Hard Power-law Tail Research Articles

Monitoring observations of SMC X-1’s excursions (MOOSE)–I. Programme description and initial high state spectral results

ABSTRACT SMC X-1 has exhibited three superorbital period excursions since the onset of X-ray monitoring beginning with the Rossi X-ray Timing Explorer's launch in 1995. TheNeutron star Interior Composition Explorer has recently probed a fourth observed excursion beginning in 2021 with our programme monitoring observations of SMC X-1’s excursions (moose). These sensitive new moose data probe different superorbital periods and phases within them. Spectral fits to the high-state continuum during 2021 April to 2022 January show that the intrinsic spectral shapes are characterized by a soft (kT ∼ 0.19 keV) disc component and a hard (Γ ∼ 0.7) power-law tail. When the 2021–2022 NICER observations, taken during an excursion, are compared to 2016 XMM–Newton observations (outside of an excursion), we find little evidence for intrinsic spectral variability across the high states, but find evidence for a >3σ change in the absorption, although we caution that there may be calibration differences between the two instruments. Thus, over different lengths of superorbital periods, we see little evidence for intrinsic spectral changes in the high state. Upcoming studies of the pulse profiles may shed light on the mechanism behind the excursions.

Exploring the inner-disc region of the atoll source 4U 1705-44 using AstroSat’s SXT and LAXPC observations

ABSTRACT For the first time, simultaneous broad-band spectral and timing study of the atoll source 4U 1705-44 was performed using AstroSat Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counter (LAXPC) data (0.8–70 keV). Based on the HID, the source was in the soft banana state during these observations. Spectral modelling was performed using the full reflection framework and an inner disc radii of 14 Rg was obtained. A hard power-law tail was noticed in the soft state and hot component fluxes and varying power-law indices point towards a varying corona/sub-Keplerian flow. Based on the spectral fits, the boundary layer radius and magnetospheric radius were constrained to be ∼14–18 and ∼9–19 km, respectively. Cross-correlation function studies were performed between the 0.8–3 keV soft SXT light curve and 10–20 keV hard LAXPC light curve and correlated and anticorrelated lags were found, which was used to constrain the coronal height to 0.6–20 km (β = 0.1). Since the inner disc radius is not varying during the observations, we conclude that the detected lags are possibly caused by a varying structure of corona/boundary layer in the inner region of the accretion disc. Based on the observations, a geometrical model is proposed for explaining the detected lags in the atoll source 4U 1705-44.

Investigating ULX accretion flows and cyclotron resonance in NGC 300 ULX1

Aims. We investigate accretion models for the newly discovered pulsating ultraluminous X-ray source (ULX) NGC 300 ULX1. Methods. We analyzed broadband XMM-Newton and NuSTAR observations of NGC 300 ULX1, performing phase-averaged and phase-resolved spectroscopy. Using the Bayesian framework, we compared two physically motivated models for the source spectrum: Non-thermal accretion column emission modeled by a power law with a high-energy exponential roll-off (AC model), and multicolor thermal emission from an optically thick accretion envelope plus a hard power-law tail (MCAE model). The AC model is an often used phenomenological model for the emission of X-ray pulsars, while the MCAE model has recently been proposed for the emission of the optically thick accretion envelope that is expected to form in ultraluminous (LX > 1039 erg s−1), highly magnetized accreting neutron stars. We combined the findings of our Bayesian analysis with qualitative physical considerations to evaluate the suitability of each model. Results. The low-energy part (< 2 keV) of the source spectrum is dominated by non-pulsating, multicolor thermal emission. The (pulsating) high-energy continuum is more ambiguous. If modeled with the AC model, a residual structure is detected that can be modeled using a broad Gaussian absorption line centered at ∼12 keV. However, the same residuals can be successfully modeled using the MCAE model, without the need for the absorption-like feature. Model comparison using the Bayesian approach strongly indicates that the MCAE model without the absorption line is the preferred model. Conclusions. The spectro-temporal characteristics of NGC 300 ULX1 are consistent with previously reported traits for X-ray pulsars and (pulsating) ULXs. All models considered strongly indicate the presence of an accretion disk that is truncated at a large distance from the central object, as has recently been suggested for a large portion of both pulsating and non-pulsating ULXs. The hard, pulsed emission is not described by a smooth spectral continuum. If modeled by a broad Gaussian absorption line, the fit residuals can be interpreted as a cyclotron scattering feature (CRSF) compatible with a ∼1012 G magnetic field. However, the MCAE model can successfully describe the spectral and temporal characteristics of the source emission, without the need for an additional absorption feature, and it yields physically meaningful parameter values. Therefore strong doubts are cast on the presence of a CRSF in NGC 300 ULX1.

The 2016 Outburst of PSR J1119-6127: Cooling and a Spin-down-dominated Glitch

We report on the aftermath of a magnetar outburst from the young, high-magnetic-field radio pulsar PSR J1119-6127 that occurred on 2016 July 27. We present the results of a monitoring campaign using the Neil Gehrels Swift X-ray Telescope, NuSTAR, and XMM-Newton. After reaching a peak luminosity of ~300 times the quiescent luminosity, the pulsar's X-ray flux declined by factor of ~50 on a time scale of several months. The X-ray spectra are well described by a blackbody and a hard power-law tail. After an initial rapid decline during the first day of the outburst, we observe the blackbody temperature rising from kT = 0.9 keV to 1.05 keV during the first two weeks of the outburst, before cooling to 0.9 keV. During this time, the blackbody radius decreases monotonically by a factor of ~4 over a span of nearly 200 days. We also report a heretofore unseen highly pulsed hard X-ray emission component, which fades on a similar timescale to the soft X-ray flux, as predicted by models of relaxation of magnetospheric current twists. The previously reported spin-up glitch which accompanied this outburst was followed by a period of enhanced and erratic torque, leading to a net spin-down of $\sim3.5\times10^{-4}$ Hz, a factor of ~24 over-recovery. We suggest that this and other radiatively loud magnetar-type glitch recoveries are dominated by magnetospheric processes, in contrast to conventional radio pulsar glitch recoveries which are dominated by internal physics.

Evolution of Thermally Driven Disk Wind in the Black Hole Binary 4U 1630–47 Observed with Suzaku and NuSTAR

We performed simultaneous observations with Suzaku and NuSTAR of the Galactic black hole binary 4U 1630−47 in the high/soft state (HSS) during the 2015 outburst. To compare our results with those observed in the HSS at lower luminosities, we re-analyze the Suzaku data taken during the 2006 outburst. The continuum can be well explained by thermal disk emission and a hard power-law tail. All spectra show strong iron-K absorption line features, suggesting that a disk wind is always developed in the HSS. We find that the degree of ionization of the wind dramatically increased at the brightest epoch in 2015, when the continuum became harder. Detailed XSTAR simulations show that this cannot be explained solely by an increase of the photoionization flux. Instead, we show that the observed behavior in the HSS is consistent with a theory of thermally-driven disk winds, where the column density and the ionization parameter of the disk wind are proportional to the luminosity and the Compton temperature, respectively.

SPECTRAL CHANGES IN THE HYPERLUMINOUS PULSAR IN NGC 5907 AS A FUNCTION OF SUPER-ORBITAL PHASE

ABSTRACT We present broadband, multi-epoch X-ray spectroscopy of the pulsating ultra-luminous X-ray source (ULX) in NGC 5907. Simultaneous XMM-Newton and NuSTAR data from 2014 are best described by a multicolor blackbody model with a temperature gradient as a function of accretion disk radius significantly flatter than expected for a standard thin accretion disk ( , with ). Additionally, we detect a hard power-law tail at energies above 10 keV, which we interpret as being due to Comptonization. We compare this observation to archival XMM-Newton, Chandra, and NuSTAR data from 2003, 2012, and 2013, and investigate possible spectral changes as a function of phase over the 78-day super-orbital period of this source. We find that observations taken around phases 0.3–0.4 show very similar temperature profiles, even though the observed flux varies significantly, while one observation taken around phase 0 has a significantly steeper profile. We discuss these findings in light of the recent discovery that the compact object is a neutron star and show that precession of the accretion disk or the neutron star can self-consistently explain most observed phenomena.

FIRST NuSTAR OBSERVATIONS OF THE BL LAC-TYPE BLAZAR PKS 2155-304: CONSTRAINTS ON THE JET CONTENT AND DISTRIBUTION OF RADIATING PARTICLES

ABSTRACT We report the first hard X-ray observations with NuSTAR of the BL Lac-type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and γ-ray data from the Fermi Large Area Telescope, obtained in 2013 April when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5–60 keV, is best described by a model consisting of a log-parabola component with curvature and a (local) photon index 3.04 ± 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 ± 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous γ-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to γ min = 1 and that there is one proton per electron, an unrealistically high total jet power of L p ∼ 1047 erg s−1 is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron–positron pairs with lepton-to-proton ratio , or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor γ br1 ∼ 100. In either case, the jet composition is expected to be strongly matter-dominated.

The origin of the hard X-ray tail in neutron-star X-ray binaries

Neutron star X-ray binaries emit a compact, optically thick, relativistic radio jet during low-luminosity, usually hard states, as Galactic black-hole X-ray binaries do. When radio emission is bright, a hard power-law tail without evidence for an exponential cutoff is observed in most systems. We have developed a jet model that explains many spectral and timing properties of black-hole binaries in the states where a jet is present. Our goal is to investigate whether our jet model can reproduce the hard tail, with the correct range of photon index and the absence of a high-energy cutoff, in neutron-star X-ray binaries. We have performed Monte Carlo simulations of the Compton upscattering of soft, accretion-disk or boundary layer photons, in the jet and computed the emergent energy spectra, as well as the time lag of hard photons with respect to softer ones as a function of Fourier frequency. We demonstrate that our jet model explains the observed power-law distribution with photon index in the range 1.8-3. With an appropriate choice of the parameters, the cutoff expected from Comptonization is shifted to energies above ~300 keV, producing a pure power law without any evidence for a rollover, in agreement with the observations. Our results reinforce the idea that the link between the outflow (jet) and inflow (disk) in X-ray binaries does not depend on the nature of the compact object, but on the process of accretion. Furthermore, we address the differences of jets in black-hole and neutron-star X-ray binaries and predict that the break frequency in the spectral energy distribution of neutron-star X-ray binaries, as a class, will be lower than that of black-hole binaries.

A low-luminosity soft state in the short-period black hole X-ray binary Swift J1753.5-0127

We present results from the spectral fitting of the candidate black hole X-ray binary Swift J1753.5-0127 in an accretion state previously unseen in this source. We fit the 0.7-78 keV spectrum with a number of models, however the preferred model is one of a multi-temperature disk with an inner disk temperature $\mathrm{k}T_\mathrm{in}=0.252\pm0.003$ keV scattered into a steep power-law with photon index $\Gamma=6.39^{+0.08}_{-0.02}$ and an additional hard power law tail ($\Gamma=1.79\pm0.02$). We report on the emergence of a strong disk-dominated component in the X-ray spectrum and we conclude that the source has entered the soft state for the first time in its ~10 year prolonged outburst. Using reasonable estimates for the distance to the source ($3$ kpc) and black hole mass ($5M_{\odot}$), we find the unabsorbed luminosity (0.1-100 keV) to be $\approx0.60$% of the Eddington luminosity, making this one of the lowest luminosity soft states recorded in X-ray binaries. We also find that the accretion disk extended towards the compact object during its transition from hard to soft, with the inner radius estimated to be $R_{\mathrm{in}}=28.0^{+0.7}_{-0.4} R_g$ or ~$12R_g$, dependent on the boundary condition chosen, assuming the above distance and mass, a spectral hardening factor $f=1.7$ and a binary inclination $i=55^{\circ}$.

A HARD X-RAY POWER-LAW SPECTRAL CUTOFF IN CENTAURUS X-4

The low-mass X-ray binary Cen X-4 is the brightest and closest (<1.2 kpc) quiescent neutron star transient. Previous 0.5-10 keV X-ray observations of Cen X-4 in quiescence identified two spectral components: soft thermal emission from the neutron star atmosphere and a hard power-law tail of unknown origin. We report here on a simultaneous observation of Cen X-4 with NuSTAR (3-79 keV) and XMM-Newton (0.3-10 keV) in 2013 January, providing the first sensitive hard X-ray spectrum of a quiescent neutron star transient. The 0.3-79 keV luminosity was 1.1 x 10^(33) erg/s (for D=1kpc), with around 60 percent in the thermal component. We clearly detect a cutoff of the hard spectral tail above 10 keV, the first time such a feature has been detected in this source class. We show that thermal Comptonization and synchrotron shock origins for the hard X-ray emission are ruled out on physical grounds. However, the hard X-ray spectrum is well fit by a thermal bremsstrahlung model with an 18 keV electron temperature, which can be understood as arising either in a hot layer above the neutron star atmosphere or in a radiatively-inefficient accretion flow (RIAF). The power-law cutoff energy may be set by the degree of Compton cooling of the bremsstrahlung electrons by thermal seed photons from the neutron star surface. Lower thermal luminosities should lead to higher (possibly undetectable) cutoff energies. We compare Cen~X-4's behavior with PSR J1023+0038, IGR J18245-2452, and XSS J12270-4859, which have shown transitions between LMXB and radio pulsar modes at a similar X-ray luminosity.

Fast transport of resonant electrons in phase space due to nonlinear trapping by whistler waves

AbstractWe present an analytical, simplified formulation accounting for the fast transport of relativistic electrons in phase space due to wave‐particle resonant interactions in the inhomogeneous magnetic field of Earth's radiation belts. We show that the usual description of the evolution of the particle velocity distribution based on the Fokker‐Planck equation can be modified to incorporate nonlinear processes of wave‐particle interaction, including particle trapping. Such a modification consists in one additional operator describing fast particle jumps in phase space. The proposed, general approach is used to describe the acceleration of relativistic electrons by oblique whistler waves in the radiation belts. We demonstrate that for a wave power distribution with a hard enough power law tail such that η &lt; 5/2, the efficiency of nonlinear acceleration could be more effective than the conventional quasi‐linear acceleration for 100 keV electrons.

RELATIVISTIC POSITRON-ELECTRON-ION SHEAR FLOWS AND APPLICATION TO GAMMA-RAY BURSTS

We present Particle-in-Cell simulation results of relativistic shear flows for hybrid positron-electron-ion plasmas and compare to those for pure e+e- and pure e-ion plasmas. Among the three types of relativistic shear flows, we find that only hybrid shear flow is able to energize the electrons to form a high-energy spectral peak plus a hard power-law tail. Such electron spectra are needed to model the observational properties of gamma-ray bursts.

The AGILE monitoring of Cygnus X-3: transient gamma-ray emission and spectral constraints

We present the AGILE-GRID monitoring of Cygnus X-3, during the period between November 2007 and July 2009. We report here the whole AGILE-GRID monitoring of Cygnus X-3 in the AGILE "pointing" mode data-taking, to confirm that the gamma-ray activity coincides with the same repetitive pattern of multiwavelength emission and to analyze in depth the overall gamma-ray spectrum by assuming both leptonic and hadronic scenarios. Seven intense gamma-ray events were detected in this period, with a typical event lasting one or two days. These durations are longer than the likely cooling times of the gamma-ray emitting particles, implying we see continuous acceleration rather than the result of an impulsive event such as the ejection of a single plasmoid which then cools as it propagates outwards. Cross-correlating the AGILE-GRID light curve with X-ray and radio monitoring data, we find that the main events of gamma-ray activity have been detected while the system was in soft spectral X-ray states (RXTE/ASM count rate > 3 counts/s), that coincide with local and often sharp minima of the hard X-ray flux (Swift/BAT count rate < 0.02 counts/cm^2/s), a few days before intense radio outbursts. [...] These gamma-ray events may thus reflect a sharp transition in the structure of the accretion disk and its corona, which leads to a rebirth of the microquasar jet and subsequent enhanced radio activity. [...] Finally, we examine leptonic and hadronic emission models for the gamma-ray events and find that both scenarios are valid. In the leptonic model - based on inverse Compton scatterings of mildly relativistic electrons on soft photons from the Wolf-Rayet companion star and from the accretion disk - the emitting particles may also contribute to the overall hard X-ray spectrum, possibly explaining the hard non-thermal power-law tail sometimes seen during special soft X-ray states in Cygnus X-3.

Further X-ray observations of EXO 0748−676 in quiescence: evidence for a cooling neutron star crust

In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748−676 started a transition from outburst to quiescence, after it actively accreted for more than 24 yr. In a previous work, we discussed Chandra and Swift observations obtained during the first 5 months of this transition. Here, we report on further X-ray observations of EXO 0748−676, extending the quiescent monitoring to 1.6 yr. Chandra and XMM–Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well fitted by a neutron star atmosphere model. An additional hard power-law tail is detected that changes non-monotonically over time, contributing between 4 and 20 per cent to the total unabsorbed 0.5–10 keV flux. The combined set of Chandra, XMM–Newton and Swift data reveals that the thermal bolometric luminosity fades from ∼ 1 × 1034 to 6 × 1033 (D/7.4 kpc)2 erg s −1, whereas the inferred neutron star effective temperature decreases from ∼124 to 109 eV. We interpret the observed decay as cooling of the neutron star crust and show that the fractional quiescent temperature change of EXO 0748−676 is markedly smaller than observed for three other neutron star X-ray binaries that underwent prolonged accretion outbursts.

SPECTRAL INDEX AS A FUNCTION OF MASS ACCRETION RATE IN BLACK HOLE SOURCES: MONTE CARLO SIMULATIONS AND AN ANALYTICAL DESCRIPTION

In this Paper, we present theoretical arguments that the observationally established index saturation effect vs mass accretion rate is a signature of the bulk (converging) flow onto the black hole. We demonstrate that the index saturation value depends on the plasma temperature of converging flow. We self-consistently calculate the Compton cloud (CC) plasma temperature as a function of mass accretion rate using the energy balance between energy dissipation and Compton cooling. We explain the observable phenomenon, index- mdot correlations using a Monte-Carlo simulation of radiative processes in the innermost part (CC) of a BH source and we account for the Comptonization processes in the presence of thermal and bulk motions, as basic types of plasma motion. We show that, when mdot increases, BH sources evolve to high and very soft states (HSS and VSS, respectively), in which the strong blackbody-like and steep power-law components are formed in the resulting X-ray spectrum. The simultaneous detections of these two components strongly depends on sensitivity of high energy instruments, given that the relative contribution of the hard power-law tail in the resulting VSS spectrum can be very low, which is why, to date {\it RXTE} observations of the VSS X-ray spectrum has been characterized by the presence of the strong BB-like component only. We also predict specific patterns for high-energy efold (cutoff) energy (E_{fold}) evolution with mdot for thermal and dynamical (bulk) Comptonization cases. For the former case, E_{fold} monotonically decreases with mdot, in the latter case, the E_{fold}-decrease is followed by its increase at high values of mdot. The observational evolution of E_{fold} vs mdot can be one more test for the presence of a converging flow effect in the formation of the resulting spectra in the close vicinity of BHs.

QUIESCENT X-RAY EMISSION FROM Cen X-4: A VARIABLE THERMAL COMPONENT

The nearby neutron star low-mass X-ray binary, Cen X-4, has been in a quiescent state since its last outburst in 1979. Typically, quiescent emission from these objects consists of thermal emission (presumably from the neutron star surface) with an additional hard power-law tail of unknown nature. Variability has been observed during quiescence in Cen X-4 on both timescales as short as hundreds of seconds and as long as years. However, the nature of this variability is still unknown. Early observations seemed to show it was all due to a variable hard X-ray tail. Here, we present new and archival observations that contradict this. The most recent Suzaku observation of Cen X-4 finds it in a historically low state, a factor of 4.4 fainter than the brightest quiescent observation. As the spectrum during the brightest observation was comprised of approximately 60% from the thermal component and 40% from the power-law component, such a large change cannot be explained by just power-law variability. Spectral fits with a variable thermal component fit the data well, while spectral fits allowing both the column density and the power-law to vary do not, leading to the conclusion that the thermal component must be variable. Interestingly, we also find that the thermal fraction remains consistent between all epochs, implying that the thermal and power-law fluxes vary by approximately the same amount. If the emitting area remains unchanged between observations, then the effective surface temperature must change. Alternatively, if the temperature remains constant, then the emitting area must change. The nature of this thermal variability is unclear, but may be explained by variable low-level accretion.

Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748—676 back to quiescence

Abstract The quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748−676 recently started the transition to quiescence following an accretion outburst that lasted more than 24 years. We report on two Chandra and 12 Swift observations performed within five months after the end of the outburst. The Chandra spectrum is composed of a soft, thermal component that fits to a neutron star atmosphere model with kT∞∼ 0.12keV, joined by a hard power-law tail that contributes ∼20 per cent of the total 0.5–10 keV unabsorbed flux. The combined Chandra/Swift data set reveals a relatively hot and luminous quiescent system with a temperature of kT∞∼ 0.11–0.13keV and a bolometric thermal luminosity of ∼8.1 × 1033–1.6 × 1034(d/7.4kpc)2ergs −1. We discuss our results in the context of cooling neutron star models.

Suzaku Spectroscopic Study of Hard X-Ray Emission in the Arches Cluster

We present the results of a Suzaku study of the Arches cluster in the Galactic center region. A high signal-to-noise ratio spectrum in the 3–12 keV band was obtained with the X-ray Imaging Spectrometer (XIS) onboard the Suzaku observatory. We found that the spectrum consists of a thermal plasma, a hard power-law tail, and two Gaussian line components. The plasma component with a temperature of $\sim 2.2 \,\mathrm{keV}$ is established from the presence of Ca XIX and Fe XXV $\mathrm{K}\alpha$ lines as well as absence of the Fe XXVI $\mathrm{K}\alpha$ line. The two Gaussian lines represent the $\mathrm{K}\alpha$ and $\mathrm{K}\beta$ lines from iron at lower ionization stages at $\sim 6.4$ and $\sim 7.1 \,\mathrm{keV}$. Both the line centers and the intensity ratio of these two lines are consistent with neutral iron. The hard power-law tail with a photon index of $\sim 0.7$ was found to have no pronounced iron K edge feature. Compared with the published Chandra spectra constructed separately for point-like and diffuse emission, we conclude that the thermal component is from an ensemble of point-like sources plus thermal diffuse emission concentrated at the cluster center, while the Gaussian and the hard tail components are from non-thermal diffuse emission extended in a larger scale. In band-limited images of the XIS field, the distribution of the 7.5–10.0 keV emission resembles that of the 6.4 keV emission, including local excess at the Arches cluster. This strongly suggests that the power-law emission is related to the 6.4 and 7.1 keV lines in the underlying physics. We discuss two ideas to explain both the hard continuum and the lines: (1) X-ray photoionization that produces fluorescence lines and the Thomson scattering continuum and (2) non-thermal electron impact ionization of iron atoms and bremsstrahlung continuum. However, whichever scenario is adopted, the photon or particle flux from the Arches cluster is too low to account for the observed lines and continuum intensity.

A Joint Suzaku and Chandra Spectroscopy Study of Hard X-Ray Emission from the Arches Cluster

We present the results of a joint Suzaku and Chandra X-ray spectroscopic study of the Arches cluster in the Galactic center region. The superb spectroscopic performance of XIS on-board Suzaku brought a tight constraint of the spectrum of the Arches X-rays. We found that the spectrum consists of a thermal plasma, a hard power-law tail, and two Gaussian components. The plasma component with a temperature of ∼2.2 keV is established from highly ionized Ca XIX and Fe XXV Kα emission lines as well as the lack of Fe XXVI Kα line. The two Gaussian lines represent the Kα and Kβ lines from iron at lower ionization stages at ∼6.4 keV and ∼7.1 keV. Both the line centers and the intensity ratio of these two lines are consistent with the neutral iron. The hard power-law tail with a photon index of ∼0.7 was found to have no pronounced iron K edge feature. In comparison with ACIS spectra constructed separately for point-like and diffuse emission by using the unprecedented spatial capability of Chandra, we conclude that the thermal component is from the ensemble of point-like sources plus thermal diffuse emission concentrated at the cluster center, while the Gaussian and the hard tail components are from the non-thermal diffuse emission extended in a larger scale. In the band-limited images of the XIS field, the distribution of the 7.5– 10.0 keV emission resembles that of the 6.4 keV emission, including the local excess at the Arches cluster. This strongly suggests that the power-law emission is related to the 6.4 and 7.1 keV lines in the underlying physics. The full discussion can be found in Tsujimoto et al. [M. Tsujimoto, Y. Hyodo and K. Koyama, Publ. Astron. Soc. Jpn. 59 (2007), S229]. For point-like X-ray emission, three Arches sources are exceptionally bright among cluster members. The brightest spectrum of them requires two temperature plasma with an extinction of 2×10 23 cm −2 . The estimated hard-band luminosity of ≈10 35 erg s −1 ,w hich is brighter than any known stellar sources in our Galaxy, might be a consequence of colliding

Comptonization and reflection of X-ray radiation and the X-ray-radio correlation in theχ-states of GRS 1915+105

We present a comprehensive X-ray study of four years of pointed RXTE observations of GRS 1915+105 in the χ-state. We interpret the behavior of the hard power law tail spectrum as coming from inverse Compton scattering of soft disk photons on a thermally dominated hybrid corona above the accretion disk. GRS 1915+105 shows a strong, variable reflection amplitude. As in other BHC and in Seyfert galaxies, a correlation between the power law slope and the reflection was found. Also, the radio fluxes at 2.25GHz and 15GHz correlate with the power law slope, thus revealing a connection between the outflowing matter and the comptonizing region in the χ-states.