arXiv subjects

Hai-Ming Zhang

Publications and source records attributed to Hai-Ming Zhang.

At least 37 records · Page 2Linked to original sources

arXiv2024

Lobe-dominated gamma-ray Emission of Compact Symmetric Objects

The $γ$-ray emitting compact symmetric objects (CSOs) PKS 1718--649, NGC 3894, and TXS 0128+554 are lobe-dominated in the radio emission. In order to investigate their $γ$-ray radiation properties, we analyze the $\sim$14-yr Fermi/LAT observation data of the three CSOs. They all show the low luminosity ($10^{41}-10^{43}$ erg s$^{-1}$) and no significant variability in the $γ$-ray band. Their $γ$-ray average spectra can be well fitted by a power-law function. These properties of $γ$-rays are clearly different from the $γ$-ray emitting CSOs CTD 135 and PKS 1413+135, for which the $γ$-rays are produced by a restarted aligned jet. In the $L_γ-Γ_γ$ plane, the three CSOs are also located at the region occupied by radio galaxies (RGs) while CTD 135 and PKS 1413+135 display the similar feature to blazars. Together with the similar radio emission property to $γ$-ray emitting RGs Cen A and Fornax A, we speculate that the $γ$-rays of the three CSOs stem from their extended mini-lobes. The broadband spectral energy distributions of the three CSOs can be well explained by the two-zone leptonic model, where their $γ$-rays are produced by the inverse Compton process of the relativistic electrons in extended region. By extrapolating the observed Fermi/LAT spectra to the very high energy band, we find that TXS 0128+554 among the three CSOs may be detected by the Cherenkov Telescope Array in future.

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arXiv2023

Fermi-LAT Detection of a GeV Afterglow from a Compact Stellar Merger

It is usually thought that long-duration gamma-ray bursts (GRBs) are associated with massive star core collapse, whereas short-duration GRBs are associated with mergers of compact stellar binaries. The discovery of a kilonova associated with a nearby (350 Mpc) long-duration GRB-GRB 211211A, however, indicates that the progenitor of this long-duration GRB is a compact object merger. Here we report the Fermi-LAT detection of gamma-ray ($>100 {\rm \ MeV}$) afterglow emission from GRB 211211A, which lasts $\sim$20,000 s after the burst, the longest event for conventional short-duration GRBs ever detected. We suggest that this gamma-ray emission results from afterglow synchrotron emission. The soft spectrum of GeV emission may arise from a limited maximum synchrotron energy of only a few hundreds of MeV at $\sim$20,000 s. The unusually long duration of the GeV emission could be due to the proximity of this GRB and the long deceleration time of the GRB jet that is expanding in a low-density circumburst medium, consistent with the compact stellar merger scenario.

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arXiv2023

Constraints on the intergalactic magnetic field strength from $γ$-ray observations of GRB 221009A

Characteristics of the cascade gamma-ray signal resulting from very-high-energy gamma-ray sources, such as gamma-ray bursts, can be used to constrain the strength and structure of intergalactic magnetic fields (IGMF). There has been a debate on whether GRB 190114C, the first gamma-ray burst with observed TeV photons, can constrain the IGMF. Recently, LHAASO detected the brightest-of-all-time GRB 221009A, which has much larger energy in TeV band and the spectrum extends to energy above 10 TeV, providing an unprecedented opportunity to studying IGMF. We perform a Monte-Carlo simulation of the cascade process with the public ELMAG code, considering the TeV data of GRB 221009A observed by LHAASO. By comparing the resulting cascade emission with the flux limit obtained from Fermi-LAT observations, we infer a limit of $B\ge 10^{-18.5}\rm G$ for IGMF. Though this limit may not be as strong as the limit from blazars, it serves as an independent constraint on IGMF from a new class of TeV sources.

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arXiv2023

GRB 221009A: revealing a hidden afterglow during the prompt emission phase with Fermi-GBM observations

Recently, LHAASO reported the detection of brightest-of-all-time GRB 221009A, revealing the early onset of a TeV afterglow. However, there is no evidence of afterglow emission at such early time at other wavelengths. Here we report the discovery of a hidden afterglow component during the prompt emission phase with Fermi Gamma-Ray Burst Monitor (GBM) observations. We analyze the spectral evolution of the X-ray/$γ$-ray emission of GRB 221009A measured by GBM during the dips of two prompt emission pulses (i.e., intervals $T_{0}+[300-328]\rm~s$ and $T_{0}+[338-378]\rm~s$, where $T_0$ is the GBM trigger time). We find that the spectra at the dips transit from the Band function to a power-law function, indicating a transition from the prompt emission to the afterglow. After $\sim T_{0}+ 660 \rm~s$, the spectrum is well described by a power-law function and the afterglow becomes dominant. Remarkably, the underlying afterglow emission at the dips smoothly connect with the afterglow after $\sim T_{0}+ 660 \rm~s$. The entire afterglow emission measured by GBM can be fitted by a power-law function $F\sim t^{-0.95\pm0.05}$, where $t$ is the time since the first main pulse at $T^*=T_0+226~{\rm s}$, consistent with the TeV afterglow decay measured by LHAASO. The start time of this power-law decay indicates that the afterglow peak of GRB 221009A should be earlier than $T_{0}+300 \rm ~s$. We also test the possible presence of a jet break in the early afterglow light curve, finding that both the jet break model and single power-law decay model are consistent with the GBM data. The two models can not be distinguished with the GBM data alone because the inferred jet break time is quite close to the end of GBM observations.

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arXiv2022

Constraints on the Model of Gamma-ray Bursts and Implications from GRB 221009A: GeV gamma rays v.s. High-energy Neutrinos

Gamma-ray bursts (GRB) are generally believed to be efficient particle accelerators. In the presence of energetic protons in a GRB jet, interactions between these protons and intense radiation field of the GRB are supposed to induce electromagnetic cascade. Electrons/positrons generated in the cascade will produce an additional spectrum of robust feature, which is in the form of a power-law distribution up to GeV regime with an index of $\lesssim 2$. We suggest that measurements of Fermi-LAT at GeV band can provide independent constraints on the key GRB model parameters such as the dissipation radius, the jet's bulk Lorentz factor, and the baryon loading factor. Taking GRB 221009A, the brightest GRB ever detected, as an example, we show that the constraints from GeV gamma-ray emission may be more stringent than that from the neutrino observation, providing us a deep insight into the origin of GRBs.

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arXiv2022

Detection of GeV emission from an ultra-long gamma-ray burst with the Fermi Large Area Telescope

GRB 220627A, detected by Fermi Gamma-ray Burst Monitor (GBM), shows two episodes of gamma-ray emission, which are separated by a {$\sim$700} s long quiescent phase. Due to similar temporal shapes and spectra in the two episodes, GRB 220627A is speculated to be a gravitationally lensed gamma-ray burst (GRB). We analyze the Fermi Large Area Telescope (LAT) data and find that about 49 gamma-ray photons above 100 MeV come from the GRB during the first episode, while there are no photons above 100 MeV in the second episode. Based on the broadband spectral study of the two episodes, the gravitationally lensing scenario can be ruled out at a high confidence level and we thus conclude that GRB 220627A is an intrinsically ultralong GRB with the prompt burst emission lasting longer than 1000 s. It is then the first case that GeV emission is detected from an ultralong GRB. We find that a short spike seen in the LAT light curve is also present in GBM detectors that see the burst, suggesting a common internal region of emission across the entire Fermi energy range. The detection of a 15.7 GeV photon during the early prompt phase places a lower limit of $Γ\ge300$ on the bulk Lorentz factor of the GRB ejecta. The constraint on the bulk Lorentz factor could shed light on the origin of ultralong GRBs.

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arXiv2022

Interpretation of the light curve of GeV-TeV emission from the 2021 outburst of the recurrent nova RS Ophiuchi

Nova outbursts take place in binary star systems comprising a white dwarf (WD) and either a low-mass Sun-like star (classical novae) or, a red giant. GeV gamma-ray emission has been detected from a dozen of classical novae and from one novae in symbiotic system (V407 Cyg) by Fermi-LAT. For classical novae, gamma-ray emission is generally thought to be related to internal shocks formed as fast outflow collides with the slow outflow. However, for V407 Cyg, the origin of the gamma-ray emission has been debated, as both internal shock and external shock, resulted from the collision between the nova ejecta and the ambient wind of the giant companion, were suggested to explain the gamma-ray data. Recently, bright GeV and TeV gamma-ray emission has been detected from a nova in symbiotic system, RS Ophiuchi, during its 2021 outburst, which shows a remarkably smooth power-law decay in time up to about one month after the outburst. We show that this temporal decay behavior can be interpreted as arising from an adiabatic external shock expanding in the red giant wind. In this interpretation, the gamma-rays are produced by shock-accelerated protons interacting with the dense wind through the hadronic process. We also derive the scaling relations for the decay slopes for both adiabatic and radiative nova shocks in the self-similar deceleration phase.

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arXiv2022

X-ray and GeV-gamma-ray emission property of TeV Compact Symmetric Object PKS 1413+135 and Implication for Episodic Jet Activity

PKS 1413+135, a compact symmetric object (CSO) with a two-side pc-scale structure in its miniature radio morphology, is spatially associated with 4FGL J1416.1+1320 and recently detected with MAGIC telescopes. We comprehensively analyze its X-ray and GeV gamma-ray observation data for revealing its high energy radiation physics. It is found that the source is in a low-flux stage before MJD 58500 and experiences violent outbursts after MJD 58500 in the GeV band. The flux at 10 GeV varies ~3 orders of magnitude, and the GeV-flux variation is accompanied by the clear spectral variation, which is characterized as a soft log-parabola spectrum in the low-flux state and a hard power-law spectrum in the bright flares. The variability amplitude of X-rays is lower than that of gamma-rays, and no correlation of variability between gamma-rays and X-rays is observed. Fitting the broadband spectral energy distribution during a GeV outburst with a multi-zone leptonic model, we show that the GeV gamma-rays are attributed to the external Compton process while the X-rays are hybrid of several components. The predicted TeV gamma-ray flux during the GeV outburst is consistent with the detection of MAGIC telescopes. These results, together with its CSO radio morphology, imply that PKS 1413+135 has the episodic nuclear jet activities. The weak gamma-ray emission before MJD 58500 may be from its sub-pc-/pc-scale jet component powered by previous activities, and the violent outbursts with short-timescale variability after MJD 58500 could be attributed to the recently restarted jet activity.

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arXiv2022

Detection of diffuse gamma-ray emission towards a massive star forming region hosting Wolf-Rayet stars

Isotopic and elemental abundances seen in Galactic cosmic rays imply that $\sim20\%$ of the cosmic-ray (CR) nuclei are probably synthesized by massive Wolf-Rayet (WR) stars. Massive star clusters hosting WR and OB-type stars have been proposed as potential Galactic cosmic-ray accelerators for decades, in particular via diffusive shock acceleration at wind termination shocks. Here we report the analysis of {\em Fermi} Large Area Telescope's data towards the direction of Masgomas-6a, a young massive star cluster candidate hosting two WR stars. We detect an extended $γ$-ray source with $\rm{TS}=183$ in the vicinity of Masgomas-6a, spatially coincident with two unassociated {\em Fermi} 4FGL sources. We also present the CO observational results of molecular clouds in this region, using the data from the Milky Way Imaging Scroll Painting project. The $γ$-ray emission intensity correlates well with the distribution of molecular gas at the distance of Masgomas-6a, indicating that these gamma rays may be produced by CRs accelerated by massive stars in Masgomas-6a. At the distance of $3.9{\rm \ kpc}$ of Masgomas-6a, the luminosity of the extended source is $(1.81\pm0.02)\times 10^{35}{\rm \ erg \ s^{-1}}$. With a kinetic luminosity of $\sim 10^{37}{\rm erg \ s^{-1}}$ in the stellar winds, the WR stars are capable of powering the $γ$-ray emission via neutral pion decay resulted from cosmic ray $pp$ interactions. The size of the GeV source and the energetic requirement suggests a CR diffusion coefficient smaller than that in the Galactic interstellar medium, indicating strong suppression of CR diffusion in the molecular cloud.

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arXiv2022

Can a Single Population Account for the Discriminant Properties in Fast Radio Bursts?

To probe this question, we perform a statistical analysis using the first Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst (CHIME/FRB) catalog and identify a few discriminant properties between repeating and non-repeating FRBs such as the repetition rate, duration, bandwidth, spectral index, peak luminosity, and potential peak frequency. If repeating and non-repeating FRBs belong to one population, their distribution distinctions for the repetition rate and duration can be explained by the selection effect due to the beamed emission as in Connor et al. (2020). However, we obtain that the distribution distinctions for the spectral index and potentially the peak frequency cannot be explained by the beamed emission within the framework of either the coherent curvature radiation or synchrotron maser emission. This indicates that there could be two populations. We further discuss three possible scenarios for the required two populations.

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arXiv2022

GeV $γ$-ray Emission of Compact Steep-Spectrum Source 4C +39.23B

Thirteen yr observation data of 4FGL J0824.9+3915 with the Large Area Telescope on board the Fermi Gamma Ray Space Telescope (Fermi/LAT) are analyzed for revisiting whether 4C +39.23B, a compact steep-spectrum (CSS) source closed to a flat-spectrum radio quasar (FSRQ) 4C +39.23A in the $γ$-ray emitting region of 4FGL J0824.9+3915, is a $γ$-ray emitter. We find that the time-integrated $γ$-ray emission of 4FGL J0824.9+3915 is overwhelmingly dominated by 4C +39.23A. It shows significant variability at a 6.7$σ$ confidence level and the average $γ$-ray flux in the 0.1--300 GeV energy band is $(1.60\pm 0.15)\times10^{-8}$ ph cm$^{-2}$ s$^{-1}$ with a power-law photon spectral index of $2.48\pm0.05$. During MJD 57500--58500, 4FGL J0824.9+3915 is in a low state with a steady $γ$-ray flux. Analyzed the Fermi/LAT observation data in this time interval, it is found that the TS values of the $γ$-ray emission from 4C +39.23A and 4C +39.23B are $\sim5$ and $\sim 31$, respectively, indicating that the $γ$-ray emission in this time interval is dominated by the CSS 4C +39.23B. The derived average flux in this time interval for 4C +39.23B is $(9.40\pm4.10)\times 10^{-9}$ ph cm $^{-2}$ s$^{-1}$ with $Γ_γ=2.45\pm0.17$. Attributing the spectral energy distribution (SED) of 4C +39.23B to the radiations from its core and extended region, we show that the SED can be represented with a two-zone leptonic model. Its $γ$-ray emission is contributed by the core region. The derived magnetic field strength and Doppler boosting factor of the core are 0.13 G and 6.5. Comparing 4C +39.23B with other $γ$-emitting CSSs and compact symmetric objects (CSOs) in the $Γ_γ-L_γ$ plane, it resembles CSSs.

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arXiv2021

Fermi-LAT detection of extended gamma-ray emission in the vicinity of SNR G045.7$-$00.4: evidence for escaping cosmic rays interacting with the surrounding molecular clouds

We present the analysis of Fermi Large Area Telecope (LAT) data of the gamma-ray emission in the vicinity of a radio supernova remnant (SNR), G045.7$-$00.4. To study the origin of the gamma-ray emission, we also make use of the CO survey data of Milky Way Imaging Scroll Painting to study the massive molecular gas complex that surrounds the SNR. The whole size of the GeV emission is significantly larger than that of the radio morphology. Above 3 GeV, the GeV emission is resolved into two sources: one is spatially consistent with the position of the SNR with a size comparable to that of the radio emission, and the other is located outside of the western boundary of the SNR and spatially coincident with the densest region of the surrounding molecular cloud. We suggest that the GeV emission of the western source may arise from cosmic rays (CRs) which have escaped the SNR and illuminated the surrounding molecular cloud. We find that the gamma-ray spectra of the western source can be consistently explained by this scenario with a total energy of $\sim 10^{50}{\rm erg}$ in escaping CRs assuming the escape is isotropic.

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arXiv2021

Highly Variable $γ$-Ray Emission of CTD 135 and Implications for its Compact Symmetric Structure

The $γ$-ray emission properties of CTD 135, a typical compact symmetric object (CSO), are investigated with $\sim$11-year Fermi/LAT observations. We show that it has bright and significantly variable GeV emission, with the $γ$-ray luminosity of $L_γ\sim10^{47}$ erg s$^{-1}$ and a variation index of TS$_{\rm var}=1002$. A quasi-periodic oscillation (QPO) with a periodicity of $\sim$460 days is detected in the global 95\% false-alarm level. These $γ$-ray emission features are similar to that of blazars. Its broadband spectral energy distribution (SED) can be attributed to the radiations of the relativistic electrons accelerated in the core region and the extended region. The SED modeling shows that the $γ$-rays are from the core region, which has a Doppler boosting factor of $δ\sim10.8$ and relativistically moves with a small viewing angle, being similar to blazar jets. On the base of the analysis results, we propose that the episodic activity of the central engine in CTD 135 results in a blazar-like jet and the bubble-like lobes as the Fermi bubbles in the Galaxy. The strong $γ$-ray emission with obvious variability is from the jet radiations and the symmetric radio structure is attributed to the bubbles. The jet radiation power and disk luminosity in units of Eddington luminosity of CTD 135 follow the same relation as other young radio galaxies, indicating that its jet radiation may be also driven by the Eddington ratio.

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arXiv2020

Magnetar giant flare origin for GRB 200415A inferred from a new scaling relation and its implications

Soft gamma-ray repeaters (SGRs) are mainly a Galactic population and originate from neutron stars with intense ($B\simeq 10^{15}{\rm \ G}$) magnetic fields ('magnetars'). Occasionally, a giant flare occurs with enormous intensity, displaying a short hard spike, followed by a weaker, oscillatory phase which exhibits the rotational period of the neutron star. If the magnetar giant flares occur in nearby galaxies, they would appear as cosmic short-hard gamma-ray bursts (GRBs) without detecting the weak oscillatory phase. Recently, a short-hard GRB named GRB 200415A was detected, with a position coincident with the Sculptor Galaxy (NGC 253), rasing the question whether it is a classic short GRB or a magentar giant flare. Here we show that magnetar giant flares follow a scaling relation between the spectral peak energy and the isotropic energy in $1\,{\rm keV} - 10\,{\rm MeV}$, i.e., $E_{\rm p}\propto E_{\rm iso}^{1/4}$, and locate in a distinct region of the $E_{\rm p}-E_{\rm iso}$ plane from that of classic short GRBs. The relation can be well understood in the model that giant flares arise from the photosphere emissions of relativistically expanding fireball. GRB 200415A, together with two other candidate giant flares (GRB 051103 and GRB 070201) follow this relation, which strongly favor the giant flare origin of these GRBs. The GeV emission detected by \emph{Fermi}/LAT from GRB 200415A at $18-285 \,$s can also be explained in the giant flare scenario. The total energy in the GeV emission implies a baryon load of $\sim 10^{23}{\rm g}$ in the giant flare fireball of GRB 200415A.

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arXiv2020

Jet Properties of Compact Steep-Spectrum Sources and an Eddington-Ratio-Driven Unification Scheme of Jet Radiation in Active Galactic Nuclei

Compact steep-spectrum sources (CSSs) likely represent a population of young radio-loud active galactic nuclei (AGNs) and have been identified as gamma-ray emitting sources. We present a comprehensive analysis of their gamma-ray emission observed with Fermi/LAT and establish their broadband spectral energy distributions (SEDs). We derive their jet properties by the SED fits with a two-zone leptonic model for radiations from the compact core and large-scale extended region, and explore the possible signature of a unification picture of jet radiation among subclasses of AGNs. We show that the observed gamma-rays of CSSs with significant variability are contributed by the radiation of their compact cores via the inverse Compton process of the torus photons. The derived power-law distribution index of the radiating electrons is p_1~1.5-1.8, magnetic field strength is B~0.15-0.6 G, and Doppler boosting factor is δ~2.8-8.9. Assuming that the jet is composed of electron-positron pairs, the compact cores of CSSs are magnetized and have a high radiation efficiency, similar to that of flat spectrum radio quasars. The six CSSs on average have higher Eddington ratio and black hole mass than those non-GeV-detected CSSs, and they follow the correlation between the jet power in units of Eddington luminosity (P_jet/L_Edd) and Eddington ratio (R_Edd) with other sub-classes of AGNs, P_jet/L_Edd~R_Edd^0.52, indicating that R_Edd would be a key physical driver for the unification scheme of AGN jet radiation.

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arXiv2020

GeV $γ$-ray emission from M33 and Arp~299

Star-forming galaxies are huge reservoirs of cosmic rays (CRs) and these CRs convert a significant fraction of their energy into $γ$-rays by colliding with the interstellar medium (ISM). Several nearby star-forming galaxies have been detected in GeV-TeV $γ$-rays. It is also found that the $γ$-ray luminosities in 0.1-100 GeV correlate well with indicators of star formation rates of the galaxies, such as the total infrared (IR) luminosity. In this paper, we report a systematic search for possible $γ$-ray emission from galaxies in the IRAS Revised Bright Galaxies Sample, using 11.4 years of $γ$-ray data taken by the Fermi Large Area Telescope (LAT). Two new galaxies, M33 and Arp 299, are detected significantly. The two galaxies are consistent with the empirical correlation between the $γ$-ray luminosity and total infrared luminosity, suggesting that their $γ$-ray emissions should mainly originate from CRs interacting with ISM. Nevertheless, there is a tentative evidence that the flux of the $γ$-ray emission from Arp~299 is variable. If the variability is true, part of the emission from Arp 299 should originate from the obscured AGN in this interacting galaxy system. In addition, we find that the $γ$-ray excess from M33 is located at the northeast region of the galaxy, where a supergiant H II region, NGC604, resides. This indicates that some bright star-forming regions in spiral galaxies could play a dominant role in the galaxy in producing $γ$-ray emission.

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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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arXiv2020

Diversity of gamma-ray and Radio Variabilities of Bright Blazars and Implications for gamma-ray Emission Location

Violent multi-wavelength variabilities are observed in gamma-ray-selected blazars. We present an analysis of long-term light curves for eight bright blazars to explore the co-variation pattern in the gamma-ray and radio bands. We extract their gamma-ray light curves and spectra with data observed by the Fermi/LAT since 2008. We find diverse co-variation patterns between the gamma-ray and radio (at 43 GHz) fluxes in these sources. The gamma-ray and radio fluxes of 3C 454.3 and PKS 1633+382 are correlated without any time-lag, suggesting that they are from the same radiation region. Similar correlation is also observed in 3C 273 and PKS 1222+216, but the radio flux is lag behind the gamma-ray flux approximately ~160 days and ~290 days, respectively. This likely suggests that their gamma-ray emission regions are located at the upstream of their radio cores at 43 GHz. The gamma-ray and radio fluxes of the other four blazars are not correlated, implying that the gamma-ray and radio emission may be from different regions in their jets. The gamma-ray light curves of the eight blazars can be decomposed into some long timescale variability components and fast spike flares. We propose that they may be attributed to the central engine activity and the magnetic reconnection process or turbulence in the local emission region, respectively.

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