arXiv subjects

Felix Aharonian

Publications and source records attributed to Felix Aharonian.

At least 91 records · Page 5Linked to original sources

arXiv2017

Glimpse of the highly obscured HMXB IGR J16318-4848 with Hitomi

We report a Hitomi observation of IGR J16318-4848, a high-mass X-ray binary system with an extremely strong absorption of N_H~10^{24} cm^{-2}. Previous X-ray studies revealed that its spectrum is dominated by strong fluorescence lines of Fe as well as continuum emission. For physical and geometrical insight into the nature of the reprocessing material, we utilize the high spectroscopic resolving power of the X-ray microcalorimeter (the soft X-ray spectrometer; SXS) and the wide-band sensitivity by the soft and hard X-ray imager (SXI and HXI) aboard Hitomi. Even though photon counts are limited due to unintended off-axis pointing, the SXS spectrum resolves Fe K{α_1} and K{α_2} lines and puts strong constraints on the line centroid and width. The line width corresponds to the velocity of 160^{+300}_{-70} km s^{-1}. This represents the most accurate, and smallest, width measurement of this line made so far from any X-ray binary, much less than the Doppler broadening and shift expected from speeds which are characteristic of similar systems. Combined with the K-shell edge energy measured by the SXI and HXI spectra, the ionization state of Fe is estimated to be in the range of Fe I--IV. Considering the estimated ionization parameter and the distance between the X-ray source and the absorber, the density and thickness of the materials are estimated. The extraordinarily strong absorption and the absence of a Compton shoulder component is confirmed. These characteristics suggest reprocessing materials which are distributed in a narrow solid angle or scattering primarily with warm free electrons or neutral hydrogen.

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arXiv2017

Hitomi Observation of Radio Galaxy NGC 1275: The First X-ray Microcalorimeter Spectroscopy of Fe-Kα Line Emission from an Active Galactic Nucleus

The origin of the narrow Fe-Kα fluorescence line at 6.4 keV from active galactic nuclei has long been under debate; some of the possible sites are the outer accretion disk, the broad line region, a molecular torus, or interstellar/intracluster media. In February-March 2016, we performed the first X-ray microcalorimeter spectroscopy with the Soft X-ray Spectrometer (SXS) onboard the Hitomi satellite of the Fanaroff-Riley type I radio galaxy NGC 1275 at the center of the Perseus cluster of galaxies. With the high energy resolution of ~5 eV at 6 keV achieved by Hitomi/SXS, we detected the Fe-Kα line with ~5.4 σ significance. The velocity width is constrained to be 500-1600 km s$^{-1}$ (FWHM for Gaussian models) at 90% confidence. The SXS also constrains the continuum level from the NGC 1275 nucleus up to ~20 keV, giving an equivalent width ~20 eV of the 6.4 keV line. Because the velocity width is narrower than that of broad Hα line of ~2750 km s$^{-1}$, we can exclude a large contribution to the line flux from the accretion disk and the broad line region. Furthermore, we performed pixel map analyses on the Hitomi/SXS data and image analyses on the Chandra archival data, and revealed that the Fe-Kα line comes from a region within ~1.6 kpc from the NGC 1275 core, where an active galactic nucleus emission dominates, rather than that from intracluster media. Therefore, we suggest that the source of the Fe-Kα line from NGC 1275 is likely a low-covering fraction molecular torus or a rotating molecular disk which probably extends from a pc to hundreds pc scale in the active galactic nucleus system.

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arXiv2017

Atmospheric gas dynamics in the Perseus cluster observed with Hitomi

Extending the earlier measurements reported in Hitomi collaboration (2016, Nature, 535, 117), we examine the atmospheric gas motions within the central 100~kpc of the Perseus cluster using observations obtained with the Hitomi satellite. After correcting for the point spread function of the telescope and using optically thin emission lines, we find that the line-of-sight velocity dispersion of the hot gas is remarkably low and mostly uniform. The velocity dispersion reaches maxima of approximately 200~km~s$^{-1}$ toward the central active galactic nucleus (AGN) and toward the AGN inflated north-western `ghost' bubble. Elsewhere within the observed region, the velocity dispersion appears constant around 100~km~s$^{-1}$. We also detect a velocity gradient with a 100~km~s$^{-1}$ amplitude across the cluster core, consistent with large-scale sloshing of the core gas. If the observed gas motions are isotropic, the kinetic pressure support is less than 10\% of the thermal pressure support in the cluster core. The well-resolved optically thin emission lines have Gaussian shapes, indicating that the turbulent driving scale is likely below 100~kpc, which is consistent with the size of the AGN jet inflated bubbles. We also report the first measurement of the ion temperature in the intracluster medium, which we find to be consistent with the electron temperature. In addition, we present a new measurement of the redshift to the brightest cluster galaxy NGC~1275.

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arXiv2017

Measurements of resonant scattering in the Perseus cluster core with Hitomi SXS

Thanks to its high spectral resolution (~5 eV at 6 keV), the Soft X-ray Spectrometer (SXS) on board Hitomi enables us to measure the detailed structure of spatially resolved emission lines from highly ionized ions in galaxy clusters for the first time. In this series of papers, using the SXS we have measured the velocities of gas motions, metallicities and the multi-temperature structure of the gas in the core of the Perseus cluster. Here, we show that when inferring physical properties from line emissivities in systems like Perseus, the resonant scattering (RS) effect should be taken into account. In the Hitomi waveband, RS mostly affects the FeXXV He$α$ line ($w$) - the strongest line in the spectrum. The flux measured by Hitomi in this line is suppressed by a factor ~1.3 in the inner ~30 kpc, compared to predictions for an optically thin plasma; the suppression decreases with the distance from the center. The $w$ line also appears slightly broader than other lines from the same ion. The observed distortions of the $w$ line flux, shape and distance dependence are all consistent with the expected effect of the resonant scattering in the Perseus core. By measuring the ratio of fluxes in optically thick ($w$) and thin (FeXXV forbidden, He$β$, Ly$α$) lines, and comparing these ratios with predictions from Monte Carlo radiative transfer simulations, the velocities of gas motions have been obtained. The results are consistent with the direct measurements of gas velocities from line broadening described elsewhere in this series, although the systematic and statistical uncertainties remain significant. Further improvements in the predictions of line emissivities in plasma models, and deeper observations with future X-ray missions will enable RS measurements to provide powerful constraints on the amplitude and anisotropy of clusters gas motions.

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arXiv2017

Diffuse gamma-ray emission in the vicinity of young star cluster Westerlund 2

We report the results of our analysis of the publicly available data obtained by the Large Area Telescope (LAT) on board of the Fermi satellite towards the direction of the young massive star cluster Westerlund 2. We found significant extended gamma-ray emission in the vicinity of Westerlund 2 with a hard power-law energy spectrum extending from 1 GeV to 250 GeV with a photon index of 2.0 +/- 0.1. We argue that amongst several alternatives, the luminous stars in Westerlund 2 are likely sites of acceleration of particles responsible for the diffuse gamma-ray emission of the surrounding interstellar medium. In particular, the young star cluster Westerlund 2 can provide sufficient non-thermal energy to account for the gamma-ray emission. In this scenario, since the gamma-ray production region is significantly larger than the area occupied by the star cluster, we conclude that the gamma-ray production is caused by hadronic interactions of the accelerated protons and nuclei with the ambient gas. In that case, the total energy budget in relativistic particles is estimated of the order of 1e50 erg.

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arXiv2017

Hitomi X-ray studies of Giant Radio Pulses from the Crab pulsar

To search for giant X-ray pulses correlated with the giant radio pulses (GRPs) from the Crab pulsar, we performed a simultaneous observation of the Crab pulsar with the X-ray satellite Hitomi in the 2 -- 300 keV band and the Kashima NICT radio observatory in the 1.4 -- 1.7 GHz band with a net exposure of about 2 ks on 25 March 2016, just before the loss of the Hitomi mission.The timing performance of the Hitomi instruments was confirmed to meet the timing requirement and about 1,000 and 100 GRPs were simultaneously observed at the main and inter-pulse phases, respectively, and we found no apparent correlation between the giant radio pulses and the X-ray emission in either the main or inter-pulse phases.All variations are within the 2 sigma fluctuations of the X-ray fluxes at the pulse peaks, and the 3 sigma upper limits of variations of main- or inter- pulse GRPs are 22\% or 80\% of the peak flux in a 0.20 phase width, respectively, in the 2 -- 300 keV band.The values become 25\% or 110\% for main or inter-pulse GRPs, respectively, when the phase width is restricted into the 0.03 phase.Among the upper limits from the Hitomi satellite, those in the 4.5-10 keV and the 70-300 keV are obtained for the first time, and those in other bands are consistent with previous reports.Numerically, the upper limits of main- and inter-pulse GRPs in the 0.20 phase width are about (2.4 and 9.3) $\times 10^{-11}$ erg cm$^{-2}$, respectively. No significant variability in pulse profiles implies that the GRPs originated from a local place within the magnetosphere and the number of photon-emitting particles temporally increases.However, the results do not statistically rule out variations correlated with the GRPs, because the possible X-ray enhancement may appear due to a $>0.02$\% brightening of the pulse-peak flux under such conditions.

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arXiv2017

Search for Thermal X-ray Features from the Crab nebula with Hitomi Soft X-ray Spectrometer

The Crab nebula originated from a core-collapse supernova (SN) explosion observed in 1054 A.D. When viewed as a supernova remnant (SNR), it has an anomalously low observed ejecta mass and kinetic energy for an Fe-core collapse SN. Intensive searches were made for a massive shell that solves this discrepancy, but none has been detected. An alternative idea is that the SN1054 is an electron-capture (EC) explosion with a lower explosion energy by an order of magnitude than Fe-core collapse SNe. In the X-rays, imaging searches were performed for the plasma emission from the shell in the Crab outskirts to set a stringent upper limit to the X-ray emitting mass. However, the extreme brightness of the source hampers access to its vicinity. We thus employed spectroscopic technique using the X-ray micro-calorimeter onboard the Hitomi satellite. By exploiting its superb energy resolution, we set an upper limit for emission or absorption features from yet undetected thermal plasma in the 2-12 keV range. We also re-evaluated the existing Chandra and XMM-Newton data. By assembling these results, a new upper limit was obtained for the X-ray plasma mass of <~ 1Mo for a wide range of assumed shell radius, size, and plasma temperature both in and out of the collisional equilibrium. To compare with the observation, we further performed hydrodynamic simulations of the Crab SNR for two SN models (Fe-core versus EC) under two SN environments (uniform ISM versus progenitor wind). We found that the observed mass limit can be compatible with both SN models if the SN environment has a low density of <~ 0.03 cm-3 (Fe core) or <~ 0.1 cm-3 (EC) for the uniform density, or a progenitor wind density somewhat less than that provided by a mass loss rate of 10-5 Mo yr-1 at 20 km s-1 for the wind environment.

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arXiv2017

Diffuse Galactic antimatter from faint thermonuclear supernovae in old stellar populations

Our Galaxy hosts the annihilation of a few $\times 10^{43}$ low-energy positrons every second. Radioactive isotopes capable of supplying such positrons are synthesised in stars, stellar remnants, and supernovae. For decades, however, there has been no positive identification of a main stellar positron source leading to suggestions that many positrons originate from exotic sources like the Galaxy's central super-massive black hole or dark matter annihilation. %, but such sources would not explain the recently-detected positron signal from the extended Galactic disk. Here we show that a single type of transient source, deriving from stellar populations of age 3-6 Gyr and yielding ~0.03 $M_\odot$ of the positron emitter $^{44}$Ti, can simultaneously explain the strength and morphology of the Galactic positron annihilation signal and the solar system abundance of the $^{44}$Ti decay product $^{44}$Ca. This transient is likely the merger of two low-mass white dwarfs, observed in external galaxies as the sub-luminous, thermonuclear supernova known as SN1991bg-like.

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arXiv2016

On the energy distribution of relativistic electrons in the young supernova remnant G1.9+0.3

The broad-band X-ray observations of the youngest known galactic supernova remnant, G1.9+0.3, provide unique information about the particle acceleration at the early stages of evolution of supernova remnants. Based on the publicly available X-ray data obtained with the Chandra and NuSTAR satellites over two decades in energy, we derived the energy distribution of relativistic electrons under the assumption that detected X-rays are of entirely synchrotron origin. The acceleration of electrons was found to be an order of magnitude slower than the maximum rate provided by the shock acceleration in the nominal Bohm diffusion regime. We discuss the implications of this result in the context of contribution of SNRs to the Galactic Cosmic Rays at PeV energies.

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arXiv2016

Detection of diffuse gamma-ray emission near the young massive cluster NGC 3603

We report the Fermi Large Area Telescope's detection of extended gamma-ray emission towards the direction of the young massive star cluster NGC 3603. The emission shows a hard spectrum with a photon index of 2.3 from 1 GeV to 250 GeV. The large size and high luminosity of this structure make it unlikely a pulsar wind nebular. On the other hand the spatial correlation with the ionised gas indicate a hadronic origin. The total cosmic ray (CR) protons energy are estimated to be of the order $10^{50} ~\rm erg$ assuming the gamma-ray are produced in the interaction of CRs with ambient gas . The environment and spectral features show significant similarity with the Cygnus cocoon. It reveals that the young star clusters may be a gamma-ray source population and they can potentially accelerate a significant fraction of the Galactic cosmic rays.

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arXiv2016

Detection of gamma rays of likely jet origin in Cygnus X-1

Aims: Probe the high-energy ($>$60 MeV) emission from the black hole X-ray binary system, Cygnus X-1, and investigate its origin. Methods: We analysed 7.5 yr of data by Fermi/LAT with the latest PASS8 software version. Results: We report the detection of a signal at $\sim$8 $σ$ statistical significance spatially coincident with Cygnus X-1 and a luminosity above 60 MeV of 5.5$\times$10$^{33}$ erg s$^{-1}$. The signal is correlated with the hard X-ray flux: the source is observed at high energies only during the hard X-ray spectral state, when the source is known to display persistent, relativistic radio emitting jets. The energy spectrum, extending up to $\sim$20 GeV without any sign of spectral break, is well fitted by a power-law function with a photon index of 2.3$\pm$0.2. There is a hint of orbital flux variability, with high-energy emission mostly coming around the superior conjunction. Conclusions: We detected GeV emission from Cygnus X-1 and probed that the emission is most likely associated with the relativistic jets. The evidence of flux orbital variability points to the anisotropic inverse Compton on stellar photons as the mechanism at work, thus constraining the emission region to a distance $10^{11}-10^{13}$ cm from the black hole.

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arXiv2016

Giant lobes of Centaurus A as seen in radio and gamma-ray images obtained with the Fermi-LAT and Planck satellites

The γ-ray data of Fermi-LAT on the giant lobes of Centaurus A are analysed together with the high frequency radio data obtained with the Planck satellite. The large γ-ray photon statistics, accumulated during seven years of observations, and the recently updated Fermi collaboration software tools allow substantial extension of the detected γ-ray emission towards higher energy, up to 30 GeV, and lower energy, down to 60 MeV. Moreover, the new γ-ray data allow us to explore the spatial features of γ-ray emission of the lobes. For the north lobe, we confirm, with higher statistical significance, our earlier finding on the extension of γ-ray emission beyond the radio image. Moreover, the new analysis reveals significant spatial variation of γ-ray spectra from both lobes. On the other hand, the Planck observations at microwave frequencies contain important information on spectra of synchrotron emission in the cutoff region, and thus allow model-independent derivation of the strength of the magnetic field and the distribution of relativistic electrons based on the combined γ-ray and radio data. The interpretation of multiwavelength spectral energy distributions (SEDs) of the lobes within a pure leptonic model requires strong enhancement of the magnetic field at the edge of the south lobe. Alternatively, a more complex, leptonic-hadronic model of the γ-ray emission, postulating a non-negligible contribution of the π^0 -decay component at highest energies, can explain the γ-ray data with a rather homogeneous distribution of the magnetic field over the giant lobes.

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arXiv2016

The Quiescent Intracluster Medium in the Core of the Perseus Cluster

Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. It can enable new insights into mechanical energy injection by the central supermassive black hole and the use of hydrostatic equilibrium for the determination of cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50 million K diffuse hot plasma filling its gravitational potential well. The Active Galactic Nucleus of the central galaxy NGC1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These likely induce motions in the intracluster medium and heat the inner gas preventing runaway radiative cooling; a process known as Active Galactic Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus cluster core, which reveal a remarkably quiescent atmosphere where the gas has a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s is found across the 60 kpc image of the cluster core. Turbulent pressure support in the gas is 4% or less of the thermodynamic pressure, with large scale shear at most doubling that estimate. We infer that total cluster masses determined from hydrostatic equilibrium in the central regions need little correction for turbulent pressure.

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arXiv2016

On the GeV excess in the diffuse γ-ray emission towards the Galactic Center

The Fermi-LAT γ-ray data have been used to study the morphological and spectral features of the so-called GeV excess - a diffuse radiation component recently discovered towards the Galactic centre. We used the likelihood method to analyze Fermi-LAT data. Our study does confirm the existence of such an extra component in the diffuse γ-ray emission at GeV energies. Based on a detailed morphological analysis, a spatial template that fits the data best was generated and adopted. Using this template, the energy distribution of γ-rays was derived in the 0.3-30 GeV energy interval. The spectrum appeared to have less distinct ('bump'-like) structure than previous reported. We argue that the morphology of this radiation component has a bipolar rather than a spherically symmetric structure as has been assumed a priori in previous studies. This finding excludes the associations of the GeV excess with Dark Matter. We briefly discuss the radiation mechanisms and possible source populations that could be responsible for this new component of diffuse gamma radiation.

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arXiv2016

On the potential of atmospheric Cherenkov telescope arrays for resolving TeV gamma-ray sources in the Galactic Plane

The potential of an array of imaging atmospheric Cherenkov telescopes to detect gamma-ray sources in complex regions has been investigated. The basic characteristics of the gamma-ray instrument have been parametrized using simple analytic representations. In addition to the ideal (Gaussian form) point spread function (PSF), the impact of more realistic non-Gaussian PSFs with tails has been considered. Simulations of isolated point-like and extended sources have been used as a benchmark to test and understand the response of the instrument. The capability of the instrument to resolve multiple sources has been analyzed and the corresponding instrument sensitivities calculated. The results are of particular interest for weak gamma-ray emitters located in crowded regions of the Galactic plane, where the chance of clustering of two or more gamma-ray sources within 1 degree is high.

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arXiv2016

Radial distribution of the diffuse gamma-ray emissivity in the galactic disk

The Fermi-LAT data accumulated over 7 years of γ-ray observations, together with the high resolution gas (CO & HI) and the dust opacity maps, are used to study the emissivity of γ-rays induced by interactions of cosmic rays (CRs) with the interstellar medium. Based on the dust opacity templates, the γ-ray emissivity was measured for 36 segments of the Galactic plane. Furthermore, the γ-ray emissivity was evaluated in six Galactocentric rings. Both the absolute emissivity and the energy spectra of γ-rays derived in the interval 0.2-100 GeV show significant variations along the galactic plane. The density of CRs, derived under the assumption that γ-rays are predominately produced in CR interactions with the interstellar gas, is characterised by a strong radial dependence. In the inner Galaxy the CR density substantially exceeds the density in the outer parts of the Galaxy: by a factor of a few at 10 GeV, and by more than an order of magnitude at 1 TeV. Remarkably, the energy distribution of CRs appears to be substantially harder than the energy spectrum obtained from direct measurements of local CRs. At the same time, the flux and the energy spectrum of multi-GeV protons derived from γ-ray data in the outskirts of the Galaxy is quite close to the measurements of local CRs.

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arXiv2015

Detection of persistent gamma-ray emission toward SS433/W50

The microquasar SS433 features the most energetic jets known in our Galaxy. A large fraction of the jet kinetic power is delivered to the surrounding W50 nebula at the jet termination shock, from which high-energy emission and cosmic-ray production have been anticipated. Here we report on the detection of a persistent gamma-ray signal from the direction of SS433/W50 with the Fermi Large Area Telescope. The steady flux and a narrow spectral energy distribution with a maximum around 250 MeV suggest that gamma-rays are rendered by the bulk jet kinetic power through proton-proton collisions at the SS433/W50 interaction regions. If the same mechanism is operating in other baryon-loaded microquasar jets, their collective contribution may represent a significant fraction of the total galactic cosmic-ray flux at GeV energies.

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arXiv2015

Fermi-LAT observations of the Sagittarius B complex

Aims. We use 5 years of Fermi-LAT data towards the Galactic-centre giant molecular cloud complex, Sagittarius B, to test questions of how well-mixed the Galactic component of cosmic rays are and what the level of the cosmic-ray sea in different parts of the Galaxy is. Methods. We use dust-opacity maps from the Planck satellite to obtain independent methods for background subtraction and an estimate for the mass of the region. We then present high-quality spectra of gamma-ray emission from 0.3 to 30 GeV and obtain an estimate of the cosmic-ray spectrum from the region. Results. We obtain an estimate of the mass of the region of $1.5 \pm 0.2 \times 10^7~\rm M_{\odot}$ using the Planck data, which agrees well with molecular-line-derived estimates for the same region. We find the the gamma-ray flux from this region is fitted well with a cosmic-ray spectrum that is the same as is observed locally, with evidence of a small over-density at intermediate (1-10 GeV) energies. Conclusions. We conclude that the gamma-ray and cosmic-ray spectrum in the region can be well-fitted using a local cosmic-ray spectrum.

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