Hard X-ray Tail Research Articles

A study of Tycho's SNR at TeV energies with the HEGRA CT-System

Tycho's supernova remnant (SNR) was observed during 1997 and 1998 with the HEGRA Cherenkov Telescope System in a search for gamma-ray emission at energies above ~1 TeV. An analysis of these data, ~65 hours in total, resulted in no evidence for TeV gamma-ray emission. The 3sigma upper limit to the gamma-ray flux (>1 TeV) from Tycho is estimated at 5.78x10^{-13} photons cm^{-2} s^{-1}, or 33 milli-Crab. We interpret our upper limit within the framework of the following scenarios: (1) that the observed hard X-ray tail is due to synchrotron emission. A lower limit on the magnetic field within Tycho may be estimated B>=22 microG, assuming that the RXTE-detected X-rays were due to synchrotron emission. However, using results from a detailed model of the ASCA emission, a more conservative lower limit B>=6 microG is derived. (2) the hadronic model of Drury, Aharonian & Voelk, and (3) the more recent time-dependent kinetic theory of Berezhko & Voelk. Our upper limit lies within the range of predicted values of both hadronic models, according to uncertainties in physical parameters of Tycho, and shock acceleration details. In the latter case, the model was scaled to suit the parameters of Tycho and re-normalised to account for a simplification of the original model. We find that we cannot rule out Tycho as a potential contributor at an average level to the Galactic cosmic-ray flux.

High-Energy X-Ray Timing Experiment Detections of Hard X-Ray Tails in Scorpius X-1

We report the detection of a nonthermal hard X-ray component from Sco X-1 based on the analysis of 20-220 keV spectra obtained with the High-Energy X-Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite. We find that the addition of a power-law component to a thermal bremsstrahlung model is required to achieve a good fit in five of 16 observations analyzed. Using Proportional Counter Array data, we were able to track the movement of the source along the Z diagram, and we found that the presence of the hard X-ray tail is not confined to a specific Z position. However, we do observe an indication that the power-law index hardens with increasing , as indicated from the position on the Z diagram. We find that the derived nonthermal luminosities are ~10% of that derived for the brightest of the atoll sources.

Broadband Spectral Energy Distributions of Active Galactic Nuclei from an Accretion Disk with Advective Coronal Flow

Recent multiwaveband observations of Seyfert nuclei and QSOs established significant deviations in the spectral shape of the big blue bump from a blackbody spectral shape; soft X-ray excess has a spectral index α (Fν ∝ ν-α) of 1.6 and hard X-ray tail with α of ~0.7. We construct a disk-corona model which accounts for such broadband spectral properties. We study the emission spectrum emerging from a vertical disk-corona structure composed of two-temperature plasma by solving hydrostatic equilibrium and radiative transfer self-consistently. A fraction f of viscous heating due to mass accretion is assumed to be dissipated in a corona with a Thomson optical depth of τc, where advective cooling is also included, and a remaining fraction, 1 - f, dissipates within a main body of the disk. Our model can nicely reproduce the soft X-ray excess with a power-law shape and the hard tail extending to ~50 keV. The different spectral slopes (α ~ 1.5 below 2 keV and ~0.5 above) are the results of different emission mechanisms and different sites; the former slope is due to unsaturated Comptonization from the innermost zone, and the latter is due to a combination of the Comptonization, bremsstrahlung, and a reflection of the coronal radiation at the disk-corona boundary from the inner to surrounding zone (≤300 Schwarzschild radii). The emergent optical spectrum is redder (α ~ 0.3) than that of the standard disk (α ~ -0.3), being consistent with observations, due to the different efficiencies of spectral hardening of disk emission at different radii. Further, we find that the cutoff frequency of the hard X-ray (~coronal electron temperature) and broadband spectral shape are insensitive to the black hole mass, while the peak frequency of the big blue bump is sensitive to the mass as the peak frequency ∝M.

HEXTE studies of Sco X-1 spectra: Detections of hard X-ray tails beyond 200 keV

Using the HEXTE experiment on-board the RXTE satellite, we performed a search for hard X-ray tails in Sco X-1 spectra. We found strong evidence for the presence of such a non-thermal component on several occasions. Using the PCA/ RXTE we were able to track the position of the source along the Z diagram, and we observed that the presence of the hard X-ray tail is not confined to a particular region. However, we found a correlation between the power law index of the non-thermal component and the position of the source in the Z diagram, suggesting that the hard X-ray spectrum (i.e., E > 50 keV) becomes flatter as the mass accretion rate increases. We were also able to study the temporal variation of the appearance/absence of the hard X-ray component. With our derived luminosities, we were also able to test the idea that X-ray luminosities can be used to distinguish between X-ray binary systems containing neutron stars and black holes.

Cassiopeia A: On the origin of the hard X-ray continuum and the implication of the observed O vııı Ly-α/Ly-β distribution

We present the rst results on the hard X-ray continuum image (up to 15 keV) of the supernova remnant Cas A measured with the EPIC cameras onboard XMM-Newton. The data indicate that the hard X-ray tail, observed previously, that extends to energies above 100 keV does not originate in localised regions, like the bright X-ray knots and laments or the primary blast wave, but is spread over the whole remnant with a rather flat hardness ratio of the 8{10 and 10{15 keV energy bands. This result does not support an interpretation of the hard X-radiation as synchrotron emission produced in the primary shock, in which case a limb brightened shell of hard X-ray emission close to the primary shock front is expected. In fact a weak rim of emission near the primary shock front is discernable in the hardest X-ray image but it contains only a few percent of the hard X-ray emissivity. The equivalent width of the Fe-K line blend varies by more than an order of magnitude over the remnant, it is hard to explain this as Fe-emission from the reverse shock heated ejecta given the ejecta temperature and the age of the remnant. The uniquely high wavelength-dispersive RGS-spectrometer has allowed, for the rst time, to extract monochromatic images in several highly ionised element species with high spectral resolution. We present here a preliminary result on the measurement of the O viii Ly- and Ly- brightness distribution and brightness ratios. The large observed decrease of the Ly-/Ly- ratio going from the N to the SE can be explained by small-scale (10 00 ) variations in the NH column over the remnant and the potential presence of resonance scattering of the O viii Ly- photons in the limb brightened shell.

Particle reacceleration in the Coma cluster: radio properties and hard X-ray emission

The radio spectral index map of the Coma halo shows a progressive steepening of the spectral index with increasing radius. Such a steepening cannot be simply justified by models involving continuous injection of fresh particles in the Coma halo or by models involving diffusion of fresh electrons from the central regions. We propose a two-phase model in which the relativistic electrons injected in the Coma cluster by some processes (starbursts, AGNs, shocks, turbulence) during a first phase in the past are systematically reaccelerated during a second phase for a relatively long time (∼1 Gyr) up to the present time. We show that for reacceleration time-scales of ∼0.1 Gyr this hypothesis can well account for the radio properties of Coma C. For the same range of parameters which explain Coma C we have calculated the expected fluxes from the inverse Compton scattering of the Cosmic Microwave Background (CMB) photons, finding that the hard X-ray tail discovered by BeppoSAX may be accounted for by the stronger reacceleration allowed by the model. The possibility of extending the main model assumptions and findings to the case of the other radio haloes is also discussed, the basic predictions being consistent with the observations.

Nonthermal Bremsstrahlung and Hard X‐Ray Emission from Clusters of Galaxies

We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray (HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these models, the HXR emission is caused by supra- thermal electrons with energies of D10¨200 keV. We consider models in which these transrelativistic suprathermal particles are the low energy end of a population of electrons being accelerated to high energies by shocks or turbulence (ii accelerating electron ˇˇ models). We also consider a model in which these electrons are the remnant of an older nonthermal population that is losing energy and rejoining the thermal distribution as a result of Coulomb interactions (ii cooling electron ˇˇ models). The supra- thermal populations are assumed to start at an electron kinetic energy 3kT , where T is the temperature of the thermal intracluster medium (ICM). The nonthermal bremsstrahlung spectra —atten at low photon energies because of the lack of low-energy nonthermal particles. The accelerating electron models have HXR spectra that are nearly power laws from D20¨100 keV. However, the spectra are brighter and —atter than given by the nonrelativistic bremsstrahlung cross section because of transrelativistic eUects. The HXR spectrum of the cooling electron model is very —at, and most of the X-ray emission in the HXR energy range (10¨100 keV) actually arises from electrons with much higher energies (D100 MeV). Under the assumption that the suprathermal electrons form part of a continuous spectrum of electrons including highly relativistic particles, we have calculated the inverse Compton (IC) extreme-ultraviolet (EUV), HXR, and radio synchrotron emission by the extensions of the same populations. For acceler- ating electron models with power-law momentum spectra (N(p) P p~k) with which are those k ( 2.7, expected from strong shock acceleration, the IC HXR emission exceeds that caused by NTB. Thus, these models are of interest only if the electron population is cut oU at some upper energy GeV. Similarly, (1 —at-spectrum accelerating electron models produce more radio synchrotron emission than is observed from clusters if the ICM magnetic —eld is kG. The cooling electron model produces vastly too B Z 1 much EUV emission as compared to the observations of clusters. We have compared these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB models require a nonthermal electron population that contains about 3% of the number of electrons in the thermal ICM. If the suprathermal electron population is cut oU at some energy above 100 keV, then the models can —t the observed HXR —uxes and spectral indices in both clusters easily. For accelerating electron models without a cutoU, the electron spectrum must be rather steep, to avoid producing too much IC HXR emission. The Z2.9, model HXR spectra are then rather steep but are marginally consistent with observations of the HXR spectrum in Abell 2199 and Coma or the radio spectrum in Coma. These models can account for the HXR and radio properties of these two clusters but do not produce enough EUV emission. Subject headings: acceleration of particlescosmic raysgalaxies: clusters: general ¨ intergalactic mediumradiation mechanisms: nonthermalX-rays: general

Hard X‐Ray Emission from Low‐Mass X‐Ray Binaries

We report on Rossi X-Ray Timing Explorer observations of four type I X-ray bursters, namely, 1E 1724-3045, GS 1826-238, SLX 1735-269, and KS 1731-260. The first three were in a low state, with 1-200 keV X-ray luminosities in the range ~0.05-0.1LEdd (LEdd: Eddington luminosity for a neutron star, LEdd = 2.5 × 1038 ergs s-1), whereas KS 1731-260 was in a high state, with luminosity ~0.35LEdd. The low-state sources have very similar power spectra, displaying high-frequency noise up to ~200 Hz. For KS 1731-260, its power spectrum is dominated by noise at frequencies 20 Hz; in addition a quasi-periodic oscillation at 1200 Hz is detected in a segment of the observation. The 1-200 keV spectra of the low-state sources are all consistent with resulting from thermal Comptonization with an electron temperature (kTe) around 25-30 keV. For KS 1731-260, the spectrum is also dominated by thermal Comptonization, but with a much lower kTe ~ 3 keV and no significant hard X-ray emission. With the exception of GS 1826-238, they each have an underlying soft component, carrying at most ~25% of the total 1-200 keV luminosity. For all sources, we have detected an iron Kα line at 6.4 keV (although it is weak and marginal in 1E 1724-3045). A reflection component is present in the spectra of GS 1826-238 and SLX 1735-269, and for both we find that the reflecting medium subtends only a small solid angle (Ω/2π ~ 0.15, 0.28). The origin of the line and the reflection component is most likely to be irradiation of the accretion disk by the X-ray source. We suggest a model in which the region of main energy release, where hard X-rays are produced, would be an optically thin boundary layer merged with an advection-dominated accretion flow (ADAF) and would be responsible for the rapid variability observed. The soft component observed probably represents the unscattered emission from an optically thick accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks and the strength of the reflected component and associated iron line increase. At the same time, the Comptonization region cools off in response to an increased cooling flux from the accretion disk and from the reprocessed/reflected component, thus leading progressively to a quenching of the hard X-ray emission. If low-state neutron stars (NSs) accrete via ADAFs, the observation of X-ray bursts, indicating that all the accreting matter actually accumulates onto the NS surface, argues against the existence of strong winds from such accretion flows. Finally, we discuss two criteria recently proposed to distinguish between nonquiescent black holes (BHs) and NSs that are not contradicted by existing observations. The first one states that, when thermal Comptonization is responsible for the hard X-ray emission, only BHs have kTe larger than ~50 keV. However, this criterion is weakened by the fact that there are NSs displaying nonattenuated power laws extending up to at least 200 keV, possibly implying nonthermal Comptonization or thermal Comptonization with kTe larger than 50 keV. The second criterion stipulates that only BHs are capable of emitting hard X-ray tails with 20-200 keV luminosities 1.5 × 1037 ergs s-1.

Hybrid accretion disks in active galactic nuclei. I - Structure and spectra

A unified treatment is presented of the two distinct states of vertically thin AGN accretion disks: a cool (about 10 to the 6th K) optically thick solution, and a hot (about 10 to the 9th K) optically thin solution. A generalized formalism and a new radiative cooling equation valid in both regimes are introduced. A new luminosity limit is found at which the hot and cool alpha solutions merge into a single solution of intermediate optical depth. Analytic solutions for the disk structure are given, and output spectra are computed numerically. This is used to demonstrate the prospect of fitting AGN broadband spectra containing both the UV bump as well as the hard X-ray and gamma-ray tail, using a single accretion disk model. Such models are found to make definite predictions about the observed spectrum, such as the relation between the hard X-ray spectral index, the UV-to-X-ray luminosity ratio, and a feature of about 1 MeV.

An evolving relativistic jet model for the BL Lacertae object Markarian 421

Coordinated observations of the nearby BL Lacertae object Mrk 421 obtained during May 1980, January 1984 and March 1984, are described. These observations give a time-frozen picture of the continuous spectrum of Mrk 421 at X-ray, ultraviolet, optical, and radio wavelengths. The observed spectra have been fitted to an inhomogeneous relativistic jet model described by Hutter and Mufson. The studies show that it is difficult to describe the data by a single jet whose properties vary as a function of time in a self-consistent manner unless the flat spectrum, hard X-ray tail present in January 1984 is due to an additional short-lived component of emission. These jet models are compared with homogeneous, spherically symmetric, synchrotron self-Compton models for this source.

Simultaneous X-ray and optical observations of GX 339-4 in an X-ray high state

Optical emission, soft X-rays, and hard X-rays have been observed simultaneously from the black hole candidate GX 3339-4 in a typical high state. Each of these components is interpreted as arising from a characteristic region of the accretion disk. Considered as a black hole, GX 339-4 lacks the variable 2 keV blackbody component that would be emitted from the 'solid' surface of a neutron star in the low-mass neutron star binaries. The X-ray emission in the high state is therefore extremely soft and stable in time. The intermediate disk region is optically thick and geometrically thin. The observed soft X-ray component with a 'disk blackbody' spectrum comes from this region. The innermost disk region is dynamically unstable due to the effect of general relativity and possibly due to radiation pressure. This region is responsible for the generation of the hard X-ray tail through Comptonization.

The X-ray spectrm of the Coma Cluster of galaxies

The HEAO 1 A-2 and OSO 8 observations of the Coma Cluster of galaxies in the 2-60 keV energy band are presented. The spectrum is not well fitted by an isothermal bremsstrahlung model with an Fe line. To model the spectrum more accurately, we have performed fits of polytropic models and find that the gas is well fitted by an equation of state with polytropic index ranging between 1.38 and 1.6. This analysis reduces the total cluster mass by a factor of 3-5 below the optically derived virial mass. Using the best-fitting parameters, the gas is found to be a significant fraction (25 percent) of the total cluster mass. The Fe K-alpha line blend is strongly detected, from which we infer 0.18-0.36 solar abundance in the X-ray emitting gas. The Fe K-beta line blend is also weakly detected. The spectrum is not consistent with the existence of a hard X-ray tail at the flux level reported by Bazzano el al. (1984). In addition, the HEAO I scanning mode data place strong constraints on possible contaminating sources in the region surrounding Coma.

Hard X-ray observation of galactic X-ray sources

A large (1455 cm 2) hard X-ray telescope was successfully launched aboard a stratospheric balloon on October 4, 1980. During this flight four galactic X-ray sources were observed, namely the transient recurrent X-ray pulsar A0535+26, the Crab Nebula, Cygnus X-1 and X Persei. Here we report the results on the latter two sources. From Cygnus X-1 we measured a photon flux in the band 30 to 200 keV, of 3.5 × 10 −2 photons cm −2 which is 6.5 times lower than that recieved from the source in a “low” intensity state in the same energy band. In addition, the photon spectrum in the same energy band was very soft and consistent with a power law with photon index α = 2.71 ± 0.14. Even if a simultaneous observation of the source at lower energies was not available, our data strongly suggest that we observed the source during a “high” intensity state. We report also positive detection in the band 30 to 200 keV of the low luminosity X-ray pulsar X Persei. In its spectrum we confirm the presence of a hard X-ray tail consistent with a power law (photon index α = 2.17 ± 0.42).