arXiv subjects

Felix Aharonian

Publications and source records attributed to Felix Aharonian.

At least 37 records · Page 2Linked to original sources

arXiv2023

Line profile of nuclear de-excitation gamma-ray emission from very hot plasma

De-excitation gamma-ray lines, produced by nuclei colliding with protons, provide information about astrophysical environments where particles have kinetic energies of $10-100$ MeV per nucleon. In general, such environments can be categorized into two types: the interaction between non-thermal MeV cosmic rays and ambient gas, and the other is thermal plasma with a temperature above a few MeV. In this paper, we focus on the latter type and investigate the production of de-excitation gamma-ray lines in very hot thermal plasma, especially the dependence of the line profile on the plasma temperature. We have calculated the line profile of prompt gamma rays from $^{12}$C and $^{16}$O and found that when nuclei have a higher temperature than protons, gamma-ray line profiles can have a complex shape unique to each nucleus species. This is caused by anisotropic gamma-ray emission in the nucleus rest frame. We propose that the spectroscopy of nuclear de-excitation gamma-ray lines may enable to probe energy distribution in very hot astrophysical plasmas. This diagnostics can be a new and powerful technique to investigate the physical state of a two-temperature accretion flows onto a black hole, especially the energy distributions of the protons and nuclei, which are difficult to access for any other diagnostics.

astro-ph.HE↗

arXiv2023

Naked forward shock seen in the TeV afterglow data of GRB221009A

We explore the implications of the light curve of the early TeV gamma-ray afterglow of GRB221009A reported by the LHAASO collaboration. We show that the reported offset of the reference time, $T_*$, allows the determination of the relativistic jet activation time, which occurs approximately $200\,\mathrm{s}$ after the GBM trigger time and closely precedes the moment at which GBM was saturated. We find that while the LHAASO data do not exclude the homogeneous circumburst medium scenario, the progenitor wind scenario looks preferable, finding excellent agreement with the expected size of the stellar bubble. We conclude that the initial growth of the light curve is dominated by processes internal to the jet or by gamma-gamma attenuation on the photons emitted during the prompt phase. Namely, either the activation of the acceleration process or the decrease of internal gamma-gamma absorption can naturally explain the initial rapid flux increase. The subsequent slow flux growth phase observed up to $T_*+18\,\mathrm{s}$ is explained by the build-up of the synchrotron radiation -- the target for inverse Compton scattering, which is also supported by a softer TeV spectrum measured during this period. The duration of this phase allows an almost parameter-independent determination of the jet's initial Lorentz factor, $Γ_0\approx600$, and magnetic field strength, $B'\sim0.3\,\mathrm{G}$. These values appear to match well those previously revealed through spectral modeling of the GRB emission.

astro-ph.HE↗

arXiv2023

On the nature of the energy-dependent morphology of the composite multi-TeV gamma-ray source HESS J1702-420

HESS J1702-420 is a multi-TeV gamma-ray source with an unusual energy-dependent morphology. The recent H.E.S.S. observations suggest that the emission is well described by a combination of point-like HESS J1702-420A (dominating at highest energies, $\gtrsim$ 30 TeV ) and diffuse ($\sim$ 0.3$^\circ$) HESS J1702-420B (dominating below $\lesssim$ 5TeV) sources with very hard ($Γ \sim 1.5$) and soft ($Γ$ ~2.6) power-law spectra, respectively. Here we propose a model which postulates that the proton accelerator is located at the position of HESS J1702-420A and is embedded into a dense molecular cloud that coincides with HESS J1702-420B. In the proposed model, the VHE radiation of HESS J1702-420 is explained by the pion-decay emission from the continuously injected relativistic protons propagating through a dense cloud. The energy-dependent morphology is defined by the diffusive nature of the low-energy protons propagation, transiting sharply to (quasi) ballistic propagation at higher energies. Adopting strong energy dependence of the diffusion coefficient, $D \propto E^β$ with $β\geq 1$, we argue that HESS J1702-420 as the system of two gamma-ray sources is the result of the propagation effect. Protons injected by a single accelerator at the rate $Q_0 \simeq 10^{38} \, (n_0/100 \, \rm cm^{-3})^{-1}\, (d/ \, 0.25\,kpc)^{-1} \rm erg/s$ can reasonably reproduce the morphology and fluxes of two gamma-ray components.

astro-ph.HE↗

arXiv2023

New estimation of the nuclear de-excitation line emission from the supernova remnant Cassiopeia A

MeV nuclear de-excitation lines serve as a unique tool to study low-energy cosmic rays (CRs), containing both spectral and elemental information of the interacting material. In this paper, we estimated the possible nuclear de-excitation lines from the young supernova remnant Cassiopeia A. Given different CR spectral shapes and interacting materials, we found the predicted fluxes of strong narrow line emissions from the remnant are highly model-dependent, ranging from about $1\times10^{-10}\,{\rm \,cm^{-2}\,s^{-1}}$ to $1\times10^{-6}\, {\rm \,cm^{-2}\,s^{-1}}$ for the 4.44 MeV narrow line and from about $4\times10^{-11}\,{\rm \,cm^{-2}\,s^{-1}}$ to $2\times10^{-7}{\rm \,cm^{-2}\,s^{-1}}$ for the 6.13 MeV narrow line, respectively. Based on the new estimation, we also discussed the detection probability of these line emissions against the MeV diffuse Galactic background under different assumptions of instrument response functions.

astro-ph.HE↗

arXiv2023

On the properties of inverse Compton spectra generated by up-scattering a power-law distribution of target photons

Relativistic electrons are an essential component in many astrophysical sources, and their radiation may dominate the high-energy bands. Inverse Compton (IC) emission is the radiation mechanism that plays the most important role in these bands. The basic properties of IC, such as the total and differential cross sections, have long been studied; the properties of the IC emission depend strongly not only on the emitting electron distribution but also on the properties of the target photons. This complicates the phenomenological studies of sources, where target photons are supplied from a broad radiation component. We study the spectral properties of IC emission generated by a power-law distribution of electrons on a power-law distribution of target photons. We approximate the resulting spectrum by a broken-power-law distribution and show that there can be up to three physically motivated spectral breaks. If the target photon spectrum extends to sufficiently low energies, $\varepsilon_{\mathrm{min}}< m_e^2c^4/E_{\mathrm{max}}$ ($m_e$ and $c$ are electron mass and speed of light, respectively; $\varepsilon_{\mathrm{min}}$ and $E_{\mathrm{max}}$ are the minimum/maximum energies of target photons and electrons, respectively), then the high energy part of the IC component has a spectral slope typical for the Thomson regime with an abrupt cutoff close to $E_{\mathrm{max}}$. The spectra typical for the Klein-Nishina regime are formed above $m_e^2c^4/\varepsilon_{\mathrm{min}}$. If the spectrum of target photons features a cooling break, i.e., a change of the photon index by $0.5$ at $\varepsilon_{\mathrm{br}}$, then the transition to the Klein-Nishina regime proceeds through an intermediate change of the photon index by $0.5$ at $m_e^2c^4/\varepsilon_{\mathrm{br}}$.

astro-ph.HE↗

arXiv2023

Pre-merger alert to detect the very-high-energy prompt emission from binary neutron-star mergers: Einstein Telescope and Cherenkov Telescope Array synergy

The current generation of very-high-energy $gamma-$ray (VHE; E above 30 GeV) detectors (MAGIC and H.E.S.S.) have recently demonstrated the ability to detect the afterglow emission of GRBs. However, the GRB prompt emission, typically observed in the 10 keV-10 MeV band, has so far remained undetected at higher energies. Here, we investigate the perspectives of multi-messenger observations to detect the prompt emission of short GRBs in VHE. Considering binary neutron star mergers as progenitors of short GRBs, we evaluate the joint detection efficiency of the Cherenkov Telescope Array (CTA) observing in synergy with the third generation of gravitational wave detectors, such as the Einstein Telescope (ET) and Cosmic Explorer (CE). In particular, we evaluate the expected capabilities to detect and localize gravitational wave events in the inspiral phase and to provide an early warning alert able to drive the VHE search. We compute the amount of possible joint detections by considering several observational strategies, and demonstrate that the sensitivities of CTA make the detection of the VHE emission possible even if it is several orders fainter than the one observed at 10 keV-10 MeV. We discuss the results in terms of possible scenarios of production of VHE photons from binary neutron star mergers by considering GRB prompt and afterglow emissions.

astro-ph.HE↗

arXiv2023

The formation of hard VHE spectra from GRB afterglow via Two-Zone Synchrotron Self-Compton Emission

Electron Compton scattering of target photons into the gamma-ray energy band (inverse Compton scattering --IC--) is commonly expected to dominate the very high energy spectra in gamma-ray bursts especially during the afterglow phase. For sufficiently large center-of-mass energies in these collisions, the effect of the electron recoil starts reducing the scattering cross section (the Klein-Nishina regime). The IC spectra generated in the Klein-Nishina regime is softer and has a smaller flux level compared to the synchrotron spectra produced by the same electrons. The detection of afterglow emission from nearby GRB 190819A in the very high energy (VHE) domain with H.E.S.S. has revealed an unexpected feature: the slope of the VHE spectrum matches well the slope of the X-ray spectra, despite expectations that for the IC production process, the impact of the Klein-Nishina effect should be strong. The multi-wavelength spectral energy distribution appears to be inconsistent with predictions of one-zone synchrotron-self-Compton models. We study the possible impact of two-zone configuration on the properties of IC emission when the magnetic field strength differs considerably between the two zones. Synchrotron photons from the strong magnetic field zone provide the dominant target for cooling of the electrons in the weak magnetic field zone, which results in a formation of hard electron distribution and consequently of a hard IC emission. We show that the two-zone model can provide a good description of the X-ray XRT and VHE H.E.S.S. data.

astro-ph.HE↗

arXiv2023

Effective Shielding of $\lesssim$ 10 GeV Cosmic Rays from Dense Molecular Clumps

The density of cosmic rays inside molecular clouds determines the ionization rate in the dense cores where stars form. It is also one of the drivers of astrochemistry leading to the creation of complex molecules. Through Fermi Large Area Telescope observations of nearby giant molecular clouds, we observed deficits (holes) in the gamma-ray residual map when modelling with the expected gamma-ray diffuse emission from uniform cosmic rays interacting with the molecular content. We propose that the deficit is due to the lack of penetration of the low-energy (sub-GeV to GeV) cosmic rays into denser regions or clumps. This differs from the prevailing view of fast cosmic ray transport in giant molecular clouds where the magnetic turbulence is suppressed by neutral-ion damping, as our results require a slow diffusion inside dense molecular clumps. Through modelling we find that while the shielding is negligible on the cloud scale, it becomes important in the denser, parsec-sized regions where the gravitational collapse is already at play, changing the initial condition of star formation and astrochemistry.

astro-ph.HE↗

arXiv2022

Gamma-ray emission from the Sagittarius Dwarf Spheroidal galaxy due to millisecond pulsars

The Fermi Bubbles are giant, gamma-ray emitting lobes emanating from the nucleus of the Milky Way discovered in ~1-100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi Bubbles that has been interpreted as a signature of collimated outflows from the Galaxy's super-massive black hole. Here we show via a spatial template analysis that much of the gamma-ray emission associated to the brightest region of substructure -- the so-called cocoon -- is likely due to the Sagittarius dwarf spheroidal (Sgr dSph) galaxy. This large Milky Way satellite is viewed through the Fermi Bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sgr dSph has no on-going star formation, but we nevertheless demonstrate that the dwarf's millisecond pulsar (MSP) population can plausibly supply the gamma-ray signal that our analysis associates to its stellar template. The measured spectrum is naturally explained by inverse Compton scattering of cosmic microwave background photons by high-energy electron-positron pairs injected by MSPs belonging to the Sgr dSph, combined with these objects' magnetospheric emission. This finding plausibly suggests that MSPs produce significant gamma-ray emission amongst old stellar populations, potentially confounding indirect dark matter searches in regions such as the Galactic Centre, the Andromeda galaxy, and other massive Milky Way dwarf spheroidals.

astro-ph.HE↗

arXiv2022

Can superbubbles accelerate ultra-high energy protons?

We critically assess limits on the maximum energy of protons accelerated within superbubbles around massive stellar clusters, considering a number of different scenarios. In particular, we derive under which circumstances acceleration of protons above peta-electronvolt (PeV) energies can be expected. While the external forward shock of the superbubble may account for acceleration of particles up to 100 TeV, internal primary shocks such as supernova remnants expanding in the low density medium or the collective wind termination shock which forms around a young compact cluster provide more favourable channels to accelerate protons up to 1 PeV, and possibly beyond. Under reasonable conditions, clustered supernovae launching powerful shocks into the magnetised wind of a young and compact massive star cluster are found to be the most promising systems to accelerate protons above 10 PeV. On the other hand, stochastic re-acceleration in the strongly turbulent plasma is found to be much less effective than claimed in previous works, with a maximum proton energy of at most a few hundred TeV.

astro-ph.HE↗

arXiv2022

Hard X-ray emission from the eastern jet of SS 433 powering the W50 `Manatee' nebula: Evidence for particle re-acceleration

We present a broadband X-ray study of W50 (`the Manatee nebula'), the complex region powered by the microquasar SS 433, that provides a test-bed for several important astrophysical processes. The W50 nebula, a Galactic PeVatron candidate, is classified as a supernova remnant but has an unusual double-lobed morphology likely associated with the jets from SS 433. Using NuSTAR, XMM-Newton, and Chandra observations of the inner eastern lobe of W50, we have detected hard non-thermal X-ray emission up to $\sim$30 keV, originating from a few-arcminute size knotty region (`Head') located $\lesssim$ 18$^{\prime}$ (29 pc for a distance of 5.5 kpc) east of SS 433, and constrain its photon index to 1.58$\pm$0.05 (0.5-30 keV band). The index gradually steepens eastward out to the radio `ear' where thermal soft X-ray emission with a temperature $kT$$\sim$0.2 keV dominates. The hard X-ray knots mark the location of acceleration sites within the jet and require an equipartition magnetic field of the order of $\gtrsim$12$μ$G. The unusually hard spectral index from the `Head' region challenges classical particle acceleration processes and points to particle injection and re-acceleration in the sub-relativistic SS 433 jet, as seen in blazars and pulsar wind nebulae.

astro-ph.HE↗

arXiv2022

On the potential of bright, young pulsars to power ultra-high gamma-ray sources

The recent discovery of a new population of ultra-high-energy gamma-ray sources with spectra extending beyond 100 TeV revealed the presence of Galactic PeVatrons - cosmic-ray factories accelerating particles to PeV energies. These sources, except for the one associated with the Crab Nebula, are not yet identified. With an extension of 1 degree or more, most of them contain several potential counterparts, including Supernova Remnants, young stellar clusters and Pulsar Wind Nebulae (PWNe), which can perform as PeVatrons and thus power the surrounding diffuse ultra-high energy gamma-ray structures. In the case of PWNe, gamma rays are produced by electrons, accelerated at the pulsar wind termination shock, through the inverse Compton scattering of 2.7 K CMB radiation. The high conversion efficiency of pulsar rotational power to relativistic electrons, combined with the short cooling timescales, allow gamma-ray luminosities up to the level of $L_γ\sim 0.1 \dot{E}$. The pulsar spin-down luminosity, $\dot E$, also determines the absolute maximum energy of individual photons: $E_{\rm γ~\rm max}\approx 0.9 \dot E_{36}^{0.65}~~\rm{PeV}$. This fundamental constraint dominates over the condition set by synchrotron energy losses of electrons for young PWNe with typical magnetic field of $\approx$100$μ$G with $\dot{E} \lesssim 10^{37}\ \rm erg/s$. We discuss the implications of $E_{\rm γ~\rm max}$ by comparing it with the highest energy photons reported by LHAASO from a dozen of ultra-high-energy sources. Whenever a PWN origin of the emission is possible, we use the LHAASO measurements to set upper limits on the nebular magnetic field.

astro-ph.HE↗

arXiv2022

Detailed study of extended gamma-ray morphology in the vicinity of the Coma cluster with Fermi-LAT

Galaxy clusters can be sources of high-energy (HE) $γ$-ray radiation, due to the efficient acceleration of particles exceeding EeV energies. At present, though, the only candidate for emitting HE $γ$-rays is the Coma cluster, towards which an excess of $γ$-ray emission has been detected by the Fermi Large Area Telescope (LAT). Using $\mathrm{\sim12.3}$ years of \textit{Fermi}-LAT data, we explored the region of the Coma cluster between energies 100 MeV and 1 TeV by detailed spectral and morphological analysis. In the region of the Coma cluster, we detected diffuse gamma-ray emission of energies between 100 MeV and 1 TeV with a 5.4$σ$ extension significance and a 68\% containment radius of $0.82^{+0.10}_{-0.05}$ degrees derived with a 2D homogeneous disk model. The corresponding gamma-ray spectrum extends up to $\sim50$ GeV, with a power-law index of $\mathrm{Γ=2.23\pm0.11}$ and flux of $\mathrm{(3.84\pm0.67)\times10^{-12}\,erg\,cm^{-2}\,s^{-1}}$. Using energy arguments we show that point-like sources such as radiogalaxies and star-forming galaxies are unlikely to explain the emission, and more likely, the emission is produced in the Coma cluster. Besides, we also identified three point-like sources in the region. However, due to the limited statistics of the detection, we could neither exclude nor conclude that the total extended emission is contributed to by these three-point like sources.

astro-ph.HE↗

arXiv2022

Multi-wavelength observations of the Blazar 4C +28.07

The active galactic nucleus 4C +28.07 is a flat spectrum radio quasar, one of the brightest at $γ$-ray energies. We study its multi-wavelength emission by analysing $\sim12.3$ years of \textit{Fermi-LAT} data in the $γ$-ray band and \textit{Swift-XRT/UVOT} available data in X-ray and Optical-to-Ultraviolet bands. In the $γ$-ray band, five flaring periods have been detected, during which the flux dramatically increases by several times (>5) compared with its average quiescent phase. Quasi-simultaneously with the flaring times, the X-ray and UVOT data detected by \textit{Swift-XRT/UVOT} have also been analysed. In one of the brightest flare periods (Flare 5; observed on Oct 12, 2018) the $γ$-ray flux reached $(6.7\pm0.81)\times 10^{-6}$ photon/cm^2/s ($\sim31\times$ higher than the mean flux over 12.3 years) with detection significance of $σ=6.1$. The apparent $γ$-ray luminosity of this flaring corresponds to $3.6\times10^{49}$ erg/s (for a distance of 8.38 Gpc), one of the highest $γ$-ray luminosities observed for blazars. Flare 5 has an estimated $\sim2$ hours time block, which can be considered the average $γ$-ray variability time. The variability time constrains the $γ$-ray emitting region size to <9e14 cm, which is close to the black hole radius. The spectral energy distributions (SEDs) in the $γ$-ray band for the $\sim12.3$ years of data show an early cut-off at $\sim14$ GeV; beyond $\sim60$ GeV, however, the spectrum hardens and is detected up to $\sim316$ GeV. Similar spectral behaviour is also noticeable for the SEDs of flares, which can be linked to the photon absorption by the emitting region's internal and external narrow-band radiation fields. Considering the significance of the obtained results from 4C\,+28.07, we compared the parameters with 3C\,279 and M87, to motivate further studies.

astro-ph.HE↗

arXiv2021

Probing the galactic cosmic-ray density with current and future $γ$-ray instruments

Cosmic Rays (CRs) propagating through dense molecular clouds (MCs) produce gamma rays which carry direct information about the CR distribution throughout the Galaxy. Observations of gamma rays in different energy bands allow exploration of the average CR density in the Galactic Disk, the so-called level of the "CR Sea". Fermi-LAT observations have demonstrated the method's feasibility based on two dozen MCs in our Galaxy. However, the potential of Fermi-LAT is limited by the most massive and relatively nearby MCs; thus, the current observations cover only a tiny fraction of the Milky Way. In this paper, we study the prospects of expanding the CR measurements to very and ultra-high energies and remote parts of the Galaxy with the current and next-generation detectors. Based on calculations of fluxes expected from MCs, we formulate the requirements to the sensitivity of the post-Fermi-LAT detectors to map GeV-TeV CRs in the Galactic Disk. We also explore the potential of the current and future air-shower and atmospheric Cherenkov telescope arrays for the extension of CR studies to multi-TeV and PeV energy bands. We demonstrate that the improvement of the Fermi-LAT sensitivity by a factor of a few would allow a dramatic increase in the number of detectable MCs covering almost the entire Galaxy. The recently completed LHAASO should be able to take the first CR probes at PeV energies in the coming five years or so.

astro-ph.HE↗

arXiv2021

Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2

Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. We analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. The spectral and morphological characteristics of the LAT source agree with the ones in the TeV regime (HESS J1023-575), allowing the description of the gamma-ray source from a few hundreds of MeV to a few tens of TeVs. We will present the results and discuss the implications of the identification with the stellar cluster and the radiation mechanism involved.

astro-ph.HE↗

arXiv2021

Probing the hadronic nature of the gamma-ray emission associated with Westerlund 2

Star-forming regions have been proposed as potential Galactic cosmic-ray accelerators for decades. Cosmic-ray acceleration can be probed through observations of gamma-rays produced in inelastic proton-proton collisions, at GeV and TeV energies. In this paper, we analyze more than 11 years of Fermi-LAT data from the direction of Westerlund 2, one of the most massive and best-studied star-forming regions in our Galaxy. In particular, we investigate the characteristics of the bright pulsar PSR J1023-5746 that dominates the gamma-ray emission below a few GeV at the position of Westerlund 2, and the underlying extended source FGES J1023.3-5747. The analysis results in a clear identification of FGES J1023.3-5747 as the GeV counterpart of the TeV source HESS J1023-575, through its morphological and spectral properties. This identification provides new clues about the origin of the HESS J1023-575 gamma-ray emission, favouring a hadronic origin of the emission, powered by Westerlund 2, rather than a leptonic origin related to either the pulsar wind nebula associated with PSR J1023-5746 or the cluster itself. This result indirectly supports the hypothesis that star-forming regions can contribute to the cosmic-ray sea observed in our Galaxy

astro-ph.HE↗

arXiv2021

Extension of the synchrotron radiation of electrons to very high energies in clumpy environments

The synchrotron cooling of relativistic electrons is one of the most effective radiation mechanisms in astrophysics. It not only accompanies the process of particle acceleration but also has feedback on the formation of the energy distribution of the parent electrons. The radiative cooling time of electrons decreases with energy as $t_{\rm syn} \propto 1/E$; correspondingly the overall radiation efficiency increases with energy. On the other hand, this effect strictly limits the maximum energy of individual photons. Even in the so-called extreme accelerators, where the acceleration proceeds at the highest possible rate, $t_{\rm acc}^{-1} = eBc/E$, allowed in an ideal magnetohydrodynamic plasma, the synchrotron radiation cannot extend well beyond the characteristic energy determined by the electron mass and the fine-structure constant: $h ν^{\rm max} \sim m_e c^2/α\sim 70 \rm\,MeV$. In this paper, we propose a model in which the formation of synchrotron radiation takes place in compact magnetic blobs located inside the particle accelerator and develop a formalism for calculations of synchrotron radiation emerging from such systems. We demonstrate that for certain combinations of parameters characterizing the accelerator and the magnetic blobs, the synchrotron radiation can extend beyond this limit by a several orders of magnitude. This scenario requires a weak magnetization of the particle accelerator, and an efficient conversion of gas internal energy into magnetic energy in sufficiently small blobs. The required size of the blobs is constrained by the magnetic mirroring effect, that can prevent particle penetration into the regions of strong magnetic field under certain conditions.

astro-ph.HE↗