arXiv subjects

Xin-Bo He

Publications and source records attributed to Xin-Bo He.

4 recordsLinked to original sources

arXiv2021

Searching for an additional high-energy component in Fermi-LAT GRB afterglows

The VHE component from at least two GRBs, i.e., GRB180720B and GRB190114C, has been detected in the afterglow phase. We systematically analyzed 199 GRBs detected by Fermi-LAT during 2008-2019. If an additional high-energy component exists in the afterglows of Fermi-LAT GRBs, the best-fit spectral model could be a broken power-law (BPL) model with an upturn above a break energy. We compare the afterglow spectra using PL and BPL representations. Out of the 30 GRBs with >10GeV photons that arrived after T90, 25 GRBs are tentatively or significantly detected at 0.1-200 GeV after 2*T90. The spectrum of GRB131231A shows an upturn above a break of 1.6+-0.8~GeV, supporting the BPL model. For GRB131231A, we performed a modeling of its X-ray and gamma-ray spectra, and found that the SSC model can explain the upturn with acceptable parameter values. In the cases of GRBs 190114C, 171210A, 150902A, 130907A, 130427A, and 090902B, the improvement of the BPL fit compared to the PL fit is tentative or marginal. There is no conclusive evidence that an additional higher energy component commonly exists in Fermi-LAT GRB afterglows, except for a group of Fermi-LAT GRBs mentioned above. Such an additional high-energy component may be explained by the synchrotron self-Compton mechanism. Current and future VHE observations will provide important constraints on the issue.

astro-ph.HE↗

arXiv2020

Is an upturn commonly seen in Fermi-LAT GRB afterglow spectra?

We analyzed 199 GRBs detected by Fermi-LAT during the years 2008-2019. We found 67 photons at energies >=10 GeV, which come from 34 GRBs. Out of these 34 GRBs, Fermi-LAT detects significant (TS>=4) afterglow 0.1-200 GeV photons from 25 GRBs. We present time-integrated 0.1-200 GeV spectra of these 25 GRBs. The spectra of a significant fraction (9/25) of these GRBs revealed a harder component above an energy break of 0.3-2 GeV. While shock synchrotron may account for the photons at the lower energy end, high energy photons above the break is naturally explained by synchrotron self-Compton (SSC) emission. We perform broadband model fit to the X-ray-LAT emission of GRB 131231A. Comparing the afterglow spectra of these 25 GRBs with other Fermi-LAT detected GRBs, we found that the power-law index distribution is similar for the two populations. This may indicate that the additional high-energy component may also exist in Fermi-LAT GRBs in general.

astro-ph.HE↗

arXiv2018

GRB 170817A: a short GRB seen off-axis

The angular distribution of GRB jets is not yet clear. The observed luminosity of GRB 170817A is the lowest among all known short GRBs, which is best explained by the fact that our line of sight is outside of the jet opening angle, $θ_{obs}$ $>$ $θ_j$, where $θ_{obs}$ is the angle between our line of sight and jet axis. Inferred by gravitational wave observations, as well as radio and X-ray afterglow modeling of GRB 170817A, it is likely that $θ_{obs}$ $\sim$ 20$^{\circ}$ -- 28$^{\circ}$. In this work, we quantitatively consider two scenarios of angular energy distribution of GRB ejecta: top-hat jet, and structured jet with a power law index $s$. For top-hat jet model, we get a large $θ_j$ (e.g., $θ_{j}$$>$$10^{\circ}$), a rather high local (i.e., z$<$0.01) short GRB rate $\sim$8--15$\times10^3$ $Gpc^{-3}$$yr^{-1}$ (estimated 90$\sim$1850 $ Gpc^{-3}$$yr^{-1}$ in Fong et al. 2015) and an extremely high $E_{peak,0}$(on-axis,rest-frame)$>$7.5$\times10^4$keV($\sim$500keV for typical short GRB). For structured jet model, we use $θ_{obs}$ to give limits on $s$ and $θ_{j}$ for a typical on-axis luminosity short GRB (e.g., $10^{49}$ erg s$^{-1}$$\sim$$10^{51}$ erg s$^{-1}$), and a low on-axis luminosity case (e.g., $10^{49}$ erg s$^{-1}$) gives more reasonable values of $s$. A structured jet model is more feasible for GRB 170817A than a top-hat jet model, due to the rather high local short GRB rate and the extremely high on-axis $E_{peak,0}$ almost rules out the top-hat jet model. GRB 170817A is likely a low on-axis luminosity GRB ($10^{49}$ erg s$^{-1}$) with a structured jet.

astro-ph.HE↗

arXiv2017

An evolving GeV spectrum from prompt to afterglow: the case of GRB 160509A

We present the high-energy emission properties of GRB 160509A, from its prompt mission to late afterglow phase. GRB 160509A contains two emission episodes: 0-40s and 280-420s after the burst onset (t0). The relatively high fluence of GRB 160509A allows us to establish an evolving spectrum above 100 MeV. During the first emission episode, the >100 MeV spectrum is soft with Γ=>3.0, which can be smoothly connected to keV energies with a Band function with a high-energy cutoff. The >100 MeV spectrum rapidly changes to a hard spectrum with Γ<=1.5 after t0+40s. The existence of very energetic photons, e.g., a 52 GeV that arrives t0+77 seconds, and a 29 GeV that arrives t0+70 ks, is hard to reconcile by the synchrotron emission from forward-shock electrons, but likely due to inverse Compton mechanism (e.g., synchrotron self-Compton emission). A soft spectrum (Γ~2) between 300s and 1000s after the burst onset is also found at a significance of about 2 standard deviations, which suggests a different emission mechanism at work for this short period of time. GRB 160509A represents the latest example where inverse Compton emission has to be taken into account in explaining the afterglow GeV emission, which had been suggested long before the launch of Fermi LAT.

astro-ph.HE↗