arXiv subjects

Fang-Kun Peng

Publications and source records attributed to Fang-Kun Peng.

21 records · Page 2Linked to original sources

arXiv2015

Measuring the bulk Lorentz factors of gamma-ray bursts with Fermi

Gamma-ray bursts (GRBs) are powered by ultra-relativistic jets. Usually a minimum value of the Lorentz factor of the relativistic bulk motion is obtained based on the argument that the observed high energy photons ($\gg {\rm MeV}$) can escape without suffering from absorption due to pair production. The exact value, rather than a lower limit, of the Lorentz factor can be obtained if the spectral cutoff due to such absorption is detected. With the good spectral coverage of the Large Area Telescope (LAT) on \textsl{Fermi}, measurements of such cutoff become possible, and two cases (GRB~090926A and GRB~100724B) have been reported to have high-energy cutoffs or breaks. We systematically search for such high energy spectral cutoffs/breaks from the LAT and the Gamma-ray burst monitor (GBM) observations of the prompt emission of GRBs detected since August 2011. Six more GRBs are found to have cutoff-like spectral feature at energies of $\sim10-500$~MeV. Assuming that these cutoffs are caused by pair-production absorption within the source, the bulk Lorentz factors of these GRBs are obtained. We further find that the Lorentz factors are correlated with the isotropic gamma-ray luminosity of the bursts, indicating that more powerful GRB jets move faster.

astro-ph.HE↗

arXiv2014

Photosphere emission in the X-Ray Flares of Swift Gamma-Ray Bursts and Implications for the Fireball Properties

X-ray flares of gamma-ray bursts (GRBs) are usually observed in the soft X-ray range and the spectral coverage is limited. In this paper, we present an analysis of 32 GRB X-ray flares that are simultaneously observed by both BAT and XRT on board the Swift mission, so a joint spectral analysis with a wider spectral coverage is possible. Our results show that the joint spectra of 19 flares are fitted with the absorbed single power-law or the Band function models. More interestingly, the joint spectra of the other 13 X-ray flares are fitted with the absorbed single power-law model plus a black body (BB) component. Phenomenally, the observed spectra of these 13 flares are analogous to several GRBs with a thermal component, but only with a much lower temperature of $kT=1\sim 3$ keV. Assuming that the thermal emission is the photosphere emission of the GRB fireball, we derive the fireball properties of the 13 flares that have redshift measurements, such as the bulk Lorentz factor $Γ_{\rm ph}$ of the outflow. The derived $Γ_{\rm ph}$ range from $50$ to $150$ and a relation of $Γ_{\rm ph}$ to the thermal emission luminosity is found. It is consistent with the $Γ_0-L_{\rm iso}$ relation that are derived for the prompt gamma-ray emission. We discuss the physical implications of these results within the content of jet composition and radiation mechanism of GRBs and X-ray flares.

astro-ph.HE↗

arXiv2014

Internal Energy Dissipation of Gamma-Ray Bursts Observed with Swift: Precursors, Prompt Gamma-rays, Extended emission and Late X-ray Flares

We jointly analyze the gamma-ray burst (GRB) data observed with BAT and XRT on board the Swift mission to present a global view on the internal energy dissipation processes in GRBs, including precursors, prompt gamma-ray emission, extended soft gamma-ray emission, and late X-ray flares. The Bayesian block method is utilized to analyze the BAT lightcurves to identify various emission episodes. Our results suggest that these emission components likely share a same physical origin, which is repeated activation of the GRB central engine. What we observe in the gamma-ray band may be the tip-of-iceberg of more extended underlying activities. The precursor emission, which is detected in about 10% of {\em Swift} GRBs, is preferably detected in those GRBs that have a massive star core-collapse origin. The soft extended emission (EE) tail, on the other hand, is preferably detected in those GRBs that have a compact star merger origin. Bright X-ray emission is detected during the BAT quiescent phases prior to subsequent gamma-ray peaks, implying that X-ray emission may be detectable prior the BAT trigger time. Future GRB alert instruments with soft X-ray capability would be essential to reveal the early stage of GRB central engine activities, sheding light into jet composition and jet launching mechanism in GRBs.

astro-ph.HE↗