arXiv subjects

J. M. Miller

Publications and source records attributed to J. M. Miller.

At least 235 records · Page 13Linked to original sources

arXiv2004

On the observed disc temperature of accreting intermediate mass black holes

Some ultraluminous X-ray sources in nearby galaxies show soft components resembling thermal disc emission. Calculations based on blackbody emission then indicate that the accreting black holes at the centres of these discs have masses of 100s to 1000s Msun. Establishing the existence of such intermediate mass black holes is of considerable importance so the assumptions and approximations lying behind blackbody spectral fits must be examined. We study here the basic assumption that the thermal emission is well-characterised by a multi-temperature blackbody spectrum. Since the opacity in the surface layers of a disc decreases at high energies the emergent spectrum is hardened. We compute the observed spectra from discs around a non-spinning 1000 Msun black hole and fit them over the XMM-Newton pn band with multi-colour disc models. The typical overall spectra hardening factor usually adopted for discs around stellar mass black hole (assuming an inclination of 60 deg and including relativistic effects for a non-spinning black hole) is found to be appropriate.

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arXiv2004

An Unveiling Event in the Type 2 AGN NGC4388: A Challenge for a Parsec Scale Absorber

We present two Rossi Xray Timing Explorer (RXTE) PCA observations of the type~2 Seyfert galaxy NGC 4388 caught in an unusual low X-ray absorption state. The observations were triggered by a detection in the 1.5-3 keV band of the RXTE All-Sky Monitor. NGC 4388 was found at a somewhat high continuum level (f(2-10 keV)=8x10(-11)erg cm(-2) s(-1)) and with a column density (N_H~3x10(22)cm(-2)), a factor ~10 lower than normal. The second PCA observation, four hours later, gave N_H<2x10(21)cm(-2) indicating, at the 3.1 sigma level, variability so rapid puts the absorber on a few 100 Schwartzschild radii scale, similar to the Broad Emission Line Region, or smaller. This small scale creates difficulties for the parsec-scale obscuring torus paradigm of Unified Schemes for type 1 and type 2 AGNs.

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arXiv2004

Multi-Temperature Blackbody Spectra of Thin Accretion Disks With and Without a Zero-Torque Inner Boundary Condition

The standard spectral model for analyzing the soft component of thermal emission from a thin accretion disk around a black hole is the multi-temperature blackbody (MTB) model. The widely used implementation of this model, which is known as ``diskbb,'' assumes nonzero torque at the inner edge of the accretion disk. This assumption is contrary to the classic and current literature on thin-disk accretion, which advocates the use of a zero-torque boundary condition. Consequently, we have written code for a zero-torque model, ``ezdiskbb,'' which we compare to the nonzero-torque model diskbb by fitting RXTE spectra of three well-known black hole binaries: 4U 1543-47, XTE J1550-564, and GRO J1655-40. The chief difference we find is that the zero-torque model gives a value for the inner disk radius that is about 2.2 times smaller than the value given by diskbb. This result has important implications, especially for the determination of black-hole angular momentum and mass accretion rate.

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arXiv2004

A Comparison of Intermediate Mass Black Hole Candidate ULXs and Stellar-Mass Black Holes

Cool thermal emission components have recently been revealed in the X-ray spectra of a small number of ultra-luminous X-ray (ULX) sources with L_X > 1 E+40 erg/s in nearby galaxies. These components can be well fitted with accretion disk models, with temperatures approximately 5-10 times lower than disk temperatures measured in stellar-mass Galactic black holes when observed in their brightest states. Because disk temperature is expected to fall with increasing black hole mass, and because the X-ray luminosity of these sources exceeds the Eddington limit for 10 Msun black holes (L_Edd = 1.3 E+39 erg/s), these sources are extremely promising intermediate-mass black hole candidates (IMBHCs). In this Letter, we directly compare the inferred disk temperatures and luminosities of these ULXs, with the disk temperatures and luminosities of a number of Galactic black holes. The sample of stellar-mass black holes was selected to include different orbital periods, companion types, inclinations, and column densities. These ULXs and stellar-mass black holes occupy distinct regions of a L_X -- kT diagram, suggesting these ULXs may harbor IMBHs. We briefly discuss the important strengths and weaknesses of this interpretation.

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arXiv2004

Simultaneous Chandra and RXTE Spectroscopy of the Microquasar H~1743-322: Clues to Disk Wind and Jet Formation from a Variable Ionized Outflow

We observed the bright phase of the 2003 outburst of the Galactic black hole candidate H 1743-322 in X-rays simultaneously with Chandra and RXTE on four occasions. The Chandra/HETGS spectra reveal narrow, variable (He-like) Fe XXV and (H-like) Fe XXVI resonance absorption lines. In the first observation, the Fe XXVI line has a FWHM of 1800 +/- 400 km/s and a blue-shift of 700 +/- 200 km/s, suggesting that the highly ionized medium is an outflow. Moreover, the Fe XXV line is observed to vary significantly on a timescale of a few hundred seconds in the first observation, which corresponds to the Keplerian orbital period at approximately 1 E+4 gravitational radii. Our models for the absorption geometry suggest that a combination of geometric effects and changing ionizing flux are required to account for the large changes in line flux observed between observations, and that the absorption likely occurs at a radius less than 1 E+4 radii for a 10 Msun black hole. Viable models for the absorption geometry include cyclic absorption due to an accretion disk structure, absorption in a clumpy outflowing disk wind, or possibly a combination of these two. If the wind in H 1743-322 has unity filling factor, the highest implied mass outflow rate is 20 percent of the Eddington mass accretion rate. This wind may be a hot precursor to the Seyfert-like, outflowing "warm absorber" geometries recently found in the Galactic black holes GX 339-4 and XTE J1650-500. We discuss these findings in the context of ionized Fe absorption lines found in the spectra of other Galactic sources, and connections to warm absorbers, winds, and jets in other accreting systems.

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arXiv2004

Spectral and Timing Evolution of the Black Hole X-ray Nova 4U 1543-47 During its 2002 Outburst

We present an X-ray spectral and timing analysis of 4U 1543-47 during its 2002 outburst based on 49 pointed observations obtained using the Rossi X-ray Timing Explorer (RXTE). The outburst reached a peak intensity of 4.2 Crab in the 2-12 keV band and declined by a factor of 32 throughout the month-long observation. A 21.9 +- 0.6 mJy radio flare was detected at 1026.75 MHz two days before the X-ray maximum; the radio source was also detected late in the outburst, after the X-ray source entered the low hard state. The X-ray light curve exhibits the classic shape of a rapid rise and an exponential decay. The spectrum is soft and dominated by emission from the accretion disk. The continuum is fit with a multicolor disk blackbody (kT_{max} = 1.04 keV) and a power-law (Gamma ~ 2.7). Midway through the decay phase, a strong low-frequency QPO (nu = 7.3-8.1 Hz) was present for several days. The spectra feature a broad Fe K alpha line that is asymmetric, suggesting that the line is due to relativistic broadening rather than Comptonization. Relativistic Laor models provide much better fits to the line than non-relativistic Gaussian models, particularly near the beginning and end of our observations. The line fits yield estimates for the inner disk radius that are within 6 R_g; this result and additional evidence indicates that this black hole may have a non-zero angular momentum.

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arXiv2004

Revealing a Cool Accretion Disk in the Ultraluminous X-ray Source M81 X-9 (Holmberg IX X-1): Evidence for an Intermediate Mass Black Hole

We report the results of an analysis of two XMM-Newton/EPIC-pn spectra of the bright ultraluminous X-ray source M81 X-9 (Holmberg IX X-1), obtained in snapshot observations. Soft thermal emission is clearly revealed in spectra dominated by hard power-law components. Depending on the model used, M81 X-9 was observed at a luminosity of L_X = 1.0-1.6 E+40 erg/s (0.3--10.0 keV). The variability previously observed in this source signals that it is an accreting source which likely harbors a black hole. Remarkably, accretion disk models for the soft thermal emission yield very low inner disk temperatures (kT = 0.17-0.29 keV, including 90% confidence errors and variations between observations and disk models), and improve the fit statistic over any single-component continuum model at the 6 sigma level of confidence. Like NGC 1313 X-1, scaling the temperatures measured in M81 X-9 to those commonly seen in stellar-mass Galactic black holes at their highest observed fluxes (kT ~ 1 keV) may imply that M81 X-9 harbors a black hole with a mass on the order of 10^3 Msun; the measured disk component normalization and broad-band luminosity imply black hole masses on the order of 10^2 Msun. It is therefore possible that these sources harbor 10^3 Msun black holes accreting such at L_X ~ 0.1*L_Edd. Radio and optical observations suggest that beaming and anisotropic emission from a stellar-mass black hole are unlikely to account for the implied luminosity. We further argue that the strength of the hard emission in these sources and well-established phenomena frequently observed in stellar-mass black holes near to the Eddington limit suggest that optically-thick photospheres are unlikely to be the origin of the cool thermal emission in bright ULXs.

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arXiv2004

XMM-Newton Spectroscopy of Bright ULX Sources in the Antennae Galaxies (NGC 4038/4039)

We report the results of spectral fits to four bright ultra-luminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/4039) observed for 41 ksec with XMM-Newton. Although emission regions are not resolved as well as in prior Chandra observations, at least four ULXs (X-11, X-16, X-37, and X-44 in the Zezas and Fabbiano scheme) are sufficiently bright and well-separated with XMM-Newton that reliable extractions and spectral analyses are possible. We find that the single component multi-color disk blackbody models cannot describe any of the spectra. Sources X-11 and X-16 are acceptably fit with simple power-law models. A thermal bremsstrahlung model provides a better fit to the spectrum of X-44. Including a disk blackbody component to the spectrum of X-37 improves the fit and reveals an apparently cool disk (kT = 0.13 +/- 0.02 keV). This would suggest a parallel to cool disks recently found in other very luminous ULXs, which may contain intermediate mass black holes; however, the complex diffuse emission of the Antennae demands this finding be regarded cautiously.

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arXiv2004

Evidence for Black Hole Spin in GX 339-4: XMM-Newton EPIC-pn and RXTE Spectroscopy of the Very High State

We have analyzed spectra of the Galactic black hole GX 339-4 obtained through simultaneous 76 ksec XMM-Newton/EPIC-pn and 10 ksec RXTE observations during a bright phase of its 2002-2003 outburst. An extremely skewed, relativistic Fe K-alpha emission line and ionized disk reflection spectrum are revealed in these spectra. Self-consistent models for the Fe K-alpha emission line profile and disk reflection spectrum rule-out an inner disk radius compatible with a Schwarzschild black hole at more than the 8 sigma level of confidence. The best-fit inner disk radius of 2-3 r_g suggests that GX 339-4 harbors a black hole with a ~ 0.8-0.9 (where r_g = GM/c^2 and a=cJ/GM^2, and assuming that reflection in the plunging region is relatively small). This confirms indications for black hole spin based on a Chandra spectrum obtained later in the outburst. The emission line and reflection spectrum also rule-out a standard power-law disk emissivity in GX 339-4; a broken power-law form with enhanced emissivity inside ~6 r_{g} gives improved fits at more than the 8 sigma level of confidence. The extreme red wing of the line and steep emissivity require a centrally--concentrated source of hard X-rays which can strongly illuminate the inner disk. Hard X-ray emission from the base of a jet - enhanced by gravitational light bending effects - could create the concentrated hard X-ray emission; this process may be related to magnetic connections between the black hole and the inner disk. We discuss these results within the context of recent results from analyses of XTE J1650-500 and MCG-6-30-15, and models for the inner accretion flow environment around black holes.

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arXiv2004

The Relativistic Fe Emission Line in XTE J1650-500 with BeppoSAX: Evidence for Black Hole Spin and Light Bending Effects?

We report spectral results from three BeppoSAX observations of the black hole candidate XTE J1650-500 during its 2001/2002 outburst. We find strong evidence for the presence of a broad and strongly relativistic Fe emission line. The line profile indicates an accretion disc extending down to two gravitational radii (or less) suggesting the presence of a rapidly rotating central Kerr black hole. Thanks to the broadband spectral coverage of BeppoSAX, we could analyze the 1.5-200 keV spectra of the three observations and report the presence of a strong reflection component from the accretion disc, which is totally consistent with the observed broad Fe emission line. The shape of the reflection component appears to be affected by the same special and general relativistic effects that produce the broad Fe line. We study the variation of the different spectral components from the first to the third observation and we find that they are well reproduced by a recently proposed light bending model.

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arXiv2004

Relativistic Iron Lines in Galactic Black Holes: Recent Results and Lines in the ASCA Archive

Recent observations with Chandra and XMM-Newton, aided by broad-band spectral coverage from RXTE, have revealed skewed relativistic iron emission lines in stellar-mass Galactic black hole systems. Such systems are excellent laboratories for testing General Relativity, and relativistic iron lines provide an important tool for making such tests. In this contribution to the Proceedings of the 10th Annual Marcel Grossmann Meeting on General Relativity, we briefly review recent developments and present initial results from fits to archival ASCA observations of Galactic black holes. It stands to reason that relativistic effects, if real, should be revealed in many systems (rather than just one or two); the results of our archival work have borne-out this expectation. The ASCA spectra reveal skewed, relativistic lines in XTE J1550-564, GRO J1655-40, GRS 1915+105, and Cygnus X-1.

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arXiv2003

Chandra/HETGS Spectroscopy of the Galactic Black Hole GX 339-4: A Relativistic Iron 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 ksec 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 K-alpha 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-like and He-like O, and He-like Ne and Mg are detected, as well as lines which 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.

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arXiv2003

Untwisting the Tornado: X-ray Imaging and Spectroscopy of G357.7-0.1

We report on the detection of X-ray emission from the unusual Galactic radio source G357.7-0.1 (the "Tornado"). Observations made with the Chandra X-ray Observatory demonstrate the presence of up to three sources of X-ray emission from the Tornado: a relatively bright region of dimensions 2'x1' coincident with and interior to the brightest radio emission at the "head" of the Tornado, plus two fainter extended regions possibly associated with the Tornado's "tail". No X-ray point sources associated with the Tornado are seen down to a 3-sigma luminosity (0.5-10 keV) of 1e33 ergs/s, for a distance to the system of 12 kpc. The spectrum of the brightest region of X-rays is consistent with a heavily absorbed (N_H ~ 1e23 cm^-2) thermal plasma of temperature kT ~ 0.6 keV; an absorbed power law can also fit the data, but implies an extremely steep photon index. From these data we tentatively conclude that the Tornado is a supernova remnant (SNR), although we are unable to rule out the possibility that the Tornado is powered either by outflows from an X-ray binary or by the relativistic wind of an unseen pulsar. Within the SNR interpretation, the head of the Tornado is a limb-brightened radio shell containing centrally-filled thermal X-rays and which is interacting with a molecular cloud. We therefore propose that the Tornado is a "mixed morphology" supernova remnant. The unusual tail component of the Tornado remains unexplained in this interpretation, but might result from expansion of the SNR into an elongated progenitor wind bubble.

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arXiv2003

Resolved Jets and Long-Period Black Hole X-ray Novae

In this brief note we point out that the four spatially resolved relativistic jets among the 14 dynamically confirmed black hole X-ray novae are all in systems with long orbital periods. Many shorter period systems show transient radio outbursts which are attributed to jets, but these jets have not been spatially resolved. Super-Eddington accretion has been suggested as a requirement for jet formation and may be consistent with our compilation of luminosities, but some super-Eddington outbursts did not form spatially resolved jets. We speculate that some as yet unknown process (or combination of processes) favors formation of substantially larger jets in long period systems. Two short period systems show evidence for extent, but have not been resolved into multiple components as the long period systems have.

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arXiv2003

XMM-Newton Spectroscopy of the Accretion-Driven Millisecond X-ray Pulsar XTE J1751-305 in Outburst

We present an analysis of the first high-resolution spectra measured from an accretion-driven millisecond X-ray pulsar in outburst. We observed XTE J1751-305 with XMM-Newton on 2002 April 7 for approximately 35 ksec. Using a simple absorbed blackbody plus power-law model, we measure an unabsorbed flux of (6.6 +/- 0.1) * 10^(-10) erg/cm^2/s (0.5--10.0 keV). A hard power-law component (Gamma = 1.44 +/- 0.01) contributes 83% of the unabsorbed flux in the 0.5-10.0 keV band, but a blackbody component (kT = 1.05 +/- 0.01 keV) is required. We find no clear evidence for narrow or broad emission or absorption lines in the time-averaged spectra, and the sensitivity of this observation has allowed us to set constraining upper-limits on the strength of important features. The lack of line features is at odds with spectra measured from some other X-ray binaries which share some similarities with XTE J1751-305. We discuss the implications of these findings on the accretion flow geometry in XTE J1751-305.

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arXiv2003

X-ray Spectroscopic Evidence for Intermediate Mass Black Holes: Cool Accretion Disks in Two Ultra--Luminous X-ray Sources

We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two "ultra-luminous" X-ray (ULX) sources. We measure isotropic luminosities of L_X = 2.0 * 10^(40) erg/s and L_X = 6.6 * 10^(39) erg/s 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 ULXs have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 sigma level of confidence for X-1; the improvement for X-2 is significant at the 3 sigma level. The soft components in these ULX spectra are well-fit by multi-color 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_sun) 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: M_BH ~ 10^(3) M_sun. Simple Eddington scaling arguments suggest a mimum mass of M_BH ~ 10^(2) M_sun. NGC 1313 X-1 and X-2 are found in optical nebulae (Pakull and Mirioni 2002), which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.

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arXiv2002

X-Ray Emission from the Jets of XTE J1550-564

We report on X-ray observations of the the large-scale jets recently discovered in the radio and detected in X-rays from the black hole candidate X-ray transient and microquasar XTE J1550-564. On 11 March 2002, X-ray emission was detected 23 arcsec to the West of the black hole candidate and was extended along the jet axis with a full width at half maximum of 1.2 arcsec and a full width at 10% of maximum intensity of 5 arcsec. The morphology of the X-ray emission matched well to that of the radio emission at the same epoch. The jet moved by 0.52 +/- 0.13 arcsec between 11 March and 19 June 2002. The apparent speed during that interval was 5.2 +/- 1.3 mas/day. This is significantly less than the average apparent speed of 18.1 +/- 0.4 mas/day from 1998 to 2002, assuming that the jet was ejected in September 1998, and indicates that the jet has decelerated. The X-ray spectrum is adequately described by a powerlaw with a photon index near 1.8 subject to interstellar absorption. The unabsorbed X-ray flux was 3.4 x 10^-13 erg cm^-2 s^-1 in the 0.3-8 keV band in March 2002, and decreased to 2.9 x 10^-13 erg cm^-2 s^-1 in June. We also detect X-rays from the eastern jet in March 2002 and show that it has decelerated and dimmed since the previous detections in 2000.

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arXiv2002

Large-Scale, Decelerating, Relativistic X-ray Jets from the Microquasar XTE J1550-654

We have discovered at x-ray and radio wavelengths large-scale moving jets from the microquasar XTE J1550-654. Plasma ejected from near the black hole traveled at relativistic velocities for at least four years. We present direct evidence for gradual deceleration in a relativistic jet. The broadband spectrum of the jets is consistent with synchrotron emission from high energy (up to 10 TeV) particles accelerated in shock waves formed within the relativistic ejecta or by the interaction of the jets with the interstellar medium. XTE J1550-654 offers a unique opportunity to study the dynamical evolution of relativistic jets on time scales inaccessible for active galactic nuclei jets, with implications for our understanding of relativistic jets from Galactic x-ray binaries and active galactic nuclei.

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