arXiv subjects

J. M. Miller

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

At least 253 records · Page 14Linked to original sources

arXiv2002

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 flux of 1 to 2 X 10-8 erg/s/cm2. Circumstantial evidence points to an accreting galactic black hole as the origin of the 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 a ~40 keV plasma (between 2 and 60 keV this component can be approximated by a power law with a photon index of 1.9 to 2.1), 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 X 10+22 cm-2, which makes it difficult 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.

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arXiv2002

Revealing the Focused Companion Wind in Cygnus X-1 with Chandra

We have analyzed a Chandra HETGS spectrum of the Galactic black hole Cygnus X-1, obtained at a source flux which is approximately twice that commonly observed in its persistent low-intensity, spectrally-hard state. We find a myriad of absorption lines in the spectrum, including Ly-alpha lines and helium-like resonance lines from Ne, Na, Mg, and Si. We calculate a flux-weighted mean red-shift of ~100 km/s and a flux-weighted mean velocity width of 800 km/s (FWHM) for lines from these elements. We also detect a number of transitions from Fe XVIII-XXIV and Ni XIX-XX in absorption; however, the identification of these lines is less certain and a greater range of shifts and breadth is measured. Our observation occurred at a binary phase of phi = 0.76; the lines observed are consistent with absorption in an ionized region of the supergiant O9.7 Iab companion wind. The spectrum is extremely complicated in that a range of temperatures and densities are implied. Prior Chandra HETGS spectra of Cygnus X-1 were obtained in a similar transition state (at phi = 0.93) and in the low/hard state (at phi= 0.84). Considered together, these spectra provide evidence for a companion wind that is focused as it flows onto the black hole primary in this system.

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arXiv2002

GX 339-4: back to life

We report preliminary results of a RossiXTE campaign on the 2002 outburst of the black-hole candidate GX 339-4. We show power density spectra of five observations during the early phase of the outburst. The first four power spectra show a smooth transition between a Low State and a Very High State. The fifth power spectrum resembles a High State, but a strong 6 Hz QPO appears suddenly within 16 seconds.

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arXiv2002

Chandra and RXTE Spectroscopy of the Galactic Microquasar XTE J1550-564 in Outburst

On two occasions, we obtained nearly simultaneous ~ 4 kilosecond snapshot observations of the Galactic black hole and microquasar XTE J1550-564 with Chandra and RXTE near the peak of its May, 2000 outburst. The low-energy sensitivity of Chandra and the resolution of the High Energy Transmission Grating Spectrometer (HETGS), coupled with the broad energy range and large collecting area of RXTE, have allowed us to place constraints on the outburst accretion flow geometry of this source in the ``intermediate'' X-ray state. The 0.65-25.0 keV continuum spectra are well-described by relatively hot (kT ~ 0.8 keV) accretion disk and hard (Gamma ~ 2.3) coronal power-law components. Broad, relatively strong Fe K-alpha emission line (EW ~170 eV) and smeared absorption edge components consistent with Fe XXV are strongly required in joint spectral fits. The resolution of the Chandra/HETGS reveals that the broad Fe K-alpha emission lines seen clearly in the individual RXTE spectra are not due to an intrinsically narrow line.

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arXiv2002

A Relativistic Fe K-alpha Emission Line in the Intermediate Luminosity BeppoSAX Spectrum of the Galactic Microquasar V4641 Sgr

Broad Fe K-alpha emission lines have recently been reported in a number of Galactic black holes. Such lines are useful accretion flow diagnostics because they may be produced at the inner accretion disk and shaped by relativistic effects, but in general they have only been observed at luminosities of L_X ~ 10^(37-38) erg/s in soft X-rays. The Galactic microquasar V4641 Sgr -- widely known for its 12.2 Crab (1.5-12 keV) outburst in 1999 September -- displayed low-level activity in 1999 March. BeppoSAX observed the source in this state and Fe K-alpha line emission was found (in 't Zand et al. 2000). In re-analyzing these data, we find strong evidence that the Fe K-alpha line profile is broadened. For the most likely values of the source distance and black hole mass measured by Orosz et al. (2001), our fits to the total spectrum indicate that the source was observed at a luminosity of L_X = 1.9 (+1.0, -0.8) * 10^(36) erg/s (2-10 keV), or L_X/L_Edd. = 1.8 (+0.9, -0.8) * 10^(-3). Advection-dominated accretion flow (ADAF) models predict a radially-recessed disk in this regime. In contrast, fits to the observed Fe K-alpha emission line profile with a relativistic line model constrain the inner disk to be consistent with the marginally stable circular orbit of a Schwarzschild black hole.

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arXiv2002

Chandra observations of the bursting X-ray transient SAX J1747.0-2853 during low-level accretion activity

We present Chandra/ACIS observations of the bursting X-ray transient SAX J1747.0-2853 performed on 18 July 2001. We detected a bright source at the position of R.A = 17^h 47^m 02.60^s and Dec. = -28 52' 58.9'' (J2000.0; with a 1 sigma error of ~0.7 arcseconds), consistent with the BeppoSAX and ASCA positions of SAX J1747.0-2853 and with the Ariel V position of the transient GX +0.2,-0.2, which was active during the 1970's. The 0.5-10 keV luminosity of the source during our observations was ~3 x 10^{35} erg/s (assuming a distance of 9 kpc) demonstrating that the source was in a low-level accretion state. We also report on the long-term light curve of the source as observed with the all sky monitor aboard the Rossi X-ray Timing Explorer. After the initial 1998 outburst, two more outbursts (in 2000 and 2001) were detected with peak luminosities about two orders of magnitude larger than our Chandra luminosity. Our Chandra observation falls in-between those two outbursts, making the outburst history for SAX J1747.0-2853 complex. Those bright 2000 and 2001 outbursts combined with the likely extended period of low level activity in-between those outbursts strongly suggest that the classification of SAX J1747.0-2853 as a faint X-ray transient was premature. It might be possible that the other faint X-ray transients also can exhibit bright, extended outbursts which would eliminate the need for a separate sub-class of X-ray transients. We discuss our results also in the context of the behavior of X-ray binaries accreting at low levels with luminosities around 10^{35} erg/s, a poorly studied accretion rate regime.

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arXiv2002

Extremely Weak Reflection Features in the X-ray Spectrum of XTE J1118+480: Possible Evidence for X-ray-Emitting Jets?

We have simultaneously fit Chandra and RXTE spectra of the Galactic black hole XTE J1118+480 with three models for X-ray reflection. We explored a range of accretion disc ionizations (log(xi)=1-4; xi=L_X/nR^{2}) and iron abundances (0.10-1.00). Our fits with the constant density ionized disc models of Ross & Fabian indicate that less than 0.5 per cent (90 per cent confidence upper-limit) of the observed flux is reflected. Fits with the "pexrav" of model Magdziarz & Zdziarski indicate that the two-dimensional solid angle (Omega/2pi) subtended by the disc relative to a central source of incident hard X-rays is 0.01 +0.06 -0.01. A combination of the high inclination (i=81 degrees), Comptonization, and bulk velocities may each contribute to the low reflection fractions we have measured. The results are also consistent with extended jets being the source of the hard X-ray flux, as the disc would then represent a small solid angle as seen from the emission region.

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arXiv2002

Evidence for Spin and Energy Extraction in a Galactic Black Hole Candidate: The XMM-Newton/EPIC-pn Spectrum of XTE J1650-500

We observed the Galactic black hole candidate XTE J1650-500 early in its Fall, 2001 outburst with the XMM-Newton European Photon Imaging pn Camera (EPIC-pn). The observed spectrum is consistent with the source having been in the "very high" state. We find a broad, skewed Fe K-alpha emission line which suggests that the primary in this system may be a Kerr black hole, and which indicates a steep emissivity profile that is hard to explain in terms of a standard accretion disk model. These results are quantitatively and qualitatively similar to those from an XMM-Newton observation of the Seyfert galaxy MCG--6-30-15. The steep emissivity in MCG--6-30-15 may be explained by the extraction and dissipation of rotational energy from a black hole with nearly- maximal angular momentum or material in the plunging region via magnetic connections to the inner accretion disk. If this process is at work in both sources, an exotic but fundamental general relativistic prediction may be confirmed across a factor of 10^6 in black hole mass. We discuss these results in terms of the accretion flow geometry in stellar-mass black holes, and the variety of enigmatic phenomena often observed in the very high state.

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arXiv2002

Resolving the Composite Fe K-alpha Emission Line in the Galactic Black Hole Cygnus X-1 with Chandra

We observed the Galactic black hole Cygnus X-1 with the Chandra High Energy Transmission Grating Spectrometer for 30 kiloseconds on 4 January, 2001. The source was in an intermediate state, with a flux that was approximately twice that commonly observed in its persistent low/hard state. Our best-fit model for the X-ray spectrum includes narrow Gaussian emission line (E = 6.415 +/- 0.007 keV, FWHM = 80 (+28, -19) eV, W = 16 (+3, -2) eV) and broad line (E = 5.82 (+0.06, -0.07) keV, FWHM = 1.9 (+0.5, -0.3) keV, W = 140 (+70, -40) eV) components, and a smeared edge at 7.3 +/- 0.2 keV (tau ~ 1.0). The broad line profile is not as strongly skewed as those observed in some Seyfert galaxies. We interpret these features in terms of an accretion disk with irradiation of the inner disk producing a broad Fe K-alpha emission line and edge, and irradiation of the outer disk producing a narrow Fe K-alpha emission line. The broad line is likely shaped predominantly by Doppler shifts and gravitational effects, and to a lesser degree by Compton scattering due to reflection. We discuss the underlying continuum X-ray spectrum and these line features in the context of diagnosing the accretion flow geometry in Cygnus X-1 and other Galactic black holes.

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arXiv2002

X-ray temporal properties of XTE J1650-500 during outburst decay

We investigated the temporal behavior of the new black hole transient XTE J1650-500 with the Rossi X-ray Timing Explorer (RXTE) as the source made a transition to the low/hard state during the decay of the 2001 outburst. We find QPOs in the 4 - 9 Hz range, enhanced time lags and reduced coherence during the state transition. We also observe a shift in the peak frequency of the noise component with energy during the transition. The evolution of the power spectrum as well as the lag and coherence behavior during the state transition are similar to the state transitions observed for other black hole sources, especially Cyg X-1. The temporal properties during the transition to the low state put constraints on the accretion geometry of XTE J1650-500 and may have implications for all black hole binary systems. We suggest a possible geometry and evolution of a jet+corona+disk system based on enhanced lags and peak frequency shift during the transition.

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arXiv2002

X-ray, Optical, and Radio Observations of the Type II Supernovae 1999em and 1998S

Observations of the Type II-P (plateau) Supernova (SN) 1999em and Type IIn (narrow emission line) SN 1998S have enabled estimation of the profile of the SN ejecta, the structure of the circumstellar medium (CSM) established by the pre-SN stellar wind, and the nature of the shock interaction. SN 1999em is the first Type II-P detected at both X-ray and radio wavelengths. The Chandra X-ray data indicate non-radiative interaction of SN ejecta with a power-law density profile (rho \propto r^{-n} with n ~ 7) with a pre-SN wind with a low mass-loss rate of ~2 \times 10^{-6} M_sun/yr for a wind velocity of 10 km/sec, in agreement with radio mass-loss rate estimates. The Chandra data show an unexpected, temporary rise in the 0.4--2.0 keV X-ray flux at ~100 days after explosion. SN 1998S, at an age of >3 years, is still bright in X-rays and is increasing in flux density at cm radio wavelengths. Spectral fits to the Chandra data show that many heavy elements (Ne, Al, Si, S, Ar, and Fe) are overabundant with respect to solar values. We compare the observed elemental abundances and abundance ratios to theoretical calculations and find that our data are consistent with a progenitor mass of approximately 15-20 M_sun if the heavy element ejecta are radially mixed out to a high velocity. If the X-ray emission is from the reverse shock wave region, the supernova density profile must be moderately flat at a velocity ~10^4 km/sec, the shock front is non-radiative at the time of the observations, and the mass-loss rate is 1-2 \times 10^{-4} M_sun/yr for a pre-supernova wind velocity of 10 km/sec. This result is also supported by modeling of the radio emission which implies that SN 1998S is surrounded by a clumpy or filamentary CSM established by a high mass-loss rate, ~2 \times 10^{-4} M_sun/yr, from the pre-supernova star.

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arXiv2001

Highly Ionized Absorption in the X-ray Spectrum of Cyg X-1

Using the Chandra High Energy Transmission Grating Spectrometer (HETGS), we have found significant absorption features in the X-ray spectrum of Cyg X-1 taken in the continuous clocking mode. These features include resonance lines of He-like ions of S, Si, and Mg; the Ly alpha lines of H-like S, Si, Mg, and Ne; and several lower ionization lines of Fe XX, XXII, and XXIV. Preliminary analysis shows that the lines are resolved in many cases, giving line widths of order 300 km/s and are redshifted by 460 +/- 10 km/s. These features are interpreted in the context of an accreting stellar wind model that is ionized by the X-ray source. In addition, there are clear absorption features due to neutral Mg, Fe, and O in the interstellar medium.

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arXiv2001

The burst behavior of the eclipsing low-mass X-ray binary MXB 1659-298

We present a detailed study of the correlations between the burst properties and the inferred mass accretion rate for the X-ray transient MXB 1659-298. The bursts which exhibited oscillations were observed when the source was at relatively high mass accretion rate, similar to what has been seen for other sources. However, due to the limited number of observations at lower mass accretion rates, no bursts were observed at such accretion rates and it is still possible that when MXB 1659-298 accretes at such low mass accretion rates, bursts can occur which might still exhibit burst oscillations. No clear correlations were found between the different burst properties and accretion rate, in contrast to what has been found for KS 1731-260 and 4U 1728-34, but similar to what has been reported for Aql X-1. However, this lack of correlation for MXB 1659-298 and Aql X-1 might be due to the limited range of mass accretion rate observed for those sources compared to KS 1731-260 and 4U 1728-34.

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arXiv2001

XMM-Newton Spectroscopy of the Galactic Microquasar GRS 1758-258 in the Peculiar Off/Soft State

We report on an XMM-Newton Reflection Grating Spectrometer observation of the black hole candidate and Galactic microquasar GRS 1758-258. The source entered a peculiar "off/soft" state in late February, 2001, in which the spectrum softened while the X-ray flux -- and the inferred mass accretion rate -- steadily decreased. We find no clear evidence for emission or absorption lines in the dispersed spectra, indicating that most of the observed soft flux is likely from an accretion disk and not from a cool plasma. The accretion disk strongly dominates the spectrum in this lower-luminosity state, and is only mildly recessed from the marginally stable orbit. These findings may be difficult to explain in terms of advection-dominated accretion flow, or "ADAF" models. We discuss these results within the context of ADAF models, simultaneous two-flow models, and observed correlations between hard X-ray flux and jet production.

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arXiv2001

High Frequency Quasi-Periodic Oscillations in the 2000 Outburst of the Galactic Microquasar XTE J1550-564

We present an analysis of the high frequency timing properties of the April-May 2000 outburst of the black hole candidate and Galactic microquasar XTE J1550-564, measured with the Rossi X-ray Timing Explorer. The rapid X-ray variability we measure is consistent with the source being in either the ``very high'' or ``intermediate'' black hole state. Strong (5-8% rms) quasi-periodic oscillations (QPOs) are found between 249-276 Hz; this represents the first detection of the same high frequency QPO in subsequent outbursts of a transient black hole candidate. We also present evidence for lower-frequency QPOs at approximately 188 Hz (3.5 sigma, single trial), also reported previously and likely present simultaneously with the higher-frequency QPOs. We discuss these findings within the context of the 1998 outburst of XTE J1550-564, and comment on implications for models of QPOs, accretion flows, and black hole spin.

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arXiv2001

The First High Resolution X-ray Spectrum of Cyg X-1: Soft X-Ray Ionization and Absorption

We observed the black hole candidate Cyg X-1 for 15 ks with the High-Energy Transmission Grating Spectrometer aboard the CHANDRA X-ray Observatory. The source was observed during a period of intense flaring activity, so it was about a factor of 2.5 brighter than usual, with a 0.5-10 keV (1-24 A) luminosity of 1.6x10^37 erg/s (at a distance of 2.5 kpc). The spectrum of the source shows prominent absorption edges, some of which have complicated substructure. We use the most recent results from laboratory measurements and and calculations to model the observed substructure of the edges. From the model, we derive a total absorption column of 6.21+/-0.22 10^21 cm^-2. Furthermore, the results indicate that there are ~ 10 - 25% abundance variations relative to solar values for neon, oxygen and iron. The X-ray continuuum is described well by a two-component model that is often adopted for black hole candidates: a soft multicolor disk component (with kT = 203 eV) and a hard power law component (with a photon index of ~2). Comparing the fit results to those of the hard and soft states, we conclude that the source was in a transitional state. Finally, the spectrum also shows the presence of faint emission lines which could be attributed to highly ionized species.

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arXiv2001

The X-ray Spectrum of the Rapid Burster using the Chandra HETGS

We present observations of the Rapid Burster (RB, also known as MXB 1730-335) using the Chandra High Energy Transmission Grating Spectrometer. The average interval between type II (accretion) bursts was about 40 s. There was one type I (thermonuclear flash) burst and about 20 "mini-bursts" which are probably type II bursts whose peak flux is 10-40% of the average peak flux of the other type II bursts. The time averaged spectra of the type II bursts are well fit by a blackbody with a temperature of kT = 1.6 keV, a radius of 8.9 km for a distance of 8.6 kpc, and an interstellar column density of 1.7e22 per sq. cm. No narrow emission or absorption lines were clearly detected. The 3 sigma upper limits to the equivalent widths of any features are < 10 eV in the 1.1-7.0 keV band and as small as 1.5 eV near 1.7 keV. We suggest that Comptonization destroys absorption features such as the resonance line of Fe XXVI.

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arXiv2001

Complete and Simultaneous Spectral Observations of the Black-Hole X-ray Nova XTE J1118+480

The X-ray nova XTE J1118+480 suffers minimal extinction (b = 62 degrees) and therefore represents an outstanding opportunity for multiwavelength studies. Hynes et al. (2000) conducted the first such study, which was centered on 2000 April 8 using UKIRT, EUVE, HST and RXTE. On 2000 April 18, the Chandra X-ray Observatory obtained data coincident with a second set of observations using all of these same observatories. A 30 ks grating observation using Chandra yielded a spectrum with high resolution and sensitivity covering the range 0.24-7 keV. Our near-simultaneous observations cover approximately 80% of the electromagnetic spectrum from the infrared to hard X-rays. The UV/X-ray spectrum of XTE J1118+480 consists of two principal components. The first of these is an approximately 24 eV thermal component which is due to an accretion disk with a large inner disk radius: > 35 Schwarzschild radii. The second is a quasi power-law component that was recorded with complete spectral coverage from 0.4-160 keV. A model for this two-component spectrum is presented in a companion paper by Esin et al. (2001).

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