Jon M. Miller

Large-Scale, Decelerating, Relativistic X-ray Jets from the Microquasar XTE J1550-564

We have detected, at x-ray and radio wavelengths, large-scale moving jets from the microquasar XTE J1550–564. Plasma ejected from near the black hole traveled at relativistic velocities for at least 4 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 tera–electron volts) particles that were accelerated in the shock waves formed within the relativistic ejecta or by the interaction of the jets with the interstellar medium. XTE J1550–564 offers a rare 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.

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).

X-Ray States and Radio Emission in the Black Hole Candidate XTE J1550–564

We report on radio and X-ray observations of the black hole candidate (BHC) XTE J1550-564 performed during its 2000 X-ray outburst. Observations were conducted with the Australia Telescope Compact Array and allowed us to sample the radio behavior of XTE J1550-564 in the X-ray low hard and intermediate/very high states. We observed optically thin radio emission from XTE J1550-564 5 days after a transition to an intermediate/very high state, but we observed no radio emission 6 days later, while XTE J1550-564 was still in the intermediate/very high state. In the low hard state, XTE J1550-564 is detected with an inverted radio spectrum. The radio emission in the low hard state most likely originates from a compact jet; optical observations suggest that the synchrotron emission from this jet may extend up to the optical range. The total power of the compact jet might therefore be a significant fraction of the total luminosity of the system. We suggest that the optically thin radio emission detected 5 days after the transition to the intermediate/very high state is due to a discrete ejection of relativistic plasma during the state transition. Subsequent to the decay of the optically thin radio emission associated with the state transition, it seems that in the intermediate/very high state the radio emission is quenched by a factor greater than 50, implying a suppression of the outflow. We discuss the properties of radio emission in the X-ray states of BHCs.

Resolving the composite Fe K alpha emission line in the Galactic black hole Cygnus X-1 with Chandra

We observed the Galactic black hole Cyg X-1 with the Chandra High Energy Transmission Grating Spectrometer for 30 ks on 2001 January 4. 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 eV, W = 16 eV) and broad-line (E = 5.82 keV, FWHM= 1.9 keV, W = 140 eV) components, and a smeared edge at 7.3 ? 0.2 keV (? ~ 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? emission line and edge, and irradiation of the outer disk producing a narrow Fe K? 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 Cyg X-1 and other Galactic black holes.

A Chandra Observation of the Long-Duration X-Ray Transient KS 1731–260 in Quiescence: Too Cold a Neutron Star?

After more than a decade of actively accreting at about a tenth of the Eddington critical mass accretion rate, the neutron star X-ray transient KS 1731-260 returned to quiescence in early 2001. We present a Chandra/Advanced CCD Imaging Spectrometer observation taken several months after this transition. We detected the source at an unabsorbed flux of ~2 × 10-13 ergs cm-2 s-1 (0.5-10 keV). For a distance of 7 kpc, this results in a 0.5-10 keV luminosity of ~1 × 1033 ergs s-1 and a bolometric luminosity approximately twice that. This quiescent luminosity is very similar to that of the other quiescent neutron star systems. However, if this luminosity is due to the cooling of the neutron star, this low luminosity may indicate that the source spends at least several hundreds of years in quiescence in between outbursts for the neutron star to cool. If true, then it might be the first such X-ray transient to be identified, and a class of hundreds of similar systems may be present in the Galaxy. Alternatively, enhanced neutrino cooling could occur in the core of the neutron star that would cool the star more rapidly. However, in that case, the neutron star in KS 1731-260 would be more massive than those in the prototypical neutron star transients (e.g., Aquila X-1 or 4U 1608-52).

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. It is the least radio luminous and one of the least X-ray luminous SNe ever detected (except for the unusual and very close SN 1987A). The Chandra X-ray data indicate nonradiative interaction of SN ejecta with a power-law density profile (ρ r-n, with n ~ 7) for a pre-SN wind with a low mass-loss rate of ~2 × 10-6 M☉ yr-1 for a wind velocity of 10 km s-1, 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 more than 3 yr, is still bright in X-rays and is increasing in flux density at centimeter 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☉ 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 ~104 km s-1, the shock front is nonradiative at the time of the observations, and the mass-loss rate is (1-2) × 10-4 M☉ yr-1 for a presupernova wind velocity of 10 km s-1. 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 × 10-4 M☉ yr-1, from the presupernova star.

Optical identification of multiple faint X-ray sources in the globular cluster NGC 6752: evidence for numerous cataclysmic variables

We report on the Chandra X-Ray Observatory ACIS-S3 imaging observation of the globular cluster NGC 6752. We detect six X-ray sources within the 105 core radius and 13 more within the 115 half-mass radius down to a limiting luminosity of LX ≈ 1030 ergs s-1 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), one to three likely RS CVn or BY Dra systems, and one or two possible background objects. Of the seven sources for which no optical identifications were made, we expect that approximately two to four 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.

INTEGRAL/RXTE high-energy observation of a state transition of GX 339–4

On 2004 August 15, we observed a fast (shorter than 10 h) state transition in the bright black hole transient GX 339-4 simultaneously with Rossi X-Ray Timing Explorer (RXTE) and INTEGRAL. This transition was evident both in timing and spectral properties. Combining the data from the Proportional Counter Array (PCA), the High-Energy X-ray Timing Experiment (HEXTE) and the Imager on Board the INTEGRAL Satellite (IBIS), we obtained good quality broad-band (3-200 keV) energy spectra before and after the transition. These spectra indicate that the hard component steepened. Also, the high-energy cut-off that was present at ∼70 keV before the transition was not detected after the transition. This is the first time that an accurate determination of the broad-band spectrum across such a transition has been measured on a short time-scale. It shows that, although some spectral parameters do not change abruptly through the transition, the high-energy cut-off increases/disappears rather fast. These results constitute a benchmark on which to test theoretical models for the production of the hard component in these systems.

X-Ray Jet Emission from the Black Hole X-Ray Binary XTE J1550-564 with Chandra in 2000

We have discovered an X-ray jet due to material ejected from the black hole X-ray transient XTE J1550-564. The discovery was first reported in 2002 by Corbel and coworkers, and here we present an analysis of the three Chandra observations made between 2000 June and September. For these observations, a source is present that moves in an eastward direction away from the point source associated with the compact object. The separation between the new source and the compact object changes from 213 in June to 234 in September, implying a proper motion of 21.2 ± 7.2 mas day-1, a projected separation of 0.31-0.85 pc, and an apparent jet velocity between 0.34 ± 0.12 and 0.93 ± 0.32 times the speed of light for a source distance range of d = 2.8-7.6 kpc. These observations represent the first time that an X-ray jet proper-motion measurement has been obtained for any accretion-powered Galactic or extragalactic source. While this work deals with the jet to the east of the compact object, the western jet has also been detected in the X-ray and radio bands. The most likely scenario is that the eastern jet is the approaching jet and that the jet material was ejected from the black hole in 1998. Along with a 1998 VLBI proper-motion measurement, the Chandra proper motion indicates that the eastern jet decelerated between 1998 and 2000. There is evidence that the eastern jet is extended by ±2-3 in the direction of the proper motion. The upper limit on the source extension in the perpendicular direction is ±15, which corresponds to a jet opening angle of less than 75. The X-ray jet energy spectrum is well but not uniquely described by a power law with an energy index of α = -0.8 ± 0.4 (Sν ∝ να) and interstellar absorption. The eastern jet was also detected in the radio band during an observation made within 7.4 days of the June Chandra observation. The overall radio flux level is consistent with an extrapolation of the X-ray power law with α = -0.6. The 0.3-8 keV X-ray jet luminosity is in the range (3-24) × 1032 ergs s-1 for the June observation using the distance range above but is a factor of ~2-3 lower for the later observations. We cannot definitively determine the X-ray emission mechanism, but a synchrotron origin is viable and may provide the simplest explanation for the observations.

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 as the source made a transition to the low/hard state during the decay of the 2001 outburst. We find quasi-periodic oscillations 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. We suggest a possible geometry and evolution of a jet + corona + disk system, based on enhanced lags and peak frequency shift during the transition.