arXiv subjects

Xiang-Yu Wang

Publications and source records attributed to Xiang-Yu Wang.

At least 91 records · Page 5Linked to original sources

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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arXiv2019

A hadronuclear interpretation of a high-energy neutrino event coincident with a blazar flare

Although many high-energy neutrinos detected by the IceCube telescope are believed to have anextraterrestrial origin, their astrophysical sources remain a mystery. Recently, an unprecedenteddiscovery of a high-energy muon neutrino event coincident with a multiwavelength flare from ablazar, TXS 0506+056, shed some light on the origin of the neutrinos. It is usually believed that ablazar is produced by a relativistic jet launched from an accreting supermassive black hole (SMBH).Here we show that the high-energy neutrino event can be interpreted by the inelastic hadronuclearinteractions between the accelerated cosmic-ray protons in the relativistic jet and the dense gasclouds in the vicinity of the SMBH. Such a scenario only requires a moderate proton power in thejet, which could be much smaller than that required in the conventional hadronic model whichinstead calls upon the photomeson process. Meanwhile, the flux of the multiwavelength flare fromthe optical to gamma-ray band can be well explained by invoking a second radiation zone in thejet at a larger distance to the SMBH. In our model, the neutrino emission lasts a shorter time thanthe multiwavelength flare so the neutrino event is not necessarily correlated with the flare but it is probably accompanied by a spectrum hardening above a few GeV.

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arXiv2019

Multimessenger Tests of Einstein's Weak Equivalence Principle and Lorentz Invariance with a High-energy Neutrino from a Flaring Blazar

The detection of the high-energy ($\sim290$ TeV) neutrino coincident with the flaring blazar TXS 0506+056, the first and only $3σ$ neutrino-source association to date, provides new, multimessenger tests of the weak equivalence principle (WEP) and Lorentz invariance. Assuming that the flight time difference between the TeV neutrino and gamma-ray photons from the blazar flare is mainly caused by the gravitational potential of the Laniakea supercluster of galaxies, we show that the deviation from the WEP for neutrinos and photons is conservatively constrained to have an accuracy of $10^{-6}-10^{-7}$, which is 3--4 orders of magnitude better than previous results placed by MeV neutrinos from supernova 1987A. In addition, we demonstrate that the association of the TeV neutrino with the blazar flare sets limits on the energy scales of quantum gravity for both linear and quadratic violations of Lorentz invariance (LIV) to $E_{\rm QG, 1}>3.2\times10^{15}-3.7\times10^{16}$ GeV and $E_{\rm QG, 2}>4.0\times10^{10}-1.4\times10^{11}$ GeV. These improve previous limits on both linear and quadratic LIV energy scales in neutrino propagation by 5--7 orders of magnitude.

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arXiv2018

On the minimum jet power of TEV BL Lac objects in the p-$γ$ model

We study the requirement on the jet power in the conventional p-$γ$ models (photopion production and Bethe-Heitler pair production) for TeV BL Lac objects. We select a sample of TeV BL Lac objects whose SEDs are difficult to be explained by the one-zone leptonic model. Based on the relation between the p-$γ$ interaction efficiency and the opacity of $γγ$ absorption, we find that the detection of TeV emission poses upper limits on the p-$γ$ interaction efficiencies in these sources and hence minimum jet powers can be derived accordingly. We find that the obtained minimum jet powers exceed the Eddington luminosity of the supermassive black holes. Implications for the accretion mode of the supermassive black hole in these BL Lac objects and the origin of their TeV emissions are discussed.

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arXiv2018

Gamma-ray Production in the Extended Halo of the Galaxy and Possible Implications for the Origin of Galactic Cosmic Rays

Various studies have implied the existence of a gaseous halo around the Galaxy extending out to 100 kpc. Galactic cosmic rays (CRs) that propagate to the halo, either by diffusion or by convection with the possibly existing large-scale Galactic wind, can interact with the gas therein and produce gamma-rays via proton-proton collision. We calculate the cosmic ray distribution in the halo and the gamma-ray flux, and explore the dependence of the result on model parameters such as diffusion coefficient, CR luminosity, CR spectral index. We find that the current measurement of isotropic gamma-ray background at $\lesssim$TeV with Fermi Large Area Telescope already approaches a level that can provide interesting constraints on the properties of Galactic cosmic ray (e.g., with CR luminosity $L_{CR}\leq 10^{41}$erg/s). We also discuss the possibilities of the Fermi bubble and IceCube neutrinos originating from the proton-proton collision between cosmic rays and gas in the halo, as well as the implication of our results for the baryon budget of the hot circumgalactic medium of our Galaxy. Given that the isotropic gamma-ray background is likely to be dominated by unresolved extragalactic sources, future telescopes may extract more individual sources from the IGRB, and hence put even more stringent restriction on the relevant quantities (such as Galactic cosmic ray luminosity and baryon budget in the halo) in the presence of a turbulent halo that we consider.

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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

Kilonova emission from black hole-neutron star mergers: observational signatures of anisotropic mass ejection

The gravitational wave event GW170817 associated with the short gamma-ray burst (GRB) 170817A confirms that binary neutron star (BNS) mergers are one of the origins of short GRBs. The associated kilonova emission, radioactively powered by nucleosynthesized heavy elements, was also detected. Black hole-neutron star (BH-NS) mergers have been argued to be another promising origin candidate of short GRBs and kilonovae. Numerical simulations show that the ejecta in BH-NS mergers is geometrically much more anisotropic than the BNS merger case. In this paper, we investigate observational signatures of kilonova emission from the anisotropic ejecta in BH-NS mergers. We find that a bump appears on the bolometric luminosity light curve due to the inhomogeneous mass distribution in the latitudinal direction. The decay slope of the single-band light curve becomes flatter and the spectrum also deviates from a single-temperature blackbody radiation spectrum due to the gradient in the velocity distribution of the ejecta. Future detection or non-detection of such signatures would be useful to test the mass ejection geometry in BH-NS mergers.

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arXiv2018

Inefficient cosmic ray diffusion around Vela X : constraints from H.E.S.S. observations of very high-energy electrons

Vela X is a nearby pulsar wind nebula (PWN) powered by a $\sim 10^4$ year old pulsar. Modeling of the spectral energy distribution of the Vela X PWN has shown that accelerated electrons have largely escaped from the confinement, which is likely due to the disruption of the initially confined PWN by the SNR reverse shock. The escaped electrons propagate to the earth and contribute to the measured local cosmic-ray (CR) electron spectrum. We find that the escaped CR electrons from Vela X would hugely exceed the measured flux by HESS at $\sim 10$ TeV if the standard diffusion coefficient for the interstellar medium is used. We propose that the diffusion may be highly inefficient around Vela X and find that a spatially-dependent diffusion can lead to CR flux consistent with the HESS measurement. Using a two-zone model for the diffusion around Vela X, we find that the diffusion coefficient in the inner region of a few tens of parsecs should be $<10^{28}{\rm cm^2 s^{-1}}$ for $\sim10$ TeV CR electrons, which is about two orders of magnitude lower than the standard value for ISM. Such inefficient diffusion around PWN resembles the case of the Geminga and Monogem PWNe, suggesting that inefficient diffusion may be common in the vicinity of PWNe spanning a wide range of ages.

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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

Can winds driven by active galactic nuclei account for the extragalactic gamma-ray and neutrino backgrounds?

Various observations are revealing the widespread occurrence of fast and powerful winds in active galactic nuclei (AGNs) that are distinct from relativistic jets, likely launched from accretion disks and interacting strongly with the gas of their host galaxies. During the interaction, strong shocks are expected to form that can accelerate non-thermal particles to high energies. Such winds have been suggested to be responsible for a large fraction of the observed extragalactic gamma-ray background (EGB) in the GeV-TeV range and the diffuse neutrino background in the PeV range, via the decay of neutral and charged pions generated in inelastic $pp$ collisions between protons accelerated by the forward shock and the ambient gas. However, previous studies did not properly account for processes such as adiabatic losses that may reduce the gamma-ray and neutrino fluxes significantly. We evaluate the production of gamma-rays and neutrinos by AGN-driven winds in some detail by modeling their hydrodynamic and thermal evolution, including the effects of their two-temperature structure. We find that they can only account for less than $\sim 30$% of the EGB flux, as otherwise the model would violate the independent upper limit derived from the diffuse isotropic gamma-ray background. If the neutrino spectral index is steep with $Γ\gtrsim 2.2$, a severe tension with the isotropic gamma-ray background would arise as long as the winds contribute more than $20$% of the IceCube neutrino flux in the $10-100$TeV range. Nevertheless, at energies $\gtrsim100$~TeV, we find that the IceCube neutrino flux may still be accountable by AGN-driven winds if the spectral index is as small as $Γ\sim2.0-2.1$. The detectability of gamma-ray point sources also provides important constraints on such scenarios.

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arXiv2018

Early soft X-ray to UV emission from double neutron star mergers: implications from the long-term radio and X-ray emissions of GW 170817

Recent long-term radio follow-up observations of GW 170817 reveals a simple power-law rising light curve, with a slope of $t^{0.78}$, up to 93 days after the merger. The latest X-ray detection at 109 days is also consistent with such a temporal slope. Such a shallow rise behavior requires a mildly relativistic outflow with a steep velocity gradient profile, so that slower material with larger energy catches up with the decelerating ejecta and re-energizes it. It has been suggested that this mildly relativistic outflow may represent a cocoon of material. We suggest that the velocity gradient profile may form during the stage that the cocoon is breaking out of the merger ejecta, resulted from shock propagation down a density gradient. The cooling of the hot relativistic cocoon material immediately after it breaks out should have produced soft X-ray to UV radiation at tens of seconds to hours after the merger. The soft X-ray emission has a luminosity of $L_{\rm X}\sim 10^{45}{\rm erg s^{-1}}$ over a period of tens of seconds for a merger event like GW 170817. The UV emission shows a rise initially and peaks at about a few hours with a luminosity of $L_{\rm UV}\sim 10^{42} {\rm erg s^{-1}}$. The soft X-ray transients could be detected by future wide-angle X-ray detectors, such as the Chinese mission Einstein Probe. This soft X-ray/UV emission would serve as one of the earliest electromagnetic counterparts of gravitation waves from double neutron star mergers and could provide the earliest localization of the sources.

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arXiv2017

Evidence of an Internal Dissipation Origin for the High-energy Prompt Emission of GRB 170214A

The origin of the prompt high-energy ($>100$MeV) emission of Gamma-ray Bursts (GRBs), detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, is still under debate, for which both the external shock origin and internal dissipation origin have been suggested. In the internal dissipation scenario, the high energy emission is expected to exhibit significant temporal variability, tracking the keV/MeV fast variable behavior. Here, we report a detailed analysis on the Fermi data of GRB~170214A, which is sufficiently bright in the high energy to enable a quantitative analysis of the correlation between high-energy emission and keV/MeV emission with high statistics. Our result shows a clear temporal correlation between high-energy and keV/MeV emission in the whole prompt emission phase as well as in two decomposed short time intervals. Such correlation behavior is also found in some other bright LAT GRBs, i.e., GRB 080916C, 090902B and 090926A. For these GRBs as well as GRB 090510, we also find the rapid temporal variability in the high-energy emission. We thus conclude that the prompt high-energy emission in these bright LAT GRBs should be due to internal origin.

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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.

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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

Modeling the gamma-ray emission in the Galactic Center with a fading cosmic-ray accelerator

Recent HESS observations of the ~200 pc scale diffuse gamma-ray emission from the central molecular zone (CMZ) suggest the presence of a PeV cosmic-ray accelerator (PeVatron) located in the inner 10 pc region of the Galactic Center. Interestingly, the gamma-ray spectrum of the point-like source (HESS J1745-290) in the Galactic Center shows a cutoff at ~10 TeV, implying a cutoff around 100 TeV in the cosmic-ray proton spectrum. Here we propose that the gamma-ray emission from the inner and the outer regions may be explained self-consistently by run-away protons from a single, yet fading accelerator. In this model, gamma rays from the CMZ region are produced by protons injected in the past, while gamma rays from the inner region are produced by protons injected more recently. We suggest that the blast wave formed in a tidal disruption event (TDE) caused by the supermassive black hole (Sgr A*) could serve as such a fading accelerator. With typical parameters of the TDE blast wave, gamma-ray spectra of both the CMZ region and HESS J1745-290 can be reproduced simultaneously. Meanwhile, we find that the cosmic-ray energy density profile in the CMZ region may also be reproduced in the fading accelerator model when appropriate combinations of the particle injection history and the diffusion coefficient of cosmic rays are adopted.

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arXiv2016

Indication of a Local "Fog" of Sub-Ankle UHECR

During their propagation through intergalactic space, ultrahigh energy cosmic rays (UHECRs) interact with the background radiation fields. These interactions give rise to energetic electron/positron pairs and photons which in turn feed electromagnetic cascades, contributing to the isotropic gamma-ray background (IGRB). The gamma-ray flux level generated in this way highly depends upon the UHECR propagation distance, as well as the evolution of their sources with redshift. Recently, the Fermi-LAT collaboration reported that the majority of the total extragalactic gamma-ray flux originates from extragalactic point sources. This posits a stringent upper limit on the IGRB generated via UHECR propagation, and subsequently constrains their abundance in the distant Universe. Focusing on the contribution of UHECR at energies below the ankle within a narrow energy band ($(1-4)\times 10^{18}$eV), we calculate the diffuse gamma-ray flux generated through UHECR propagation, normalizing the total cosmic ray energy budget in this band to that measured. We find that in order to not over-produce the new IGRB limit, a local "fog" of UHECR produced by nearby sources may exist, with a possible non-negligible contribution from our Galaxy. Following the assumption that a given fraction of the observed IGRB at 820GeV originates from UHECR, we obtain a constraint on the maximum distance for the majority of their sources. With other unresolved source populations still contaminating the new IGRB limit, and UHECR above the ankle invariably contributing also to this background, the results presented here are rather conservative.

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