Disk Blackbody Research Articles

XMM‐NewtonObservations of an Intermediate X‐Ray Object in NGC 2276

We present the results from a approx. 53 ksec XMM observation of NGC 2276. This galaxy has an unusual optical morphology with the disk of this spiral appearing to be truncated along the western edge. This XMM observation shows that the X-ray source at the western edge is a bright Intermediate X-ray Object (IXO). Its spectrum is well fit by a multi-color disk blackbody model used to fit optically thick standard accretion disks around black holes. The luminosity derived for this IXO is 1.1 x 10(exp 41) erg/s in the 0.5 - 10 keV band making it one of the most luminous discovered to date. The large source luminosity implies a large mass black hole if the source is radiating at the Eddington rate. On the other hand, the inner disk temperature determined here is too high for such a massive object given the standard accretion disk model. In addition to the IXO we find that the nuclear source in this galaxy has dimmed by at least a factor of several thousand in the eight years since the ROSAT HRI observations.

Chandraobservations of five ultraluminous X-ray sources in nearby galaxies

We report the results of a programme of dual-epoch Chandra ACIS-S observations of five ultraluminous X-ray sources (ULXs) in nearby spiral galaxies. All five ULXs are detected as unresolved, point-like X-ray sources by Chandra, though two have faded below the 10^39 erg/s luminosity threshold used to first designate these sources as ULXs. Using this same criterion, we detect three further ULXs within the imaged regions of the galaxies. The ULXs appear to be related to the star forming regions of the galaxies, indicating that even in ``normal'' spiral galaxies the ULX population is predominantly associated with young stellar populations. A detailed study of the Chandra ACIS-S spectra of six of the ULXs shows that five are better described by a powerlaw continuum than a multi-colour disc blackbody model, though there is evidence for additional very soft components to two of the powerlaw continua. The measured photon indices in four out of five cases are consistent with the low/hard state in black hole binaries, contrary to the suggestion that powerlaw-dominated spectra of ULXs originate in the very high state. A simple interpretation of this is that we are observing accretion onto intermediate-mass black holes, though we might also be observing a spectral state unique to very high mass accretion rates in stellar-mass black hole systems. Short-term flux variability is only detected in one of two epochs for two of the ULXs, with the lack of this characteristic arguing that the X-ray emission of this sample of ULXs is not dominated by relativistically-beamed jets. The observational characteristics of this small sample suggest that ULXs are a distinctly heterogeneous source class.

X-ray spectral evolution of SAX J1747.0-2853 during outburst activity and confirmation of its transient nature

SAX J1747.0-2853 is an X-ray transient which exhibited X-ray outbursts yearly between 1998 and 2001, and most probably also in 1976. The outburst of 2000 was the longest and brightest. We have analyzed X-ray data sets that focus on the 2000 outburst and were obtained with BeppoSAX, XMM-Newton and RXTE. The data cover unabsorbed 2–10 keV fluxes between 0.1 and $5.3\times10^{-9}$ erg s -1 cm -2 . The equivalent luminosity range is $6\times10^{35}$ to $2\times10^{37}$ erg s -1 . The 0.3–10 keV spectrum is well described by a combination of a multi-temperature disk blackbody, a hot Comptonization component and a narrow Fe–K emission line at 6.5 to 6.8 keV with an equivalent width of up to 285 eV. The hydrogen column density in the line of sight is $(8.8\pm0.5)\times10^{22}$ cm -2 . The most conspicuous spectral changes in this model are represented by variations of the temperature and radius of the inner edge of the accretion disk, and a jump of the equivalent width of the Fe–K line in one observation. Furthermore, 45 type-I X-ray bursts were unambiguously detected between 1998 and 2001 which all occurred during or close to outbursts. We derive a distance of $7.5\pm1.3$ kpc which is consistent with previous determinations. Our failure to detect bursts for prolonged periods outside outbursts provides indirect evidence that the source returns to quiescence between outbursts and is a true transient.

The Luminous X‐Ray Source Population in M51 Observed withChandra

We present the results of two Chandra observations (separated by 1 yr) of the population of X-ray sources in the spiral galaxy M51 (NGC 5194 and NGC 5195). One hundred and thirteen X-ray sources have been detected in an 84 × 84 (20.4 × 20.4 kpc) region, and 84 and 12 of them project within the disks of NGC 5194 and NGC 5195, respectively. Nine and 28 sources have luminosities exceeding 1 × 1039 ergs s-1 (ultraluminous X-ray sources or ULXs) and 1 × 1038 ergs s-1 in the 0.5-8 keV band, respectively, assuming that they are associated with M51. The number of ULXs is much higher than found in most normal spiral and elliptical galaxies. Most of the X-ray sources and all seven of the ULXs in NGC 5194 are located in, or close to, a spiral arm, suggesting a connection with recent star formation. The cumulative luminosity function of the X-ray sources in NGC 5194 with L(0.5-8 keV) > 1038 ergs s-1 is well described by a power law, N[>L(0.5-8 keV)] ∝ L(0.5- 8 keV)-α with α = 0.91. The X-ray spectra of most of the detected sources are consistent with a power law with a photon index between 1 and 2, with a few sources showing harder or softer spectra. The spectra of most ULXs are consistent with both a power law and a multicolor disk blackbody (MCD) model, while a power-law model is preferable to an MCD model in two ULXs. One ULX (NGC 5194 source 69) shows drastic spectral steepening accompanied by a decline in luminosity by a factor of 3460 in the 2-10 keV band between the two observations. This source also exhibited a possible period of 2.1 hr in the 2000 observation. Another ULX (NGC 5194 source 26) shows strong emission lines from highly ionized species. The masses of the compact objects and mass accretion rates in ULXs and other X-ray sources are not well constrained by these observations. If we adopt an MCD interpretation, their MCD parameters imply that most of the X-ray sources are stellar mass (~5-10 M☉) black holes accreting near or above the Eddington rate, although other possibilities (intermediate-mass black holes and relativistically beamed emission) cannot be excluded. The power-law sources may instead represent Comptonized disk or nonthermal emission. Two ULXs have very soft spectra; MCD models require kT ≈ 0.1 keV. We discuss the possibility that this soft emission originates in an accretion disk around an intermediate-mass black hole. We also present a study of the nucleus of, and discrete sources (including two ULXs) in, the companion galaxy NGC 5195.

Chandra/High Energy Transmission Grating Spectrometer Spectroscopy of the Galactic Black Hole GX 339−4: A Relativistic Iron Emission Line and Evidence for a Seyfert‐like Warm Absorber

We observed the Galactic black hole GX 339-4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ks during the decline of its 2002-2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT~=0.6 keV) and power-law (Gamma~=2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe Kalpha emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The breadth of the line is sufficient to suggest that GX 339-4 may harbor a black hole with significant angular momentum. Absorption lines from H- and He-like O and He-like Ne and Mg are detected, as well as lines that are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to an intrinsic AGN-like warm-absorber geometry-perhaps produced by a disk wind in an extended disk-dominated state-may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650-500 and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes.

A Study of X-Ray Sources in the Central Region of NGC 1569 with Chandra

The dwarf starburst galaxy NGC 1569 was observed for 96.8 ks with Chandta. We detected several variable X-ray point sources in the central region of NGC 1569. Inparticular, two bright point sources, CXOU 043048.1+645050 and CXOU 043048.6+645058, exhibit a prominent variability in time. Fitting with the disk blackbody model, we find that CXOU 043048.1+645050 may be a stellar mass black hole with ∼ 20M. From the fact that CXOU 043048.6+645058 has a short time variability and flat spectrum, we concluded that this source may be a neutron star binary. We calculated the luminosity ratio of the soft X-ray band to other energy bands, and find that NGC 1569 has a typical luminosity ratio found in starburst galaxies.

The eclipsing massive X-ray binary M 33 X-7: New X-ray observations and optical identification

The eclipsing X-ray binary M 33 X 7 was in the field of view during several observations of our XMM-Newton M 33 survey and in the archival Chandra observation 1730 which cover a large part of the 3.45 d orbital period. We detect emission of M 33 X 7 during eclipse and a soft X-ray spectrum of the source out of eclipse that can best be described by bremsstrahlung or disk blackbody models. No significant regular pulsations of the source in the range 0.25-1000 s were found. The average source luminosity out of eclipse is 5 10 37 erg s 1 (0.5-4.5 keV). In a special analysis of DIRECT observations we identify as optical counterpart a B0I to O7I star of 18.89 mag in V which shows the ellipsoidal heating light curve of a high mass X-ray binary with the M 33 X 7 binary period. The location of the X-ray eclipse and the optical minima allow us to determine an improved binary period and ephemeris of mid-eclipse as HJD (2 451 760:61 0:09) N (3:45376 0:00021). The mass of the compact object derived from orbital parameters and the optical companion mass, the lack of pulsations, and the X-ray spectrum of M 33 X 7 may indicate that the compact object in the system is a black hole. M 33 X 7 would be the first detected eclipsing high mass black hole X-ray binary.

Radiative Processes, Spectral States and Variability of Black-Hole Binaries

We review radiative processes responsible for X-ray emission in hard (low) and soft (high) spectral states of black-hole binaries. The main process in the hard state appears to be scattering of blackbody photons from a cold disk by thermal electrons in a hot inner flow, and any contribution from nonthermal synchrotron emission is at most small. In the soft states, blackbody disk emission dominates energetically, and its high-energy tail is due to scattering by hybrid, thermal/nonthermal electrons, probably in active regions above the disk surface. State transitions appear to correspond to a variable inner radius of the cold disk driven by changes of the accretion rate. The existence of two accretion solutions, hot and cold, in a range of the accretion rate leads to hysteresis in low-mass X-ray binaries.

X‐Ray–Extreme‐Ultraviolet Simultaneous Observations of NGC 5548: Fast Extreme‐Ultraviolet Dip without X‐Ray Variation

The bright Seyfert 1 galaxy NGC 5548 was monitored in X-rays by ASCA for 9 days and in EUV by EUVE for 12 days continuously in 1996. The periods of X-ray observations were really simultaneously covered by EUV observations. The 0.1-10 keV combined continuum of X-ray and EUV is well presented by a simple power law with a photon index Γ = 1.78 plus the Wien part of a disk blackbody with kT = 17 eV. On long timescales of days, the variability of the EUV flux is in good accordance with that of the X-rays. Furthermore, there is a clear positive correlation between Γ and the 2-10 keV flux. These results are consistent with the scenario that the X-rays are produced by inverse Compton scattering of soft photons in hot electron clouds. On the other hand, focusing on the short-term variability, we found that the EUV flux shows 20,000 s variability with 40% amplitude with neither X-ray flux nor spectral changes when the source is at its maximum flux state. This seems to conflict with the ordinary framework of the inverse Compton model. To explain such fast variability of the EUV flux accompanied by no X-ray change, we suggest that the EUV emission consists of two components; relatively stable disk emission and a variable component that is dominant at EUV flares.

XMM--Newton observation of the X-ray point source population of the starburst galaxy IC 342

We present the results of an XMM–Newton observation of the starburst galaxy IC 342. Thirty-seven X-ray point sources were detected down to a luminosity limit of ∼1037 erg s−1. Most of the sources are located near the spiral arms. The X-ray point source luminosity function is consistent with a power-law shape with a slope of 0.55, typical of starburst galaxies. We also present the energy spectra of several ultraluminous X-ray sources (ULXs), including the luminous X-ray source in the galactic nucleus. Except for the nucleus and a luminous supersoft X-ray source, other ULXs can generally be fitted with a simple power-law spectral model. The nucleus is very luminous (∼1040 erg s−1 in 0.2–12 keV) and requires disc blackbody and power-law components to describe the X-ray emission. The spectral fit reveals a cool accretion disc (kT= 0.11 keV) and suggests that the source harbours either an intermediate-mass black hole or a stellar-mass black hole with outflow.

First INTEGRAL observations of GRS 1915+105

We present data from the first of six monitoring Open Time observations of GRS 1915+105 undertaken with the orbiting INTEGRAL satellite. The source was clearly detected with all three X-ray and gamma-ray instruments on board. GRS 1915+105 was in a highly variable state, as demonstrated by the JEM X-2 and ISGRI lightcurves. These and simultaneous RXTE/PCA lightcurves point to a novel type of variability pattern in the source. In addition, we fit the combined JEM X-2 and ISGRI spectrum between 3-300 keV with a disk blackbody + powerlaw model leading to typical parameter values found earlier at similar luminosity levels. A new transient, IGR J19140+098, was discovered during the present observation.

X-ray spectral evolution of low-mass X-ray binary GX 349+2

We present the results of a systematic investigation of spectral evolution in the Z source GX 349+2, using data obtained during 1998 with Proportional-Counter-Array (PCA) on-board the RXTE satellite. The source traced a extended normal branch (NB) and flaring branch (FB) in the color-color diagram (CD) and hardness-intensity diagram (HID) during these observations. The spectra at different positions of Z-track were best fitted by a model consisting of a disk blackbody and a comptonized spectrum. A broad (gaussian) iron line at $\sim 6.7$ keV is also required to improve the fit. The spectral parameters showed a systematic and significant variation with the position along the Z-track. The evolution in spectral parameters is discussed in the view of increasing mass accretion rate scenario, proposed to explain the motion of Z sources in the CD and HID.

AChandraX‐Ray Study of NGC 1068. II. The Luminous X‐Ray Source Population

We present an analysis of the compact X-ray source population in the Seyfert 2 galaxy NGC 1068, imaged with a approx. 50 ks Chandra observation. We find a total of 84 compact sources on the S3 chip, of which 66 are located within the 25.0 B-mag/arcsec isophote of the galactic disk of NGC 1068. Spectra have been obtained for the 21 sources with at least 50 counts and modeled with both multicolor disk blackbody and power-law models. The power-law model provides the better description of the spectrum for 18 of these sources. For fainter sources, the spectral index has been estimated from the hardness ratio. Five sources have 0.4 - 8 keV intrinsic luminosities greater than 10(exp 39)ergs/ s, assuming that their emission is isotropic and that they are associated with NGC 1068. We refer to these sources as intermediate-luminosity X-ray objects (ISOs). If these five sources are X-ray binaries accreting with luminosities that are both sub-Eddington and isotropic, then the implied source masses are approx greater than 7 solar mass, and so they are inferred to be black holes. Most of the spectrally modeled sources have spectral shapes similar to Galactic black hole candidates. However, the brightest compact source in NGC 1068 has a spectrum that is much harder than that found in Galactic black hole candidates and other ISOs. The brightest source also shows large amplitude variability on both short-term and long-term timescales, with the count rate possibly decreasing by a factor of 2 in approx. 2 ks during our Chundra observation, and the source flux decreasing by a factor of 5 between our observation and the grating observations taken just over 9 months later. The ratio of the number of sources with luminosities greater than 2.1 x 10(exp 38) ergs/s in the 0.4 - 8 keV band to the rate of massive (greater than 5 solar mass) star formation is the same, to within a factor of 2, for NGC 1068, the Antennae, NGC 5194 (the main galaxy in M51), and the Circinus galaxy. This suggests that the rate of production of X-ray binaries per massive star is approximately the same for galaxies with currently active star formation, including starbursts.

Discovery of Bright Variable X-Ray Sources in NGC 1569 with Chandra

From the analysis of a ~100 ks Chandra observation of the dwarf starburst galaxy NGC 1569, we have found that the X-ray point sources CXOU 043048.1+645050 and CXOU 043048.6+645058 show significant time variability. During this observation, the X-ray flux of CXOU 043048.1+645050 increased by 10 times in only 2 × 104 s. Since the spectrum in its bright phase was fitted with a disk blackbody model with kTin ≈ 0.43 keV and the bolometric luminosity is Lbol ≈ 1038 ergs s-1, this source is an X-ray binary with a stellar mass black hole. Since the spectrum in its faint phase was also fitted with a disk blackbody model, the time variability can be explained by a change of the accretion rate onto the black hole. The other variable source, CXOU 043048.6+645058, had a flat spectrum with a photon index of ~1.6. This source may be an X-ray binary with an X-ray luminosity of several times 1037 ergs s-1. In addition, three other weak sources showed possible time variability. Taking all of the variability into account may suggest an abundant population of compact X-ray sources in NGC 1569.

Multiwavelength Spectrum of the Black Hole XTE J1118+480 in Quiescence

We present an X-ray/UV/optical spectrum of the black hole primary in the X-ray nova XTE J1118+480 in quiescence at L_x = 4 x 10^{-9} of the Eddington luminosity. The Chandra, HST and MMT spectroscopic observations were performed simultaneously on 2002 January 12 UT. Because this 4.1-hr binary is located at b = 62 deg, the transmission of the ISM is very high (e.g., 70% at 0.3 keV). We present many new results for the quiescent state, such as the first far-UV spectrum and evidence for an 0.35 mag orbital modulation in the near-UV flux. However, the centerpiece of our work is the multiwavelength spectrum of XTE J1118+480, which we argue represents the canonical spectrum of a stellar-mass black hole radiating at L_x = 4 x 10^{-8.5} of the Eddington luminosity. This spectrum is comprised of two apparently disjoint components: a hard X-ray spectrum with a photon index Gamma = 2.02 +/- 0.16, and an optical/UV continuum that resembles a 13,000 K disk blackbody spectrum punctuated by several strong emission lines. We present a model of the source in which the accretion flow has two components: (1) an X-ray-emitting interior region where the flow is advection-dominated, and (2) a thin, exterior accretion disk with a truncated inner edge (R_tr ~ 10^4 Schwarzschild radii) that is responsible for the optical/UV spectrum. For D = 1.8 kpc, the luminosity of the X-ray component is L_x = 3.5 x 10^{30} erg/s (0.3-7 keV); the bolometric luminosity of the optical/UV component is 20 times greater.

Discovery of X-Ray Quasi-periodic Oscillations from an Ultraluminous X-Ray Source in M82: Evidence against Beaming

We report the discovery with the European Photon Imaging Camera CCDs on board XMM-Newton of a 54 mHz quasi-periodic oscillation (QPO) in the greater than 2 keV X-ray flux from an ultraluminous X-ray source (ULX) in the starburst galaxy M82. This is the first detection of a QPO in the X-ray flux from an extragalactic ULX and confirms that the source is a compact object. On the basis of the QPO strength and previous Chandra observations, it appears likely that the QPO is associated with the most luminous object in the central region of M82, CXO M82 J095550.2+694047; however, XMM imaging alone is not sufficient to unambiguously confirm this. The other plausible candidate is CXO M82 J095551.1+694045; however, the QPO luminosity is comparable to the peak luminosity of this object in Chandra observations, which argues against it being the source of the QPO. The QPO had a centroid frequency of 54.3 ± 0.9 mHz, a coherence Q ≡ ν0/Δνfwhm ≈ 5, and an amplitude (rms) in the 2-10 keV band of 8.5%. Below 0.2 Hz, the power spectrum can be fitted by a power law with index ≈1 and amplitude (rms) of 13.5%. The X-ray spectrum requires a curving continuum, with a disk blackbody at T = 3.1 keV providing an acceptable fit. A broad Fe line centered at 6.55 keV is required in all fits, but the equivalent width is sensitive to the continuum model. There is no evidence of a reflection component. The implied bolometric luminosity is ≈ × 1040 ergs s-1. Archival Rossi X-Ray Timing Explorer pointings at M82 also show evidence for QPOs in the 50-100 mHz frequency range. We discuss the implications of our findings for models of ULXs.

X-Ray Spectroscopic Evidence for Intermediate-Mass Black Holes: Cool Accretion Disks in Two Ultraluminous X-Ray Sources

We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two "ultraluminous" X-ray (ULX) sources. We measure isotropic luminosities of LX = 2.0 × 1040 ergs s-1 and LX = 6.6 × 1039 ergs s-1 for NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc). The spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULX sources have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 σ level of confidence for X-1; the improvement for X-2 is significant at the 3 σ level. The soft components in these ULX spectra are well fitted by multicolor disk blackbody models with color temperatures of kT ≃ 150 eV. This temperature differs markedly from those commonly measured in the spectra of stellar mass (10 M☉) black holes in their brightest states (kT ≃ 1 keV). It is expected that the temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: MBH ≃ 103 M☉. Simple Eddington scaling arguments suggest a minimum mass of MBH ~ 102 M☉. NGC 1313 X-1 and X-2 are found in optical nebulae, which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.

Greenhouse Effect of Cloudy Accretion Disks

Abstract When an accretion disk is not naked, but covered with an optically-thick cloud, mutual irradiation between the disk and the cloud layer becomes important. We calculated the temperature distribution and the continuous spectra of the clouds as well as the disk. The temperature distribution of the cloud becomes somewhat flat in the central region, and gradually decreases in the outer part, while that of the disk is greatly enhanced, compared with the initial standard disk. The continuous spectrum emitted from the clouds is also modified, and resembles a blackbody spectrum with a single temperature, compared with the disk blackbody spectrum.

Broad-band X-ray measurements of the black hole candidate XTE J1908+094

XTE J1908+094 is an X-ray transient that went into outburst in February 2002. After two months it reached a 2- 250 keV peak fl ux of 1t o 2 10 8 erg cm 2 s 1 . Circumstantial evidence points to an accreting galactic black hole as the origin of the X-radiation: pulsations nor thermonuclear flashes were detected that would identify a neutron star and the spectrum was unusually hard for a neutron star at the outburst onset. We report on BeppoSAX and RXTE All Sky Monitor observations of the broad-band spectrum of XTE J1908+094. The spectrum is consistent with a model consisting of a Comptonization component by a40 keV plasma, a multicolor accretion disk blackbody component with a temperature just below 1 keV and a broad emission line at about 6 keV. The spectrum is heavily absorbed by cold interstellar matter with an equivalent hydrogen column density of 2:5 10 22 cm 2 , which makes it dicult to study the black body component in detail. The black body component exhibits strong evolution about 6 weeks into the outburst. Two weeks later this is followed by a swift decay of the power law component. The broadness of the 6 keV feature may be due to relativistic broadening or Compton scattering of a narrow Fe-K line.

ChandraObservation of Low‐Mass X‐Ray Binaries in the Elliptical Galaxy M84

We present characteristics of the X-ray point-source population in the M84 galaxy observed by the Chandra Advanced CCD Imaging Spectrometer S-Array. We find an excess in the number of sources centered on M84 with a spatial distribution closely corresponding to the M84 stellar light. Given an absence of recent star formation, accreting binaries are the only candidates for the M84 X-ray sources. The majority of M84 sources (with luminosities exceeding 1038 ergs s-1) exhibit hardness ratios expected from multitemperature blackbody disk emission. The most luminous sources, which we attribute to accreting black holes, exhibit X-ray colors typical of a blackbody spectrum. We also identify the sources whose X-ray colors match the expectations for constituents of the cosmic X-ray background. The number of such sources agrees with that expected to be background sources. After correcting for incompleteness in the source detection, we find a log N- log S for M84 similar to that of the elliptical galaxy NGC 4697, i.e., having a break at a luminosity of Lb = 2.4 × 1038 ergs s-1, approximately the Eddington limit on the isotropic luminosity for accretion onto a neutron star. The slope of the luminosity function above the break provides evidence for a mass distribution in the M84 accreting black holes.