Observed X-ray Luminosity Research Articles

DETECTION OF X-RAY ELONGATED EMISSION FROM AN ULTRALUMINOUS X-RAY SOURCE IN THE INTERACTING PAIR OF GALAXIES NGC 5953/5954

We present radio through X-ray observations of a bright (>1040 erg s–1 in the 0.5-8.0 keV band) ultraluminous X-ray source (ULX), CXOU J153434.9+151149, in the starburst, interacting pair of galaxies NGC 5953/5954. A Chandra image of this ULX shows that it is elongated. From HST/WFPC2/F606W data we have detected a counterpart of the ULX system with M F606W ~ –7.1 ± 0.7 mag. This optical counterpart may be either an O-type supergiant star or a young star cluster. From our Fabry-Perot interferometric observations, we have detected Hα and [N II] (6584 A) diffuse emission, with velocity gradients up to 60 km s–1 at the astrometric corrected Chandra position of the ULX. Different scenarios have been invoked as to explain the possible nature of CXOU J153434.9+151149. Based on the observed X-ray morphology of the ULX, we estimate that the inclination between the line of sight to the observer and the direction of a possibly beamed outflow is ~53°. Beaming with this geometry from a stellar-mass black hole system will be inadequate to explain the observed X-ray luminosity of this ULX. Finally, we suggest that mild-beaming from a binary black hole with mass more than 50 M ☉ associated with a young star cluster, is the most favorable scenario that describes the multiwavelength properties of this ULX. Future observations are definitely needed to determine the nature of this rare object.

X‐Ray Observations and Infrared Identification of the Transient 7.8 s X‐Ray Binary Pulsar XTE J1829−098

XMM-Newton and Chandra observations of the transient 7.8 s pulsar XTE J1829-098 are used to characterize its pulse shape and spectrum, and to facilitate a search for an optical or infrared counterpart. In outburst, the absorbed, hard X-ray spectrum with Γ = 0.76 ± 0.13 and NH = (6.0 ± 0.6) × 1022 cm-2 is typical of X-ray binary pulsars. The precise Chandra localization in a faint state leads to the identification of a probable infrared counterpart at R.A. = 18h29m43.98s, decl. = -09°51'23'' (J2000.0), with magnitudes K = 12.7, H = 13.9, I > 21.9, and R > 23.2. If this is a highly reddened O or B star, we estimate a distance of 10 kpc, at which the maximum observed X-ray luminosity is 2 × 1036 ergs s-1, typical of Be X-ray transients or wind-fed systems. The minimum observed luminosity is 3 × 1032(d/10 kpc)2 ergs s-1. We cannot rule out the possibility that the companion is a red giant. The two known X-ray outbursts of XTE J1829-098 are separated by ≈1.3 yr, which may be the orbital period or a multiple of it, with the neutron star in an eccentric orbit. We also studied a late M giant long-period variable that we found only 9'' from the X-ray position. It has a pulsation period of ≈1.5 yr, but is not the companion of the X-ray source.

Type IIP supernova SN 2004et: a multiwavelength study in X-ray, optical and radio

We present X-ray, broad band optical and low frequency radio observations of the bright type IIP supernova SN 2004et. The \cxo observed the supernova at three epochs, and the optical coverage spans a period of $\sim$ 470 days since explosion. The X-ray emission softens with time, and we characterise the X-ray luminosity evolution as $\Lx \propto t^{-0.4}$. We use the observed X-ray luminosity to estimate a mass-loss rate for the progenitor star of $\sim \ee{2}{-6} M_\odot \mathrm{yr}^{-1}$. The optical light curve shows a pronounced plateau lasting for about 110 days. Temporal evolution of photospheric radius and color temperature during the plateau phase is determined by making black body fits. We estimate the ejected mass of $^{56}$Ni to be 0.06 $\pm$ 0.03 M$_\odot$. Using the expressions of Litvinova & Nad\{e}zhin (1985) we estimate an explosion energy of (0.98 $\pm$ 0.25) $\times 10^{51}$ erg. We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier et al. (2006). These multi-wavelength studies suggest a main sequence progenitor mass of $\sim$ 20 M$_\odot$ for SN 2004et.

Why are accreting T Tauri stars observed to be less luminous in X-rays than non-accretors?

Abstract Accreting T Tauri stars are observed to be less luminous in X-rays than non-accretors, an effect that has been detected in various star-forming regions. To explain this we have combined, for the first time, a radiative transfer code with an accretion model that considers magnetic fields extrapolated from surface magnetograms obtained from Zeeman–Doppler imaging. Such fields consist of compact magnetic regions close to the stellar surface, with extended field lines interacting with the disc. We study the propagation of coronal X-rays through the magnetosphere and demonstrate that they are strongly absorbed by the dense gas in accretion columns. The reduction in the observed X-ray emission depends on the field geometry, which may explain why accreting T Tauri stars show a larger scatter in their observed X-ray luminosity compared with non-accreting stars.

Populations of Bright X-ray Sources in the Starburst Galaxies NGC 4038/4039

Assuming a naive star formation history, we construct synthetic X-ray source populations, using a population synthesis code, for comparison with the observed X-ray luminosity function (XLF) of the interacting galaxies NGC 4038/4039. We have included high- and intermediate-mass X-ray binaries, young rotation-powered pulsars and fallback disk-fed black holes in modeling the bright X-ray sources detected. We find that the majority of the X-ray sources are likely to be intermediate-mass X-ray binaries, but for typical binary evolution parameters, the predicted XLF seems to be steeper than observed. We note that the shape of the XLFs depends critically on the existence of XLF break for young populations, and suggest super-Eddington accretion luminosities or the existence of intermediate-mass black holes to account for the high luminosity end and the slope of the XLF in NGC 4038/4039.

The Young Stellar Population in M17 Revealed by Chandra

We report here results from a Chandra ACIS observation of the stellar populations in and around the M17 H II region. The field reveals 886 sources with observed X-ray luminosities (uncorrected for absorption) between similar to 29.3 ergs s(-1) < log L(X) < 32.8 ergs s(-1), 771 of which have stellar counterparts in infrared images. In addition to comprehensive tables of X-ray source properties, several results are presented:

Pulsations Detected from Accreting High‐Mass X‐Ray Binaries at Low Luminosities

We report the detection of pulsations from two accreting transient high-mass X-ray binaries (HMXBs) at low X-ray luminosities (≈1-4 × 1034 ergs s-1). While one (4U 1145-619) has been detected pulsing previously at a lower luminosity, the second (1A 1118-615) has not, making it the third such transient HMXB from which pulsations are detected at low luminosity. The pulsars exhibit broadband rms variability (22.3% ± 0.1% and 9.2% ± 0.1%, respectively). While the present observations do not permit measurement of a spin frequency derivative, spin-down is implied in comparison with previous Compton Gamma Ray Observatory/BATSE observations. If spin-down occurred at the presently observed X-ray luminosity of 4U 1145-619, the required mass accretion rate is insufficient to produce the observed X-ray luminosity from a shock at the magnetosphere, and only a fraction of the mass accretion makes its way to the neutron star surface, with most of the mass leaving the system, carrying away angular momentum to affect spin-down.

Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque in IGR J00291+5934

We performed a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of 2004 December. We corrected the arrival times of all the events for the orbital (Doppler) effects and performed a timing analysis of the resulting phase delays. In this way we are able to study, for the first time in this class of sources, the spin-up of a millisecond pulsar as a consequence of accretion torques during the X-ray outburst. The accretion torque gives us for the first time an independent estimate of the mass accretion rate onto the neutron star, which can be compared with the observed X-ray luminosity. We also report a revised value of the spin period of the pulsar.

Relativistic Effects on the Observed AGN Luminosity Distribution

Abstract Recently, Zhang (2005 ApJ, 618, L79) has proposed a model to account for the well-established effect that the fraction of type-II AGNs is anti-correlated with the observed X-ray luminosity; the model consists of an X-ray emitting accretion disk coaligned to the dusty torus within the standard AGN unification model. In this paper the model is refined by including relativistic effects of the observed X-ray radiation from the vicinity of the supermassive black hole in an AGN. The relativistic corrections improve the combined fitting results of the observed luminosity distribution and the type-II AGN fraction, though the improvement is not significant. The type-II AGN fraction prefers non–or mildly spinning black hole cases, and rules out the extremely spinning case.

An X‐Ray Census of Young Stars in the Massive Southern Star‐forming Complex NGC 6357

We present the first high spatial resolution X-ray study of the massive star-forming region NGC 6357, obtained in a 38 ks Chandra/ACIS observation. Inside the brightest constituent of this large H II region complex is the massive open cluster Pismis 24. It contains two of the brightest and bluest stars known, yet remains poorly studied; only a handful of optically bright stellar members have been identified. We investigate the cluster extent and initial mass function and detect ~800 X-ray sources with a limiting sensitivity of ~1030 ergs s-1; this provides the first reliable probe of the rich intermediate-mass and low-mass population of this massive cluster, increasing the number of known members from optical study by a factor of ~50. The high-luminosity end (log Lh[2-8 keV] ≥ 30.3 ergs s-1) of the observed X-ray luminosity function in NGC 6357 is clearly consistent with a power-law relation as seen in the Orion Nebula Cluster and Cepheus B, yielding the first estimate of NGC 6357's total cluster population, a few times the known Orion population. We investigate the structure of the cluster, finding small-scale substructures superposed on a spherical cluster with 6 pc extent, and discuss its relationship to the nebular morphology. The long-standing LX - 10-7Lbol correlation for O stars is confirmed. Twenty-four candidate O stars and one possible new obscured massive YSO or Wolf-Rayet star are presented. Many cluster members are estimated to be intermediate-mass stars from available infrared photometry (assuming an age of ~1 Myr), but only a few exhibit K-band excess. We report the first detection of X-ray emission from an evaporating gaseous globule at the tip of a molecular pillar; this source is likely a B0-B2 protostar.

Timing an Accreting Millisecond Pulsar: Measuring the Accretion Torque in IGR J00291+5934

We present here a timing analysis of the fastest accreting millisecond pulsar IGR J00291+5934 using RXTE data taken during the outburst of December 2004. We corrected the arrival times of all the events for the orbital (Doppler) effects and performed a timing analysis of the resulting phase delays. In this way we find a clear parabolic trend of the pulse phase delays showing that the pulsar is spinning up as a consequence of accretion torques during the X-ray outburst. The accretion torque gives us for the first time an independent estimate of the mass accretion rate onto the neutron star, which can be compared with the observed X-ray luminosity. We also report a revised value of the spin period of the pulsar.

Large Scatter in X-Ray Emission among Elliptical Galaxies: Correlations with Mass of Host Halo

Optically similar elliptical galaxies have an enormous range of X-ray luminosities. We show that this range can be attributed to large variations in the dark halo mass Mvir determined from X-ray observations. The K-band luminosity of ellipticals varies with virial mass, LK ∝ M, but with considerable scatter, probably due to the stochastic incidence of massive satellite galaxies that merge by dynamical friction to form group-centered ellipticals. Both the observed X-ray luminosity LX ∝ M and LX/LK ∝ M are sufficiently sensitive to the virial mass to explain the wide variation observed in LX among galaxies of similar LK. The central galaxy supernova energy per particle of diffuse gas increases dramatically with decreasing virial mass, and elliptical galaxies with the lowest X-ray luminosities (and Mvir) are easily explained by supernova-driven outflows.

Circumstellar Emission from Type Ib and Ic Supernovae

The presumed Wolf-Rayet star progenitors of Type Ib/c supernovae have fast, low-density winds, and the shock waves generated by the supernova interaction with the wind are not expected to be radiative at typical times of observation. The injected energy spectrum of radio-emitting electrons typically has an observed index p = 3, which is suggestive of acceleration in cosmic-ray-dominated shocks. The early, absorbed part of the radio light curves can be attributed to synchrotron self-absorption, which leads to constraints on the magnetic field in the emitting region and on the circumstellar density. The range of circumstellar densities inferred from the radio emission is somewhat broader than that for Galactic Wolf-Rayet stars, if similar efficiencies of synchrotron emission are assumed in the extragalactic supernovae. For the observed and expected ranges of circumstellar densities to roughly overlap, a high efficiency of magnetic field production in the shocked region is required (B ≈ 0.1). For the expected densities around a Wolf-Rayet star, a nonthermal mechanism is generally required to explain the observed X-ray luminosities of Type Ib/c supernovae. Inverse Compton emission is a candidate for the emission, if the observations are near optical maximum. In other cases we suggest that the mechanism is X-ray synchrotron emission in a situation in which the shock wave is cosmic-ray-dominated so that the electron energy spectrum flattens at high energy. More comprehensive X-ray observations of a Type Ib/c supernova are needed to determine whether this suggestion is correct.

Hydrostatic Modeling of the Integrated Soft X‐Ray and Extreme Ultraviolet Emission in Solar Active Regions

Many studies of the solar corona have shown that the observed X-ray luminosity is well correlated with the total unsigned magnetic flux. In this paper we present results from the extensive numerical modeling of active regions observed with the \textit{Solar and Heliospheric Observatory} (\textit{SOHO}) Extreme Ultraviolet Telescope (EIT), the \textit{Yohkoh} Soft X-ray Telescope (SXT), and the \textit{SOHO} Michelson Doppler Imager (MDI). We use potential field extrapolations to compute magnetic field lines and populate these field lines with solutions to the hydrostatic loop equations assuming steady, uniform heating. Our volumetric heating rates are of the form $\epsilon_H\sim \bar{B}^\alpha/L^\beta$, where $\bar{B}$ is the magnetic field strength averaged along a field line and $L$ is the loop length. Comparisons between the observed and simulated emission for 26 active regions suggest that coronal heating models that scale as $\epsilon_H\sim \bar{B}/L$ are in the closest argreement the observed emission at high temperatures. The field-braiding reconnection model of Parker, for example, is consistent with our results. We find, however, that the integrated intensities alone are insufficent to uniquely determine the parameterization of the volumetric heating rate. Visualizations of the emission are also needed. We also find that there are significant discrepancies between our simulation results and the lower temperature emission observed in the EIT channels.

A Simple Model of Radiative Emission in M87

We present a simple physical model of the central source emission in the M87 galaxy. It is well known that the observed X-ray luminosity from this galactic nucleus is much lower than the predicted one, if a standard radiative efficiency is assumed. Up to now the main model invoked to explain such a luminosity was the ADAF (advection-dominated accretion flow) model. Our approach supposes only a simple axisymmetric adiabatic accretion with a low angular momentum together with the bremsstrahlung emission process in the accreting gas. With no other special hypothesis on the dynamics of the system, this model agrees well enough with the luminosity value measured by Chandra.

Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. II. Optical Study and Interpretation

Our X-ray study of the nuclear activity in a new sample of six quiescent early-type galaxies, as well as in a larger sample from the literature, confirmed (Paper I) that the Bondi accretion rate of diffuse hot gas is not a good indicator of the SMBH X-ray luminosity. Here we suggest that a more reliable estimate of the accretion rate must include the gas released by the stellar population inside the sphere of influence of the SMBH, in addition to the Bondi inflow of hot gas across that surface. We use optical surface brightness profiles to estimate the mass-loss rate from stars in the nuclear region: we show that for our sample of galaxies it is an order of magnitude higher (~10-4 to 10-3 M☉ yr-1) than the Bondi inflow rate of hot gas, as estimated from Chandra (Paper I). Only by taking into account both sources of fuel can we constrain the true accretion rate, the accretion efficiency, and the power budget. Radiatively efficient accretion is ruled out, for quiescent SMBHs. For typical radiatively inefficient flows, the observed X-ray luminosities of the SMBHs imply accretion fractions ~1%-10% (i.e., ~90%-99% of the available gas does not reach the SMBH) for at least five of our six target galaxies and most of the other galaxies with known SMBH masses. We discuss the conditions for mass conservation inside the sphere of influence, so that the total gas injection is balanced by accretion plus outflows. We show that a fraction of the total accretion power (mechanical plus radiative) would be sufficient to sustain a self-regulating, slow outflow that removes from the nuclear region all the gas that does not sink into the BH (``BH feedback''). The rest of the accretion power may be carried out in a jet or advected. We also discuss scenarios that would lead to an intermittent nuclear activity.

Massive Stars and the Energy Balance of the Interstellar Medium. II. The 35M⊙Star and a Solution to the “Missing Wind Problem”

We continue our numerical analysis of the morphological and energetic influence of massive stars on their ambient interstellar medium for a 35 M☉ star that evolves from the main-sequence through red supergiant and Wolf-Rayet phases, until it ultimately explodes as a supernova. We find that structure formation in the circumstellar gas during the early main-sequence evolution occurs as in the 60 M☉ case but is much less pronounced because of the lower mechanical wind luminosity of the star. On the other hand, since the shell-like structure of the H II region is largely preserved, effects that rely on this symmetry become more important. At the end of the stellar lifetime 1% of the energy released as Lyman continuum radiation and stellar wind has been transferred to the circumstellar gas. From this fraction 10% is kinetic energy of bulk motion, 36% is thermal energy, and the remaining 54% is ionization energy of hydrogen. The sweeping up of the slow red supergiant wind by the fast Wolf-Rayet wind produces remarkable morphological structures and emission signatures, which are compared with existing observations of the Wolf-Rayet bubble S308, whose central star has probably evolved in a manner very similar to our model star. Our model reproduces the correct order of magnitude of observed X-ray luminosity, the temperature of the emitting plasma, and the limb brightening of the intensity profile. This is remarkable, because current analytical and numerical models of Wolf-Rayet bubbles fail to consistently explain these features. A key result is that almost the entire X-ray emission in this stage comes from the shell of red supergiant wind swept up by the shocked Wolf-Rayet wind rather than from the shocked Wolf-Rayet wind itself as hitherto assumed and modeled. This offers a possible solution to what is called the missing wind problem of Wolf-Rayet bubbles.

An analytic investigation of the scatter in the integrated X-ray properties of galaxy groups and clusters

We revisit the scaling relationships between the dark matter mass and observed X-ray luminosity and temperature of galaxy clusters and groups in the local Universe. Specifically, we compare recent observations with analytic models of the intracluster medium in which the gas entropy distribution has been shifted by a variable amount, K o , to investigate the origin of the scatter in these scaling relations, and its influence on the luminosity and temperature functions. We find that variations in halo concentration or formation epoch (which might determine the time available for low-entropy gas to cool out) are insufficient to explain the amount of scatter in the mass and X-ray luminosity (M-L) relation. Instead, a range of entropy floors at a fixed halo mass, spanning approximately ∼50-700 keV cm 2 , is required to match the data. This range is likely related to the variance in heating and/or cooling efficiency from halo to halo. We demonstrate that these models are consistent with the observed temperature and luminosity functions of clusters, with a normalization of σ 8 ∼ 0.8 in agreement with Wilkinson Microwave Anisotropy Probe (WMAP) measurements (for h = 0.7 and Ω m = 0.3); in particular, the scatter in the M-L relation has an important influence on the shape of the luminosity function, and must be accounted for to provide a consistent result. Finally, we present predictions for the redshift evolution of these scaling relations and luminosity/temperature functions. Comparison with recent data at z < 0.7 shows reasonable agreement with a model that assumes a median entropy floor of K = 200 keV cm 2 . When observations are extended to group scales (kT ≤ 1 keV). this evolution will have the potential to discriminate between an entropy floor that is independent of redshift (e.g. in a preheating scenario) and one that depends on the cooling time of the halo.

Optical/X-ray correlations as a signature of jet-disc coupling in the accreting black hole XTE J1118+480

We interpret the rapid correlated UV/optical/X-ray variability of XTE J1118+480 as a signature of the coupling between the X-ray corona and a jet emitting synchrotron radiation in the optical band. We propose a scenario in which the jet and the X-ray corona are fed by the same energy reservoir where large amounts of accretion power are stored before being channelled into either the jet or the high energy radiation. This time-dependent model reproduces the main features of the rapid multi-wavelength variability of XTE J1118+480. A strong requirement of the model is that the total jet power should be at least a few times larger than the observed X-ray luminosity, implying a radiative efficiency for the jet ϵ j ≳ 3 × 10 −3. This would be consistent with the overall low radiative efficiency of the source. We present independent arguments showing that the jet probably dominates the energetic output of all accreting black holes in the low-hard state.

Tracing the X-ray emitting intra-cluster medium of clusters of galaxies beyondR200

We present in this paper a study of a sample of 14 nearby clusters of galaxies (0.06 r200. In the center (r 0.4r200 we find that the ScEM profiles agree well within the error bars, suggesting self-similarity. Fitting beta-models to the overall ScEM profiles, we find for the different subsamples rc = 0.15−0.18r200 and β = 0.8, which is higher than the canonical value of β = 2/3 often found. The beta-model is generally a better representation for hotter than for cooler clusters. We see indications for continuous steepening of the profiles with increasing radius: at radii r > 0.8r200 the profiles are systematically below the beta-model curve with β = 0.8. We discuss our results with respect to the observed X-ray luminosity LX − T relation, the gas mass Mgas − T relation and the total mass M − T relation. We also address implications on the origin of the observed S − T relation. Furthermore, we discuss the observed steepness of the X-ray profiles, which falls off more rapidly than predicted from the NFW-profile for cold dark matter halos.