arXiv subjects

D. Pooley

Publications and source records attributed to D. Pooley.

44 records · Page 3Linked to original sources

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

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

Optical Identification of Multiple Faint X-ray Sources in the Globular Cluster NGC 6752: Evidence for Numerous Cataclysmic Variables

We report on the Chandra ACIS-S3 imaging observation of the globular cluster NGC 6752. We detect 6 X-ray sources within the 10.5" core radius and 13 more within the 115" half-mass radius down to a limiting luminosity of Lx approx 10^{30} erg/s for cluster sources. We reanalyze archival data from the Hubble Space Telescope and the Australia Telescope Compact Array and make 12 optical identifications and one radio identification. Based on X-ray and optical properties of the identifications, we find 10 likely cataclysmic variables (CVs), 1-3 likely RS CVn or BY Dra systems, and 1 or 2 possible background objects. Of the 7 sources for which no optical identifications were made, we expect that ~2-4 are background objects and that the rest are either CVs or some or all of the five millisecond pulsars whose radio positions are not yet accurately known. These and other Chandra results on globular clusters indicate that the dozens of CVs per cluster expected by theoretical arguments are being found. The findings to date also suggest that the ratio of CVs to other types of X-ray sources is remarkably similar in clusters of very different structural parameters.

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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

Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440

After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright transient X-ray source turning on in August 2001, as noted with the RXTE All-Sky Monitor. We carried out a short target of opportunity observation with the Chandra X-ray Observatory and are able to associate the transient with the brightest of 24 X-ray sources detected during quiescence in July 2000 with Chandra. Furthermore, we securely identify the optical counterpart and determine that the 1998 X-ray outburst in NGC 6440 was from the same object. This is the first time that an optical counterpart to a transient in a globular cluster is securely identified. Since the transient is a type I X-ray burster, it is established that the compact accretor is a neutron star. Thus, this transient provides an ideal case to study the quiescent emission in the optical and X-ray of a transiently accreting neutron star while knowing the distance and reddening accurately. One model that fits the quiescent spectrum is an absorbed power law plus neutron star hydrogen atmosphere model. We find an intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is consistent with predictions for a cooling neutron star.

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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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arXiv2000

Relativistic Iron Emission and Disk Reflection in Galactic Microquasar XTE J1748-288

We report evidence for an Fe K-alpha fluorescence line feature and disk reflection in the Very High, High, and Low State X-ray spectra of the galactic microquasar XTE J1748-288 during its June~1998 outburst. Spectral analyses are made on data gathered throughout the outburst by the Rossi X-ray Timing Explorer Proportional Counter Array. Gaussian line, relativistic disk emission line, and ionized disk reflection models are fit to the data. In the Very High State the line profile appears strongly redshifted, consistent with disk emission from the innermost stable orbits around a maximally rotating Kerr black hole. In the High State the line profile is less redshifted and increasingly prominent. The Low State line profile is very strong (~0.5 keV equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model fits indicate that the inner edge of the disk fluctuates between ~20 and ~100 gravitational radii in this state. The disk reflection fraction is traced through the outburst; reflection from an ionized disk is preferred in the VHS and HS, and reflection from a relatively neutral disk is preferred in the LS. We discuss the implications of our findings for the binary system dynamics and accretion flow geometry in XTE J1748-288.

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