Edward H. Morgan

First Results from the All-Sky Monitor on the Rossi X-Ray Timing Explorer

The all-sky monitor on the Rossi X-Ray Timing Explorer has been monitoring the sky in the 1.5-12 keV band since late February. The instrument consists of three coded-aperture cameras that can be rotated to view different regions by a motorized drive assembly. Intensities of ~100 known sources are obtained via least-squares fits of shadow patterns to the data and compiled to form X-ray light curves. Six orbital periodicities and four long-term periodicities, all previously known, have been detected in these light curves. Searches for additional sources have also been conducted. X-ray light curves for the Crab Nebula, Cyg X-1, 4U 1705-44, GRO J1655-40, and SMC X-1 are reported. They illustrate the quality of the results and the range of observed phenomena.

In-orbit performance and calibration of the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA)

The proportional counter array (PCA) is designed to perform microsecond timing of bright galactic sources and broad band, confusion limited, studies of faint extragalactic sources in the 2 - 60 keV x-ray band. The PCA was launched as part of the Rossi X-ray Timing Explorer (RXTE) satellite into a circular orbit of altitude 580 km and 23 degrees inclination on December 30, 1995. The mission contains three experiments: a set of large area xenon proportional counters sensitive from 2 - 60 keV (proportional counter array: PCA), a set of large area sodium iodide scintillators sensitive from 15 - 200 keV (high energy x-ray timing experiment: HEXTE), and three wide field of view scanning detectors which monitor most of the sky each orbit (all sky monitor: ASM). The goals of the mission are summarized by Swank et al. We present performance and calibration data on the measured and predicted in-orbit background, energy response, relative and absolute timing performance, and the operational possibilities made available with the high performance experiment data system (EDS) designed and built by MIT.

Nuclear-powered millisecond pulsars and the maximum spin frequency of neutron stars

Millisecond pulsars are neutron stars that are thought to have been spun-up by mass accretion from a stellar companion. It is not known whether there is a natural brake for this process, or if it continues until the centrifugal breakup limit is reached at submillisecond periods. Many neutron stars that are accreting mass from a companion star exhibit thermonuclear X-ray bursts that last tens of seconds, caused by unstable nuclear burning on their surfaces. Millisecond-period brightness oscillations during bursts from ten neutron stars (as distinct from other rapid X-ray variability that is also observed) are thought to measure the stellar spin, but direct proof of a rotational origin has been lacking. Here we report the detection of burst oscillations at the known spin frequency of an accreting millisecond pulsar, and we show that these oscillations always have the same rotational phase. This firmly establishes burst oscillations as nuclear-powered pulsations tracing the spin of accreting neutron stars, corroborating earlier evidence. The distribution of spin frequencies of the 11 nuclear-powered pulsars cuts off well below the breakup frequency for most neutron-star models, supporting theoretical predictions that gravitational radiation losses can limit accretion torques in spinning up millisecond pulsars.

The two-hour orbit of a binary millisecond X-ray pulsar

Typical radio pulsars are magnetized neutron stars that are born rapidly rotating and slow down as they age on time scales of 10 to 100 million years. In contrast, millisecond radio pulsars spin very rapidly even though many are billions of years old. The most compelling explanation is that they have been ‘spun up’ by the transfer of angular momentum during the accretion of material from a companion star in so-called low-mass X-ray binary systems, LMXBs. (LMXBs consist of a neutron star or black hole accreting matter from a companion with mass less than one solar mass.) The recent detection of coherent X-ray pulsations with a millisecond period from a suspected low-mass X-ray binary system appears to confirm this link. Here we report observations showing that the orbital period of this binary system is two hours, which establishes it as an LMXB. We also find an apparent modulation of the X-ray flux at the orbital period (at the two per cent level), with a broad minimum when the pulsar is behind the low-mass companion star. This system seems closely related to the ‘black-widow’ millisecond radio pulsars, which are evaporating their companions through irradiation. It may appear as an eclipsing radio pulsar during periods of X-ray quiescence.

Quasi-periodic Oscillations and Spectral States in GRS 1915+105

We present results from the analysis of X-ray energy spectra and quasi-periodic oscillations (QPOs) from a set of observations that samples a broad range of time variability in GRS 1915+105. We first demonstrate that the frequency and integrated amplitude of a 0.5-10 Hz QPO is correlated with the apparent temperature of the accretion disk for the majority of observations. We then show that the behavior of GRS 1915+105 exhibits two distinct modes of accretion. In the first mode, the QPO is present between 0.5 and 10 Hz and variability in the source luminosity is dominated by the power-law component. In the second mode, the QPO is absent and the changes in the luminosity are dominated by thermal emission from the accretion disk. We find that the color radius and temperature of the inner accretion disk are empirically related by Rcol ∝ T + const. We discuss these results in terms of ongoing efforts to explain the origin of both the QPOs and the hard X-ray component in the spectrum of GRS 1915+105.

Science with the Murchison Widefield Array

Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.

Discovery of submillisecond quasi-periodic oscillations in the x-ray flux of scorpius X-1

We report the discovery, with NASAs Rossi X-Ray Timing Explorer (RXTE), of the first submillisecond oscillations found in a celestial X-ray source. The quasi-periodic oscillations (QPOs) come from Sco X-1 and have a frequency of ~1100 Hz and amplitudes of 0.6%-1.2% (rms) and are relatively coherent, with Q up to ~102. The frequency of the QPOs increases with accretion rate, rising from 1050 to 1130 Hz when the source moves from top to bottom along the normal branch in the X-ray color-color diagram, and shows a strong, approximately linear correlation with the frequency of the well-known 6-20 Hz normal/flaring-branch QPOs. We also report the discovery of QPOs with a frequency near 800 Hz that occur, simultaneously with the 1100 Hz QPOs, in the upper normal branch. We discuss several possible interpretations, one involving a millisecond X-ray pulsar whose pulses we see reflected off accretion flow inhomogeneities. Finally, we report the discovery of ~45 Hz QPOs, most prominent in the middle of the normal branch, which might be magnetospheric beat-frequency QPOs.

Global Characteristics of X-Ray Flashes and X-Ray-Rich Gamma-Ray Bursts Observed by HETE-2

We describe and discuss the global properties of 45 gamma-ray bursts (GRBs) observed by HETE-2 during the first 3 years of its mission, focusing on the properties of X-ray flashes (XRFs) and X-ray-rich GRBs (XRRs). We find that the numbers of XRFs, XRRs, and GRBs are comparable, and that the durations and the sky distributions of XRFs and XRRs are similar to those of GRBs. We also find that the spectral properties of XRFs and XRRs are similar to those of GRBs, except that the values of the peak energy E of the burst spectrum in νFν, the peak energy flux Fpeak, and the energy fluence SE of XRFs are much smaller (and those of XRRs are smaller) than those of GRBs. Finally, we find that the distributions of all three kinds of bursts form a continuum in the [SE(2-30 keV), SE(30-400) keV] plane, the [SE(2-400 keV), Epeak] plane, and the [Fpeak(50-300 keV), Epeak] plane. These results provide strong evidence that all three kinds of bursts arise from the same phenomenon.

Complete RXTE Spectral Observations of the Black Hole X-ray Nova XTE J1550–564

We report on the X-ray spectral behavior of XTE J1550(564 during its 1998¨1999 outburst. XTE J1550(564 is an exceptionally bright X-ray nova and is also the third Galactic black hole candi- date known to exhibit quasi-periodic X-ray oscillations above 50 Hz. Our study is based on 209 pointed observations using the PCA and HEXTE instruments on board the Rossi X-Ray T iming Explorer (RXT E) spanning 250 days and covering the entire double-peaked eruption that occurred from 1998 September until 1999 May. The spectra are —tted to a model including multicolor blackbody disk and power-law components. The spectra from the —rst half of the outburst are dominated by the power-law component, whereas the spectra from the second half are dominated by the disk component. The source is observed in the very high and high/soft outburst states of black hole X-ray novae. During the very high state, when the power-law component dominated the spectrum, the inner disk radius is observed to vary by more than an order of magnitude; the radius decreased by a factor of 16 in one day during a 6.8 crab —are. If the larger of these observed radii is taken to be the last stable orbit, then the smaller observed radius would imply that the inner edge of the disk is inside the event horizon! However, we conclude that the apparent variations of the inner disk radius observed during periods of increased power-law emission are probably caused by the failure of the multicolor disk/power-law model; the actual physical radius of the inner disk may remain fairly constant. This interpretation is supported by the fact that the observed inner disk radius remains approximately constant over 120 days in the high state, when the power-law component is weak, even though the disk —ux and total —ux vary by an order of magnitude. The mass of the black hole inferred by equating the approximately constant inner disk radius observed in the high/soft state with the last stable orbit for a Schwarzschild black hole is M BH \ 7.4 kpc)(cos i)~1@2. M _ (D/6 Subject headings: black hole physicsstars: individual (XTE J1550(564) ¨ X-rays: stars On-line material: machine-readable tables

RXTE observations of 0.1-300 Hz QPOs in the microquasar GRO J1655-40

We have investigated 52 Rossi X-Ray T iming Explorer pointed observations of GRO J1655(40 span- ning the X-ray outburst that commenced on 1996 April 25 and lasted for 16 months. Our X-ray timing analyses reveal four types of quasi-periodic oscillations (QPOs): three with relatively stable central fre- quencies near 300, 9, and 0.1 Hz, and a fourth QPO that varied over the range 14¨28 Hz. The 300 and 0.1 Hz QPOs appear only when the power-law component dominates the X-ray spectrum and the esti- mated unabsorbed X-ray luminosity is above At lower luminosity the thermal D0.2L Edd .( L X D 0.1L Edd ), component dominates the spectrum; the disk appears somewhat cooler (D1.3 keV), and its inner radius appears larger. In this state only two of the QPOs are observed: the broad and spectrally ii soft ˇˇ 9 Hz QPO and the narrow, ii hard ˇˇ QPO that varies from 14 to 28 Hz as the hard —ux decreases. At still lower luminosities the power-law component contributes less than 30% of the total lumi- (L X \ 0.1L Edd ), nosity, the inner disk appears both larger and cooler, the 9 Hz QPO vanishes, and only a very weak (rms 0.3%) and narrow QPO at 28 Hz remains. The 300 Hz QPO is likely to be analogous to the stationary QPO at 67 Hz seen in the microquasar GRS 1915)105. We discuss four models of these high-frequency QPOs that depend on eUects due to general relativity. The models suggest that these rapid QPOs may eventually provide a measure of the mass and rotation of the accreting black hole. The 9 Hz QPO displays a spectrum consistent with a thermal origin, but this frequency is not uniquely con- sistent with any of the natural timescales associated with the disk. The mechanism for the 14¨28 Hz QPOs appears to be linked to the power-law component, similar to the 1¨10 Hz QPOs in GRS 1915)105. Thus these low-frequency QPOs have the potential to lead us to the origin of the energetic electrons that radiate the power-law spectral component. Finally, we show data for GRO J1655(40 and GRS 1915)105 as each source teeters between relative stability and a state of intense oscillations at 0.1 Hz. A comparison of the respective spectral parameters allows us to speculate that the black hole mass in GRS 1915)105 is very large, possibly D100 M _ . Subject headings: black hole physicsstars: individual (GRO J1655(40) ¨ stars: oscillations ¨ X-rays: stars