaxies

Ultra-luminous infrared galaxies (ULIRGs) are the most luminous and intense starburst galaxies in the Universe. Both their star-formation rate (SFR) and gas surface mass density are very high, implying a high supernovae rate and an efficient energy conversion of energetic protons. A small fraction of these supernovae is the so-called hypernovae with a typical kinetic energy ~1e52 erg and a shock velocity >=1e9 cm/s. The strong shocks driven by hypernovae are able to accelerate cosmic ray protons up to 1e17 eV. These energetic protons lose a good fraction of their energy through proton-proton collision when ejected into very dense interstellar medium, and as a result, produce high energy neutrinos (<=5 PeV). Recent deep infrared surveys provide solid constraints on the number density of ULIRGs across a wide redshift range 0<z<2.3, allowing us to derive the flux of diffuse neutrinos from hypernovae. We find that at PeV energies, the diffuse neutrinos contributed by ULIRGs are comparable with the atmosphere neutrinos with the flux of 2e-9GeV cm^-2/s/sr, by assuming the injected cosmic ray power law spectrum with an index of -2.

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arXiv2013

PAMELA/Fermi-LAT electron cosmic ray spectrum at $\sim$100 GeV: implication for dark matter annihilation signal in accordance with the 130 GeV $γ$-ray line

Recently, a tentative 130 GeV $γ$-ray line signal was identified by quite a few groups. If correct it would constitute a ``smoking gun'' for dark matter annihilations. Interestingly, the spectra of the cosmic ray electrons detected by PAMELA and Fermi-LAT both show tiny wiggle-like structure at $\sim 100$ GeV, which might indicate a weak signal of the annihilation of $\sim 130$ GeV dark matter particles into electrons/positrons with a velocity-weighted cross section $\langleσv\rangle_{\rm χχ\rightarrow e^{+}e^{-}} \sim 4\times10^{-26}~{\rm cm^{3}~s^{-1}}$. The prospect of identifying such a potential weak dark-matter-annihilation electron and/or positron component by AMS-02, a mission to measure the high energy cosmic ray spectra with unprecedented accuracy, is investigated.

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arXiv2012

Statistical interpretation of the spatial distribution of current 130 GeV gamma-ray line signal within the dark matter annihilation scenario

Recently, several groups identified a tentative $γ$-ray line signal with energy $\sim 130$ GeV in the central Galaxy from the Fermi-LAT data. %The morphology study shows that the signal is consistent with dark matter %annihilation, but with an offset $\sim 220$ pc ($1.5^{\circ}$) of the %center from the Galactic center Sgr A$^{\star}$, Such a $γ-$ray line can be interpreted as the signal of dark matter annihilation. However, the offset $\sim 220$ pc ($1.5^{\circ}$) of the center of the most prominent signal region from the Galactic center Sgr A$^{\star}$ has been thought to challenge the dark matter annihilation interpretation. Considering the fact that such a 130 GeV $γ$-ray line signal consists of only $\sim14$ photons, we suggest that the "imperfect" consistency of these photons with the expected dark matter distribution is due to the limited statistics. The offset will be smaller as more signal photons have been collected in the near future. Our Monte Carlo simulation supports the above speculation.

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arXiv2012

GRB 120422A: A Low-luminosity Gamma-ray Burst Driven by Central Engine

GRB 120422A is a low-luminosity Gamma-ray burst (GRB) associated with a bright supernova, which distinguishes itself by its relatively short T90 ~ 5 s and an energetic X-ray tail. We analyze the Swift BAT and XRT data and discuss the physical implications. We show that the early steep decline in the X-ray light curve can be interpreted as the curvature tail of a late emission episode around 58-86 s, with a curved instantaneous spectrum at the end of the emission episode. Together with the main activity in the first ~ 20 s and the weak emission from 40 s to 60 s, the prompt emission is variable, which points towards a central engine origin, in contrast to the shock breakout origin as invoked to interpret some other nearby low-luminosity supernova GRBs. The curvature effect interpretation and interpreting the early shallow decay as the coasting external forward shock emission in a wind medium both give a constraint on the bulk Lorentz factor Γto be around several. Comparing the properties of GRB 120422A and other supernova GRBs, we found that the main criterion to distinguish engine-driven GRBs from the shock breakout GRBs is the time-averaged luminosity, with a separation line of about ~ 10^48 erg s^-1.

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arXiv2012

The photospheric radiation model for the prompt emission of Gamma-ray Bursts: Interpreting four observed correlations

We show that the experiential $E_{\rm p}-L$, $Γ-L$, $E_{\rm p}-Γ$ and ${\rm \barη_γ}-E_{\rm p}$ correlations (where $L$ is the time-averaged luminosity of the prompt emission, $E_{\rm p}$ is the spectral peak energy, $Γ$ is the bulk Lorentz factor and $\barη_γ$ is the emission efficiency of Gamma-ray bursts) are well consistent with the relations between the resembling parameters predicted in the photospheric radiation model of the prompt emission of Gamma-ray bursts. The time-resolved thermal radiation of GRB 090902B does follow the $E_{\rm p}-L$ and $Γ-L$ correlations. A reliable interpretation of the four correlations in alternative models is still lacking. These facts may point towards a photospheric origin of prompt emission of some Gamma-ray Bursts.

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arXiv2012

Fermi Large Area Telescope observations of GRB 110625A

Gamma-ray bursts (GRBs) that emit photons at GeV energies form a small but significant population of GRBs. However, the number of GRBs whose GeV-emitting period is simultaneously observed in X-rays remains small. We report gamma-ray observations of GRB 110625A using Fermi's Large Area Telescope (LAT) in the energy range 100 MeV to 20 GeV. Gamma-ray emission at these energies was clearly detected using data taken between 180s and 580s after the burst, an epoch after the prompt emission phase. The GeV light curve differs from a simple power-law decay, and probably consists of two emission periods. Simultaneous Swift/XRT observations did not show flaring behaviors as in the case of GRB 100728A. We discuss the possibility that the GeV emission is the synchrotron self-Compton radiation of underlying ultraviolet flares.

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arXiv2011

Dark Matter Mini-halo around the Compact Objects: the Formation, Evolution and Possible Contribution to the Cosmic Ray Electrons/Positrons

Dark matter particles may be captured by a star and then thermalized in the star's core. At the end of its life a massive star collapses suddenly and a compact object is formed. The dark matter particles redistribute accordingly. In the inelastic dark matter model, an extended dense dark matter mini-halo surrounding the neutron star may be formed. Such mini-halos may be common in the Galaxy. The electron/positron flux resulting in the annihilation of dark matter particles, however, is unable to give rise to observable signal unless a nascent mini-halo is within a distance \sim a few 0.1 pc from the Earth.

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arXiv2011

GRB 081029: A Gamma-Ray Burst with a Multi-Component Afterglow

We present an analysis of the unusual optical light curve of the gamma-ray burst GRB 081029, a long-soft burst with a redshift of z = 3.8479. We combine X-ray and optical observations from the Swift X-Ray Telescope and the Swift UltraViolet/Optical Telescope with ground-based optical and infrared data obtained using the REM, ROTSE, and CTIO 1.3-m telescopes to construct a detailed data set extending from 86 s to approximately 100,000 s after the BAT trigger. Our data cover a wide energy range, from 10 keV to 0.77 eV (1.24 to 16,000 Angstrom). The X-ray afterglow shows a shallow initial decay followed by a rapid decay starting at about 18,000 s. The optical and infrared afterglow, however, shows an uncharacteristic rise at about 3000 s that does not correspond to any feature in the X-ray light curve. Our data are not consistent with synchrotron radiation from a jet interacting with an external medium, a two-component jet, or continuous energy injection from the central engine. We find that the optical light curves can be broadly explained by a collision between two ejecta shells within a two-component jet. A growing number of gamma-ray burst afterglows are consistent with complex jets, which suggests that some (or all) gamma-ray burst jets are complex and will require detailed modelling to fully understand them.

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arXiv2011

Very Old Isolated Compact Objects as Dark Matter Probes

Very old isolated neutron stars and white dwarfs have been suggested to be probes of dark matter. To play such a role, two requests should be fulfilled, i.e., the annihilation luminosity of the captured dark matter particles is above the thermal emission of the cooling compact objects (request-I) and also dominate over the energy output due to the accretion of normal matter onto the compact objects (request-II). Request-I calls for very dense dark matter medium and the critical density sensitively depends on the residual surface temperature of the very old compact objects. The accretion of interstellar/intracluster medium onto the compact objects is governed by the physical properties of the medium and by the magnetization and rotation of the stars and may outshine the signal of dark matter annihilation. Only in a few specific scenarios both requests are satisfied and the compact objects are dark matter burners. The observational challenges are discussed and a possible way to identify the dark matter burners is outlined.

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arXiv2011

Implications of Understanding Short Gamma-Ray Bursts Detected by {\it Swift}

In an effort to understand the puzzle of classifying gamma-ray bursts (GRBs), we perform a systematic study of {\it Swift} GRBs and investigate several short GRB issues. Though short GRBs have a short ($\lesssim2$ s) prompt duration as monitored by the Burst Alert Telescope, the composite light curves including both the prompt and afterglow emission suggest that most of the short GRBs have a similar radiative feature to long GRBs. Further, some well-studied short GRBs might also have an intrinsically long prompt duration, which renders them as a type of short GRB imposters. Genuine short GRBs detected by {\it Swift} might be rare that discriminating the observed short GRBs is, not surprisingly, troublesome. In particular, the observational biases in the host identification and redshift measurement of GRBs should be taken with great caution. The redshift distribution which has been found to be different for long and short GRBs might have been strongly affected by the measurement methods. We find that the redshifts measured from the presumed host galaxies of long and short GRBs appear to have a similar distribution.

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