Broad-Band High-Energy Emission of the Gamma-ray Binary System LS 5039: Spectral and Temporal Features using NuSTAR and Fermi Observations

We report on detailed analysis of the hard X-ray and GeV gamma-ray spectra of LS 5039, one of the brightest gamma-ray binary system in the Galaxy. The NuSTAR observation covering its entire orbit in 2016 allowed us for the first time to study the orbital variability of the spectrum above 10 keV. The hard X-ray spectrum is well described with a single power-law component up to 78 keV. The X-ray flux showed a slight deviation from those observed previously with Suzaku in 2007. The fast X-ray brightening observed with Suzaku, around the inferior conjunction, was not observed in this observation. We also analyzed 11 years of Fermi Large Area Telescope data of LS 5039. The GeV spectrum around the inferior conjunction was well described with two non-thermal components; a power law with a photon index of $sim 3$ and a cut-off power law with a cutoff energy of $sim 2$ GeV. The orbital flux variability also changed gradually around a few GeV. These results indicate that there are two emission components in the GeV band, and the dominant component above $sim 1$ GeV does not depend on the orbital phase. By combining these results, we update the spectral energy distribution of LS 5039 with the highest available statistics. Theoretical models proposed so far cannot explain the obtained multi-wavelength spectrum, especially the emission from $sim$ 1 MeV to $sim$ 400 MeV, and we discuss a possibility that particle acceleration in LS 5039 is different from the shock acceleration.

High-frequency radio synchrotron maser emission from relativistic shocks

We study the conditions required for the production of the synchrotron maser emission downstream of a relativistic shock. We show that for weakly magnetized shocks, synchrotron maser emission can be generated at frequencies significantly exceeding the relativistic gyrofrequency. This high-frequency maser emission seems to be the most suitable for interpreting peculiar GHz radio sources. To illustrate this, we consider a magnetar flare model for FRBs. Our analysis shows that the maser emission is radiated away from the central magnetar, which guarantees a short duration of bursts independently of the shock wave radius. If FRBs are produced by the high-frequency maser emission then one can significantly relax the requirements for several key parameters: the magnetic field strength at the production site, luminosity of the flare, and the production site bulk Lorentz factor. To check the feasibility of this model, we study the statistical relation between powerful magnetar flares and the rate of FRBs. The expected ratio is derived by convoluting the redshift-dependent magnetar density with their flare luminosity function above the energy limit determined by the FRB detection threshold. We obtain that only a small fraction, (sim10^{-5}), of powerful magnetar flares trigger FRBs. This ratio agrees surprisingly well with our estimates: we obtained that (10%) of magnetars should be in the evolutionary phase suitable for the production of FRBs, and only (10^{-4}) of all flares are expected to be weakly magnetized, which is a necessary condition for the high-frequency maser emission.

Gamma-ray and Neutrino Signals from Accretion Disk Coronae of Active Galactic Nuclei

To explain X-ray spectra of active galactic nuclei (AGN), non-thermal activity in AGN coronae such as pair cascade models has been extensively discussed in the past literature. Although X-ray and gamma-ray observations in the 1990s disfavored such pair cascade models, recent millimeter-wave observations of nearby Seyferts establish the existence of weak non-thermal coronal activity. Besides, the IceCube collaboration reported NGC 1068, a nearby Seyfert, as the hottest spot in their 10-yr survey. These pieces of evidence are enough to investigate the non-thermal perspective of AGN coronae in depth again. This article summarizes our current observational understandings of AGN coronae and describes how AGN coronae generate high-energy particles. We also provide ways to test the AGN corona model with radio, X-ray, MeV gamma-ray, and high-energy neutrino observations.

Systematic Study of Acceleration Efficiency in Young Supernova Remnants with Nonthermal X-ray Observations

Cutoff energy in a synchrotron radiation spectrum of a supernova remnant (SNR) contains a key parameter of ongoing particle acceleration. We systematically analyze 11 young SNRs, including all historical SNRs, to measure the cutoff energy, thus shedding light on the nature of particle acceleration at the early stage of SNR evolution. The nonthermal (synchrotron) dominated spectra in filament-like outer rims are selectively extracted and used for spectral fitting because our model assumes that accelerated electrons are concentrated in the vicinity of the shock front due to synchrotron cooling. The cutoff energy parameter ($varepsilon_0$) and shock speed ($v_{rm sh}$) are related as $ varepsilon_0 propto v_{rm sh}^2 eta^{-1}$ with a Bohm factor of $eta$. Five SNRs provide us with spatially resolved $varepsilon_0$-$v_{rm sh}$ plots across the remnants, indicating a variety of particle acceleration. With all SNRs considered together, the systematic tendency of $eta$ clarifies a correlation between $eta$ and an age of $t$ (or an expansion parameter of $m$) as $eta propto t^{-0.4}$ ($eta propto m^{4}$). This might be interpreted as the magnetic field becomes more turbulent and self-generated, as particles are accelerated at a greater rate with time. The maximum energy achieved in SNRs can be higher if we consider the newly observed time dependence on $eta$.

The Crab nebula variability at short timescales with the Cherenkov Telescope Array

Since 2009, several rapid and bright flares have been observed at high energies (>100 MeV) from the direction of the Crab Nebula. Several hypotheses have been put forward to explain this phenomenon, but the origin is still unclear. The detection of counterparts at higher energies with the next generation of Cherenkov telescopes will be determinant to constrain the underlying emission mechanisms. We aim at studying the capability of the Cherenkov Telescope Array (CTA) to explore the physics behind the flares, by performing simulations of the Crab Nebula spectral energy distribution, both in flaring and steady state, for different parameters related to the physical conditions in the nebula. In particular, we explore the data recorded by Fermi during two particular flares that occurred in 2011 and 2013. The expected GeV and TeV gamma-ray emission is derived using different radiation models. The resulting emission is convoluted with the CTA response and tested for detection, obtaining an exclusion region for the space of parameters that rule the different flare emission models. Our simulations show different scenarios that may be favourable for achieving the detection of the flares in Crab with CTA, in different regimes of energy. In particular, we find that observations with low sub-100 GeV energy threshold telescopes could provide the most model-constraining results.

Detection of Small Flares from the Crab Nebula with Fermi-LAT

Gamma radiation from the Crab pulsar wind nebula (PWN) shows significant variability at $sim100$ MeV energies, recently revealed with spaceborne gamma-ray telescopes. Here we report the results of a systematic search for gamma-ray flares using a 7.4-year data set acquired with the Fermi Large Area Telescope. Analyzing the off-pulse phases of the Crab pulsar, we found seven previously unreported low-intensity flares (small flares). The small flares originate from the variable synchrotron component of the Crab PWN and show clearly different features from the steady component of the Crab PWN emission. They are characterized by larger fluxes and harder photon indices, similar to previously reported flares. These flares show day-scale time variability and imply a strong magnetic field of $B_{rm min}approx 1~mathrm{mG}$ at the site of the gamma-ray production. This result seems to be inconsistent with the typical values revealed with modeling of the non-thermal emission from the nebula. The detection of the small flares gives a hint of production of gamma rays above $100$ MeV in a part of the nebula with properties which are different from the main emitters, e.g., due to bulk relativistic motion.

On the Origin of High Energy Neutrinos from NGC 1068: The Role of Non-Thermal Coronal Activity

NGC 1068, a nearby type-2 Seyfert galaxy, is reported as the hottest neutrino spot in the 10-year survey data of IceCube. Although there are several different possibilities for the generation of high-energy neutrinos in astrophysical sources, feasible scenarios allowing such emission in NGC 1068 have not yet been firmly defined. We show that the flux level of GeV and neutrino emission observed from NGC 1068 implies that the neutrino emission can be produced only in the vicinity of the supermassive black hole in the center of the galaxy. The coronal parameters, such as magnetic field strength and corona size, making this emission possible are consistent with the spectral excess registered in the millimeter range. The suggested model and relevant physical parameters are similar to those revealed for several nearby Seyferts. Due to the internal gamma-ray attenuation, the suggested scenario cannot be verified by observations of NGC 1068 in the GeV and TeV gamma-ray energy bands. However, the optical depth is expected to become negligible for MeV gamma rays, thus future observations in this band will be able to prove our model.

On high-energy particles in accretion disk coronae of supermassive black holes: implications for MeV gamma rays and high-energy neutrinos from AGN cores

Recent observations with ALMA have revealed evidence for non-thermal synchrotron emission from the core regions of two nearby Seyfert galaxies. This suggests that the coronae of accretion disks in active galactic nuclei (AGNs) can be conducive to the acceleration of non-thermal electrons, in addition to the hot, thermal electrons responsible for their X-ray emission through thermal Comptonization. Here we investigate the mechanism of such particle acceleration, based on observationally inferred parameters for AGN disk coronae. One possibility to account for the observed non-thermal electrons is diffusive shock acceleration, as long as the gyrofactor $eta_g$ does not exceed $sim10^6$. These non-thermal electrons can generate gamma rays via inverse Compton scattering of disk photons, which can appear in the MeV band, while those with energies above $sim100$ MeV would be attenuated via internal $gammagamma$ pair production. The integrated emission from all AGNs with thermal and non-thermal Comptonization can reproduce the observed cosmic background radiation in X-rays as well as gamma-rays up to $sim 10$ MeV. Furthermore, if protons are accelerated in the same conditions as electrons and $eta_gsim30$, our observationally motivated model is also able to account for the diffuse neutrino flux at energies below 100-300 TeV. The next generation of MeV gamma-ray and neutrino facilities can test these expectations by searching for signals from bright, nearby Seyfert galaxies such as NGC 4151 and IC 4329A.

Short-Term Variability of X-rays from Accreting Neutron Star Vela X-1: II. Monte-Carlo Modeling

We develop a Monte Carlo Comptonization model for the X-ray spectrum of accretion-powered pulsars. Simple, spherical, thermal Comptonization models give harder spectra for higher optical depth, while the observational data from Vela X-1 show that the spectra are harder at higher luminosity. This suggests a physical interpretation where the optical depth of the accreting plasma increases with mass accretion rate. We develop a detailed Monte-Carlo model of the accretion flow, including the effects of the strong magnetic field ($sim 10^{12}$ G) both in geometrically constraining the flow into an accretion column, and in reducing the cross section. We treat bulk-motion Comptonization of the infalling material as well as thermal Comptonization. These model spectra can match the observed broad-band {it Suzaku} data from Vela X-1 over a wide range of mass accretion rates. The model can also explain the so-called low state, in which the uminosity decreases by an order of magnitude. Here, thermal Comptonization should be negligible, so the spectrum instead is dominated by bulk-motion Comptonization.

Short-Term Variability of X-rays from Accreting Neutron Star Vela X-1: I. Suzaku Observations

We have analyzed the time variability of the wide-band X-ray spectrum of Vela X-1, the brightest wind-fed accreting neutron star, on a short timescale of 2 ks by using {it Suzaku} observations with an exposure of 100 ks. During the observation, the object showed strong variability including several flares and so-called low states, in which the X-ray luminosity decreases by an order of magnitude. Although the spectral hardness increases with the X-ray luminosity, the majority of the recorded flares do not show any significant changes of circumstellar absorption. However, a sign of heavy absorption was registered immediately before one short flare that showed a significant spectral hardening. In the low states, the flux level is modulated with the pulsar spin period, indicating that even at this state the accretion flow reaches the close proximity of the neutron star. Phenomenologically, the broad-band X-ray spectra, which are integrated over the entire spin phase, are well represented by the NPEX function (a combination of negative and positive power laws with an exponential cutoff by a common folding energy) with a cyclotron resonance scattering feature at 50 keV. Fitting of the data allowed us to infer a correlation between the photon index and X-ray luminosity. Finally, the circumstellar absorption shows a gradual increase in the orbital phase interval 0.25--0.3, which can be interpreted as an impact of a bow shock imposed by the motion of the compact object in the supersonic stellar wind.