arXiv subjects

Fang-Kun Peng

Publications and source records attributed to Fang-Kun Peng.

At least 19 recordsLinked to original sources

arXiv2026

Imprints of gravitational waves from magnetar spindown in GRB X-ray afterglows

Given that newborn magnetars are considered potential central engines of gamma-ray bursts (GRBs), there is strong motivation to identify gravitational wave (GW) signatures within GRB samples. If the X-ray afterglow of a GRB is powered by a magnetar, and the initial spindown of the magnetar is dominated by the GW radiation induced by $r$-mode instability or magnetic-field-induced deformation, the decay of the X-ray flux would record the information of the GW radiation. We find that GRB 130603B potentially represents a rare and precious case where the spindown of the central magnetar is dominated in-turn by $r$-mode and magnetic distortion-induced GW radiation. By fitting the X-ray light curve of GRB 130603B in this model, we obtain the initial spin period of magnetar $\sim 5.3\times 10^{-4}$ s, the effective dipole magnetic field strength $\sim 5.2\times 10^{14}$ G, the ellipticity of the magnetar $\sim 1.3\times 10^{-4}$, and the amplitude of $r$-mode oscillation $\sim3.3\times 10^{-2}$. It may serve as a reliable approach for investigating neutron star physics by comparing the parameters estimated using the method presented in this manuscript with those obtained from future GW observations.

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arXiv2024

The physical properties of Fermi-4LAC low-synchrotron-peaked BL Lac objects

Previous studies on the fitting of spectral energy distributions (SEDs) often apply the external-Compton process to interpret the high-energy peak of low-synchrotron-peaked (LSP) BL Lac objects (LBLs), despite the lack of strong broad emission lines observed for LBLs. In this work, we collect quasi-simultaneous multi-wavelength data of 15 LBLs from the Fermi fourth LAT AGN catalog (4LAC). We propose an analytical method to assess the necessity of external photon fields in the framework of one-zone scenario. Following derived analytical results, we fit the SEDs of these LBLs with the conventional one-zone leptonic model and study their jet physical properties. Our main results can be summarized as follows. (1)We find that most LBLs cannot be fitted by the one-zone synchrotron self-Compton (SSC) model. This indicates that external photons play a crucial role in the high-energy emission of LBLs, therefore we suggest that LBLs are masquerading BL Lacs. (2) We suggest that the $γ$-ray emitting regions of LBLs are located outside the broad-line region and within the dusty torus. (3) By extending the analytical method to all types of LSPs in Fermi-4LAC (using historical data), we find that the high-energy peaks of some flat spectrum radio quasars and blazar candidates of unknown types can be attributed to the SSC emission, implying that the importance of external photons could be minor. We suggest that the variability timescale may help distinguish the origin of the high-energy peak.

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arXiv2023

Broadband multi-wavelength study of LHAASO detected AGN

Recently, the Large High Altitude Air Shower Observatory (LHAASO) collaboration presented the first catalog of $γ$-ray sources using 508 days of LHAASO data, from March 2021 to September 2022. This catalog contains four blazars and a possible liner-type AGN counterpart. In this work, we establish averaged multi-wavelength SEDs by combining data from the \textsl{Fermi}-Large Area Telescope, \textsl{Swift}, ZTF, and WISE covering the same period as the LHAASO detection. In general, these five AGNs are found in low states at all wavelengths. To study the multi-wavelength properties of these AGNs, several jet emission models, including the one-zone leptonic model, the one-zone leptonic and hadronuclear ($pp$) model, the one-zone proton-synchrotron model, and the spine-layer model are applied to reproduce their averaged SEDs, respectively. We find that the one-zone leptonic model can reproduce most of the SEDs, except for the high-energy tail of the LHAASO spectra of Mrk 421 and Mrk 501. To improve the fitting, emission from $pp$ interactions is favoured in the framework of a one-zone model. The spine-layer model, which can be treated as a multi-zone scenario, can also provide good spectral fits. The influence of different extragalactic background light models on fitting LHAASO energy spectrum is also discussed.

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arXiv2022

Searching for candidates of coalescing binary black holes formed through chemically homogeneous evolution in GWTC-3

The LIGO, Virgo, and KAGRA (LVK) collaboration has announced 90 coalescing binary black holes (BBHs) with $p_{\rm astro} > 50\%$ to date, however, the origin of their formation channels is still an open scientific question. Given various properties of BBHs (BH component masses and individual spins) inferred using the default priors by the LVK, independent groups have been trying to explain the formation of the BBHs with different formation channels. Of all formation scenarios, the chemically homogeneous evolution (CHE) channel has stood out with distinguishing features, namely, nearly-equal component masses and preferentially high individual spins aligned with the orbital angular momentum. We perform Bayesian inference on the BBH events officially reported in GWTC-3 with astrophysically-predicted priors representing different formation channels of the isolated binary evolution (CEE: common-envelope evolution channel; CHE; SMT: stable mass transfer). Given assumed models, we report strong evidence for GW190517\_055101 being most likely to have formed through the CHE channel. Assuming the BBH events in the subsample are all formed through one of the isolated binary evolution channels, we obtain the lower limits on the local merger rate density of these channels at $11.45 ~\mathrm{Gpc^{-3}~yr^{-1}}$ (CEE), $0.18 ~\mathrm{Gpc^{-3}~yr^{-1}}$ (CHE), and $0.63 ~\mathrm{Gpc^{-3}~yr^{-1}}$ (SMT) at $90\%$ credible level.

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arXiv2020

Diffuse gamma-ray emission toward the massive star-forming region, W40

We report the detection of high-energy gamma-ray signal towards the young star-forming region, W40. Using 10-year Pass 8 data from the Fermi Large Area Telescope (Fermi-LAT), we extracted an extended gamma-ray excess region with a significance of about 18sigma. The radiation has a spectrum with a photon index of 2.49 +/- 0.01. The spatial correlation with the ionized gas content favors the hadronic origin of the gamma-ray emission. The total cosmic-ray (CR) proton energy in the gamma-ray production region is estimated to be the order of 10^47 erg. However, this could be a small fraction of the total energy released in cosmic rays (CRs) by local accelerators, presumably by massive stars, over the lifetime of the system. If so, W40, together with earlier detections of gamma-rays from Cygnus cocoon, Westerlund 1, Westerlund 2, NGC 3603, and 30 Dor C, supports the hypothesis that young star clusters are effective CR factories. The unique aspect of this result is that the gamma-ray emission is detected, for the first time, from a stellar cluster itself, rather than from the surrounding "cocoons".

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arXiv2019

Evidence of AGN activity in the gamma-ray emission from two starburst galaxies

Starburst galaxies are huge reservoirs of cosmic rays (CRs) and these CRs convert a significant fraction of their energy into gamma-rays by colliding with the interstellar medium (ISM). The produced GeV gamma-ray emission is temporally stable and the flux is found to correlate well with indicators of star formation rates, such as the total infrared (IR) luminosity $L_{\rm IR}$ and monochromatic radio continuum luminosity at 1.4 GHz $L_{\rm 1.4\ GHz}$, i.e., following $L_γ-L_{\rm IR}$ and $L_γ-L_{\rm 1.4\ GHz}$ relations. Recently, gamma-ray excesses are reported to be spatially coincident with two starburst galaxies NGC 3424 and UGC 11041 in the fourth \textsl{Fermi} Large Area Telescope (LAT) source catalog (4FGL). Different from other starburst galaxies detected by \textsl{Fermi}-LAT, we find that the gamma-ray emission associated with NGC 3424 and UGC 11041 show significant flux variability. With relatively weak infrared and radio emission, NGC 3424 and UGC 11041 appear as outliers of the $L_γ-L_{\rm IR}$ and $L_γ-L_{\rm 1.4\ GHz}$ relations of starburst galaxies. These results suggest that NGC 3424 and UGC 11041 may harbor obscured active galactic nuclei (AGNs) and the AGN activities provide the dominant contribution to the gamma-ray emission as compared to that provided by the starburst activities.

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arXiv2019

Effect of magnetic field on neutrino annihilation efficiency in gamma-ray bursts

Neutrino annihilation process on a hyperaccreting disk is one of the leading models to explain the generation of relativistic jets of gamma-ray bursts (GRBs). However, there is still uncertainty regarding the NAE of neutron star-accretion disc (NS-disc) system because of complicated microphysics processes and effects of strong magnetic field. In this paper, we investigate the NAE by assuming that the prompt jet of GRB 070110 is driven by neutrino pair annihilation in the NS-disc system. Our calculation shows $η_{ν\barν}> 1.2 \times 10^{-3}$ under the estimated accretion rate $\dot{M}\simeq 0.04\rm M_{\odot}\cdot s^{-1}$. Independent of the detailed accretion disc models, our result shows that the magnetic field may play an important role in the neutrino annihilation process on the hyperaccreting magnetized accretion disc. Compared with the theoretical value of $η_{\rm ν\barν}$ of the nonmagnetized BH-disc system, the NAE should increase significantly in the case of the NS-disc system if the GRB is powered by magnetar-disc system.

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arXiv2019

Measuring the speed of gravitational waves with the distorted pulsars

The measurement of the speed of gravitational waves (GWs) is useful to distinguish general relativity from massive gravity. We propose a new model-independent strategy to measure the speed of GWs with the distorted pulsars. Theoretically, when the the GW frequencies from a distorted pulsar are twice the frequencies of EM pulses, they should be emitted at the same time. By measuring the arrival times of these two signals emitted at the same time, the speed of GWs can be calculated with the time difference. Specifically, when the glitches of pulsars are taken into consideration, some pulsars in our Galaxy and nearby galaxies are potential to test our new strategy at high accuracy in the foreseeable future. On the other hand, the new method is meaningful as a motivation for future design of instruments.

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arXiv2019

Interpreting the relation between the gamma-ray and infrared luminosities of star-forming galaxies

It has been found that there is a quasi-linear scaling relationship between the gamma-ray luminosity in GeV energies and the total infrared luminosity of star-forming galaxies, i.e. $L_γ\propto L_{\rm IR}^α$ with $α\simeq 1$. However, the origin of this linear slope is not well understood. Although extreme starburst galaxies can be regarded as calorimeters for hadronic cosmic ray interaction and thus a quasi-linear scaling may hold, it may not be the case for low star-formation-rate (SFR) galaxies, as the majority of cosmic rays in these galaxies are expected to escape. We calculate the gamma-ray production efficiency in star-forming galaxies by considering realistic galaxy properties, such as the gas density and galactic wind velocity in star-forming galaxies. We find that the slope for the relation between gamma-ray luminosity and the infrared luminosity gets steeper for low infrared luminosity galaxies, i.e. $α\rightarrow 1.6$, due to increasingly lower efficiency for the production of gamma-ray emission. We further find that the measured data of the gamma-ray luminosity is compatible with such a steepening. The steepening in the slope suggests that cosmic-ray escape is very important in low-SFR galaxies.

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arXiv2018

Pre-merger electromagnetic counterparts of binary compact stars

We investigate emission signatures of binary compact star gravitational wave sources consisting of strongly magnetized neutron stars (NSs) and/or white dwarfs (WDs) in their late-time inspiral phase. Because of electromagnetic interactions between the magnetospheres of the two compact stars, a substantial amount of energy will be extracted, and the resultant power is expected to be $\sim 10^{38} - 10^{44}$ erg/s in the last few seconds before the two stars merge, when the binary system contains a NS with a surface magnetic field $10^{12}$ G. The induced electric field in the process can accelerate charged particles up to the EeV energy range. Synchrotron radiation is emitted from energetic electrons, with radiative energies reaching the GeV energy for binary NSs and the MeV energy for NS - WD or double WD binaries. In addition, a blackbody component is also presented and it peaks at several to hundreds keV for binary NSs and at several keV for NS - WD or double WD binaries. The strong angular dependence of the synchrotron radiation and the isotropic nature of the blackbody radiation lead to distinguishable modulation patterns between the two emission components. If coherent curvature radiation is presented, fast radio bursts could be produced. These components provide unique simultaneous electromagnetic signatures as precursors of gravitational wave events associated with magnetized compact star mergers and short gamma ray bursts (e.g., GRB 100717).

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arXiv2018

Comparative study of gamma-ray emission from molecular clouds and star-forming galaxies

Star-forming regions on different scales, such as giant molecular clouds in our Galaxy and star-forming galaxies, emit GeV gamma-rays. These are thought to originate from hadronic interactions of cosmic-ray (CR) nuclei with the interstellar medium. It has recently been shown that the gamma-ray luminosity ($L_γ$) of star-forming galaxies is well correlated with their star formation rates (SFR). We investigated \textsl{Fermi} data of eight Galactic molecular clouds in the Gould belt and found that molecular clouds do not follow the $L_γ-{\rm SFR}$ correlation of star-forming galaxies. We also compared the scaling relations of gamma-ray luminosity, SFR, and the gas mass for molecular clouds and star-forming galaxies. Using a multiple-variable regression analysis, we found different dependences of gamma-ray emission on SFR or mass for molecular clouds and star-forming galaxies. This suggests that different mechanisms may govern the production of gamma-rays in these two types of sources. Specifically, the strong dependence on mass supports that gamma-ray emission of molecular clouds primarily comes from {\em \textup{passive}} interaction by diffuse Galactic CRs, whereas the strong dependence on SFR supports that gamma-ray emission of star-forming galaxies originates from CRs that are accelerated by local {\em \textup{active}} sources.

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arXiv2018

Detection of gamma-ray emission from the Coma cluster with Fermi Large Area Telescope and tentative evidence for an extended spatial structure

Many galaxy clusters have giant halos of non-thermal radio emission, indicating the presence of relativistic electrons in the clusters. Relativistic protons may also be accelerated by merger and/or accretion shocks in galaxy clusters. These cosmic-ray (CR) electrons and/or protons are expected to produce gamma-rays through inverse-Compton scatterings or inelastic $pp$ collisions respectively. Despite of intense efforts in searching for high-energy gamma-ray emission from galaxy clusters, conclusive evidence is still missing so far. Here we report the discovery of $\ge 200$ MeV gamma-ray emission from the Coma cluster direction with an unbinned likelihood analysis of the 9 years of {\it Fermi}-LAT Pass 8 data. The gamma-ray emission shows a spatial morphology roughly coincident with the giant radio halo, with an apparent excess at the southwest of the cluster. Using the test statistic analysis, we further find tentative evidence that the gamma-ray emission at the Coma center is spatially extended. The extended component has an integral energy flux of $\sim 2\times 10^{-12}{\rm \ erg\ cm^{-2}\ s^{-1}}$ in the energy range of 0.2 - 300 GeV and the spectrum is soft with a photon index of $\simeq-2.7$. Interpreting the gamma-ray emission as arising from CR proton interaction, we find that the volume-averaged value of the CR to thermal pressure ratio in the Coma cluster is about $\sim 2\%$. Our results show that galaxy clusters are likely a new type of GeV gamma-ray sources, and they are probably also giant reservoirs of CR protons.

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arXiv2018

Gravitational waves induced by the asymmetric jets of gamma-ray bursts

We study the gravitational wave (GW) production induced by the asymmetric jets of gamma-ray bursts (GRBs). The asymmetric jets result in a recoil force acted on the central compact object, whose motion leads to emission of GW. Under reasonable assumptions and simplifications, we derive the analytic form of the produce GWs. The amplitude of emitted GWs is estimated to be relatively low, but possibility exists that they can be detected by future experiments such as the Einstein Telescope. We find the dynamical properties of the central object, which is difficult to be studied via the electromagnetic (EW) channel, can be inferred by measuring the emitted GWs. Moreover, we find the emitted GWs can be used determine whether the relativistic jets is launched by the neutrino annihilation process or the Blandford-Znajek process, which cannot be clearly distinguished by the current GRB observations. Our work manifests the importance of the GW channel in multi-messenger astronomy. The physical information encoded in the GW and EW emissions of an astrophysical object is complementary to each other; in case some physics can not be effectively investigated using the EW channel alone, including the GW channel can be very helpful.

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arXiv2017

Evidence of a spectral break in the gamma-ray emission of the disk component of Large Magellanic Cloud: a hadronic origin?

It has been suggested that high-energy gamma-ray emission ($>100{\rm MeV}$) of nearby star-forming galaxies may be produced predominantly by cosmic rays colliding with the interstellar medium through neutral pion decay. The pion-decay mechanism predicts a unique spectral signature in the gamma-ray spectrum, characterized by a fast rising spectrum and a spectral break below a few hundreds of MeV. We here report the evidence of a spectral break around 500 MeV in the disk emission of Large Magellanic Cloud (LMC), which is found in the analysis of the gamma-ray data extending down to 60 MeV observed by {\it Fermi}-Large Area Telescope. The break is well consistent with the pion-decay model for the gamma-ray emission, although leptonic models, such as the electron bremsstrahlung emission, cannot be ruled out completely.

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arXiv2017

Search for GeV counterparts to fast radio bursts with Fermi

The non-repeating fast radio bursts (FRBs) could arise from catastrophic stellar explosions or magnetar giant flares, so relativistic blast waves might be produced in these events. Motivated by this, we here search for GeV counterparts to all non-repeating FRBs with Fermi Large Area Telescope (LAT), including FRB 131104 that is claimed to be possibly associated with a gamma-ray transient candidate detected by Swift Burst Alert Telescope (BAT). FRB 131104 enters the field of view (FoV) of LAT ~5000s after the burst time, so we are only able to search for the GeV afterglow emission during this period, but no significant GeV emission is detected. We also perform a search for GeV emissions from other FRBs, but no significant GeV emissions are detected either. Upper limit fluences in the range of (4.7-29.2)x10^(-7) erg/cm2 are obtained, and then the upper limits of the isotropic blast wave kinetic energy of about (1-200)x10^(53) erg are inferred under certain assumptions. Although the current limits on the isotropic blast wave energy are not sufficiently stringent to rule out the connection between FRBs and GRB-like transients, future more sensitive observations with Fermi or Imaging Atmospheric Cherenkov Telescopes might be able to constrain the connection.

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arXiv2016

Search for GeV and X-ray flares associated with the IceCube track-like neutrinos

Dozens of high-energy neutrinos have been detected by the IceCube neutrino telescope, but no clear association with any classes of astrophysical sources has been identified so far. Recently, Kadler et al. (2016) report that a PeV cascade-like neutrino event occurs in positional and temporal coincidence with a giant gamma-ray flare of the blazar PKS B1424-418. Since IceCube track-like events have much better angular resolution, we here search for possible short-term gamma-ray flares that are associated with the IceCube track-like events with Fermi Large Area Telescope (LAT) observations. Among them, three track-like neutrino events occur within the field of view of Fermi-LAT at the time of the detection, so search for the {\em prompt} gamma-ray emission associated with neutrinos are possible. Assuming a point source origin and a single power law spectrum for the possible gamma-ray sources associated with neutrinos, a likelihood analysis of 0.2-100 GeV photons observed by Fermi-LAT on the timescales of $\sim 12$ hours and one year are performed, and for the three special neutrinos, the analysis are also performed on the timescales of thousand of seconds before and after the neutrino detection. No significant GeV excesses over the background are found and the upper limit fluxes at 95\% confidence level are obtained for different timescales. We also search for possible hard X-ray transient sources associated with the IceCube track-like neutrino events, but the search also yields null results. We discuss the implication of the non-detection of gamma-ray flares for the constraints on the neutrino source density.

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arXiv2016

Search for high energy gamma-ray emission from tidal disruption events with the Fermi Large Area Telescope

Massive black holes at galaxy center may tear apart a star when the star passes occasionally within the disruption radius, which is the so-called tidal disruption event(TDE). Most TDEs radiate with thermal emission resulted from the accretion disk, but three TDEs have been detected in bright non-thermal X-ray emission, which is interpreted as arising from the relativistic jets. Search for high-energy gamma-ray emission from one relativistic TDE (Swift J164449.3+573451) with the \textsl{Fermi} Large Area Telescope (LAT) has yielded non-detection. In this paper, we report the search for high energy emission from the other two relativistic TDEs (Swift J2058.4+0516 Swift J1112.2-8238) during the flare period. No significant GeV emission is found, with an upper limit fluence in LAT energy range being less than $1\%$ of that in X-rays. Compared with gamma-ray bursts (GRBs) and blazars, these TDEs have the lowest flux ratio between GeV emission and X-ray emission. The non-detection of high-energy emission from relativistic TDEs could be due to that the high-energy emission is absorbed by soft photons in the source. Based on this hypothesis, upper limits on the bulk Lorentz factors, $Γ\la 30$, are then obtained for the jets in these TDEs. We also search for high-energy gamma-ray emission from the nearest TDE discovered to date, ASASSN-14li. No significant GeV emission is found and an upper limit of $L(\rm 0.1-10 GeV)\le 4.4\times 10^{42}$ erg s$^{-1}$ (at $95\%$ confidence level) is obtained for the first $10^{7}$ s after the disruption.

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arXiv2016

First detection of GeV emission from an ultraluminous infrared galaxy: Arp 220 as seen with the Fermi Large Area Telescope

Cosmic rays (CRs) in starburst galaxies produce high energy gamma-rays by colliding with the dense interstellar medium (ISM). Arp 220 is the nearest ultra luminous infrared galaxy (ULIRG) that has star-formation at extreme levels, so it has long been predicted to emit high-energy gamma-rays. However, no evidence of gamma-ray emission was found despite intense efforts of search. Here we report the discovery of high-energy gamma-ray emission above 200 MeV from Arp 220 at a confidence level of $\sim 6.3 σ$ using 7.5 years of \textsl {Fermi} Large Area Telescope observations. The gamma-ray emission shows no significant variability over the observation period and it is consistent with the quasi-linear scaling relation between the gamma-ray luminosity and total infrared luminosity for star-forming galaxies, suggesting that these gamma-rays arise from CR interactions. As the high density medium of Arp 220 makes it an ideal CR calorimeter, the gamma-ray luminosity can be used to measure the efficiency of powering CRs by supernova (SN) remnants given a known supernova rate in Arp 220. We find that this efficiency is about $4.2\pm2.6\%$ for CRs above 1 GeV.

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