Alan P. Smale

A short γ-ray burst apparently associated with an elliptical galaxy at redshift z = 0.225

Gamma-ray bursts (GRBs) come in two classes: long (> 2 s), soft-spectrum bursts and short, hard events. Most progress has been made on understanding the long GRBs, which are typically observed at high redshift (z ≈ 1) and found in subluminous star-forming host galaxies. They are likely to be produced in core-collapse explosions of massive stars. In contrast, no short GRB had been accurately (< 10″) and rapidly (minutes) located. Here we report the detection of the X-ray afterglow from—and the localization of—the short burst GRB 050509B. Its position on the sky is near a luminous, non-star-forming elliptical galaxy at a redshift of 0.225, which is the location one would expect if the origin of this GRB is through the merger of neutron-star or black-hole binaries. The X-ray afterglow was weak and faded below the detection limit within a few hours; no optical afterglow was detected to stringent limits, explaining the past difficulty in localizing short GRBs.

Millisecond X-Ray Variability from an Accreting Neutron Star System

We report the detection with the Proportional Counter Array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE) of millisecond variability in the X-ray emission from the low-mass X-ray binary 4U 1728-34. Pulsations at 363 Hz with amplitudes (rms) of 2.5%-10% are present in six of the eight bursts analyzed to date. The strongest were seen in two successive bursts recorded on 1996 February 16 when the quiescent count rate was near the highest seen by PCA. The pulsations during these bursts show frequency changes of 1.5 Hz during the first few seconds but become effectively coherent as the burst decays. We interpret the 363 Hz pulsations as rotationally induced modulations of inhomogeneous burst emission. This represents the first compelling evidence for a millisecond spin period in a low-mass X-ray binary. Complex, intensity-dependent, millisecond X-ray variability is also present in all the quiescent flux intervals we examined. Most interesting was the behavior as the count rate approached its highest observed level. Two quasi-periodic oscillations (QPOs) were simultaneously observed in the 650-1100 Hz range. Both QPOs increased in frequency together, maintaining a nearly constant frequency separation of about 363 Hz, the spin period inferred from the burst oscillations. This phenomenology is strongly suggestive of the magnetospheric beat frequency model proposed for the horizontal-branch oscillations (HBOs) seen in Z sources. We discuss this and several other possible physical interpretations for the observed X-ray variability.

Multiepoch multiwavelength spectra and models for blazar 3C 279

Of the blazars detected by EGRET in GeV γ-rays, 3C 279 is not only the best observed by EGRET but also one of the best monitored at lower frequencies. We have assembled 11 spectra, from GHz radio through GeV γ-rays, from the time intervals of EGRET observations. Although some of the data have appeared in previous publications, most are new, including data taken during the high states in early 1999 and early 2000. All of the spectra show substantial γ-ray contribution to the total luminosity of the object; in a high state, the γ-ray luminosity dominates over that at all other frequencies by a factor of more than 10. There is no clear pattern of time correlation; different bands do not always rise and fall together, even in the optical, X-ray, and γ-ray bands. The spectra are modeled using a leptonic jet, with combined synchrotron self-Compton plus external Compton γ-ray production. Spectral variability of 3C 279 is consistent with variations of the bulk Lorentz factor of the jet, accompanied by changes in the spectral shape of the electron distribution. Our modeling results are consistent with the UV spectrum of 3C 279 being dominated by accretion disk radiation during times of low γ-ray intensity.

Correlation between energy spectral states and fast time variability and further evidence for the marginally stable orbit in 4u 1820-30

We report the results from a long-term monitoring data set obtained with the Rossi X-Ray Timing Explorer (RXTE) from the bright globular cluster binary source 4U 1820-30. During this monitoring, the source flux varied from 2.9?10 -->?9 to 8.5?10 -->?9 ergs cm-2 s-1 in the 2-10 keV band, which correspond to, respectively, count rates of 1600 and 4500 counts s-1 in the nominal 2-60 keV RXTE proportional counter array band. Its energy spectral state was observed to encompass both the banana branches and the island state. Kilohertz quasi-periodic oscillations (kHz QPOs) were observed to exist in the upper banana and lower banana branches, as well as in the island state. In particular, we report that the kHz QPOs were present only when the soft color (defined as the flux ratio of the 3.9-5.0 keV band to the 1.7-3.9 keV band) was below 0.78 without regard to the overall count rate and without regard to whether the source is in any one of the three branches or states. Their centroid frequency was well correlated with the overall count rate below 2500 counts s-1. When the count rate was above 2500 counts s-1, the QPO frequency was consistent with a constant independent of the overall count rate. We take this as further evidence that the inner edge of the accretion had reached the marginally stable orbit, and therefore the constant 1060?20 Hz is the marginally stable orbit frequency.

A Type I Burst with Radius Expansion Observed from Cygnus X-2 with the Rossi X-Ray Timing Explorer

We present the results of a Rossi X-Ray Timing Explorer Proportional Counter Array observation of Cygnus X-2 in its high-intensity state, during which the source passed through the normal and horizontal branches of its Z diagram and mapped out a new, pronounced upturn on its horizontal branch. During this upturn state, a bright X-ray burst was observed, with rise and decay times of ~1 and ~5 s, respectively, and a peak flux of 1.52×10 -->−8 ergs cm-2 s-1. The energy-dependent profile, energetics, and spectral evolution allow us to identify this as a type I X-ray burst, with evidence for photospheric radius expansion in its first 2 s and spectral cooling in its later stages. The detection of these phenomena removes the previous ambiguity about the nature of bursts from Cyg X-2 and, in addition, allows us to make an independent determination of the source distance of 11.6±0.3 kpc.

A new X-ray spectral observation of NGC 1068

A new X-ray observation of NGC 1068, in which improved spectral resolution (R is approximately equal to 40) and broad energy range provide important new constraints on models for this galaxy, is reported. The observed X-ray continuum of NGC 1068 from 0.3 to 10 keV is well fitted as the sum of two power-law spectra with no evidence for absorption intrinsic to the source. Strong Fe K emission lines with a total equivalent width of 2700 eV were detected due to iron less ionized than Fe XX and to iron more ionized than Fe XXIII. No evidence was seen for lines due to the recombination of highly ionized oxygen with an upper limit for the O Ly-alpha emission line of 40 eV. The discovery of multiple Fe K and Fe L emission lines indicates a broad range of ionization states for this gas. The X-ray emission from the two components is modeled for various geometries using a photoionization code that calculates the temperature and ionization state of the gas. Typical model parameters are a total Compton depth of a few percent, an inner boundary of the hot component of about 1 pc, and an inner boundary of the warm component of about 20 pc.

X-Ray and Radio Observations of the Cygnus OB2 Association

Several OB stars in the Cygnus OB2 association are among the strongest stellar X-ray and radio sources in the Galaxy. The radio emission is particularly unusual, displaying a high level of variability and nonthermal behavior. We obtained two ROSAT PSPC observations, a ROSAT HRI observation, and three VLA observations of the association during a 2 yr time span. Our study will focus on four stars, Cyg OB2 No. 5, No. 8A, No. 9, and No. 12. Three of the four (Cyg No. 5, No. 9, and No. 12) were relatively constant in their X-ray emission over the 2 yr time frame. The fourth, Cyg OB2 No. 8A, increased in intensity by ~34%. No short-term (hourly) variability was detected. The observed X-ray characteristics (e.g., luminosity, temperature) are found to be consistent with the X-ray properties of other OB stars. The exception is Cyg OB2 No. 12, whose X-ray characteristics are found to be inconsistent with its spectral classification. Detailed spectral analyses of the PSPC data are presented for two absorption models: (1) ISM (cold absorber) and (2) Wind + ISM (warm absorber). The spectral fits suggest that the X-ray sources are located within the stellar wind, and estimates of the X-ray locations are presented. Adopting the radio-derived mass-loss rates, these X-ray locations are found to be consistent with the shock scenario proposed for OB stars. As expected, the radio emission has continued to be highly variable. Nonthermal characteristics are observed in Cyg OB2 No. 8A and No. 12. One of the most unusual nonthermal radio sources, Cyg OB2 No. 9, was found to be thermal in one of our observations. An observation of Cyg OB2 No. 5 also displayed a thermal radio spectrum. A comparison of the observed and intrinsic X-ray fluxes with the observed radio fluxes suggests that these quantities are anticorrelated; the strongest X-ray source is the weakest radio source. This is contrary to normal expectations for a wind-generated model of X-ray and radio emission. We investigate the long-term temporal behavior of both the X-ray and radio emission by comparing our newly acquired data with the previous X-ray (IPC) and radio data over the past 15 yr. Except for three events observed in Cyg OB2 No. 5, which displayed significant increases in its X-ray emission, the X-ray emission has remained relatively constant with a variability level less than 20% over this time span, whereas the radio emission has stayed highly variable with various levels of nonthermal behavior. We present a model to investigate the case in which the X-ray and radio emission are controlled by stellar wind properties and find that the predicted variability should be comparable in both emission processes. This is not observed. It is very intriguing that every time we observe the radio emission, it is different, whereas the X-ray emission always appears to be constant. If the stellar wind is as variable as suggested by the radio data, we believe it is highly implausible that we just happened to miss all periods of X-ray variability. We also investigate the implications of the observed nonthermal radio spectrum of Cyg OB2 No. 8A. Using the synchrotron emission model of White, we find that this nonthermal radio spectrum predicts a mass-loss rate almost 2 orders of magnitude less than that expected for a thermal radio spectrum. This lower mass-loss rate is consistent with an X-ray source located at the base of the stellar wind, contrary to the basic shock scenario. Since these stars show evidence of changing from thermal to nonthermal radio characteristics, it is difficult to understand how such a large change in mass loss did not produce a significant change in the observed X-rays.

Day-Scale Variability of 3C?279 and Searches for Correlations in Gamma-Ray, X-Ray, and Optical Bands

Light curves of 3C 279 are presented in optical (R band), X-rays (RXTE/PCA), and γ rays (CGRO/EGRET) for 1999 January-February and 2000 January-March. During both of those epochs the γ-ray levels were high and all three observed bands demonstrated substantial variation, on timescales as short as 1 day. Correlation analyses provided no consistent pattern, although a rather significant optical/γ-ray correlation was seen in 1999, with a γ-ray lag of ~2.5 days, and there are other suggestions of correlations in the light curves. For comparison, correlation analysis is also presented for the γ-ray and X-ray light curves during the large γ ray flare in 1996 February and the two γ-bright weeks leading up to it; the correlation at that time was strong, with a γ-ray/X-ray offset of no more than 1 day.

Long-term variability in low-mass X-ray binaries - A study using data from Vela 5B

A complete sample of low-mass X-ray binaries in uncrowded fields obtained from Vela 5B is studied here for periodic and aperiodic time variability on time scales of more than one day. The strong 176-day periodicity in X1820 - 303 is confirmed, and evidence is found for a significant 77-day period in Cyg X-2 and for variability on time scales of less than about 200 days in Cyg X-3. Limits are set on the possible amplitude of candidate periodicities in the literature for Ser X-1 and X0614 + 091. The evidence reported here suggests that while long-term cyclic variability is common among high-mass binaries, it is a rare occurrence in low-mass systems. 53 refs.

Discovery of Kilohertz Quasi-periodic Oscillations from 4U 1820–303 with Rossi X-Ray Timing Explorer

We have detected high-frequency (HF) quasi-periodic oscillations (QPOs) from the low-mass X-ray binary 4U 1820-303 during observations performed in 1996 October using the Rossi X-ray Timing Explorer. The QPOs are visible when the source occupies the low-state luminosity range LX = 2.4-3.1 × 1037 ergs s-1 (2-20 keV, at 6.4 kpc); the centroid frequency of the main QPO peak varies between 546 ± 2 Hz and 796 ± 6 Hz and is tightly correlated with the source count rate. The measured QPO widths are typically ~20 Hz, with mean rms amplitude 4.1% ± 0.3%. At the upper end of this luminosity range a second significant QPO peak appears with frequency 1065 ± 7 Hz, width 40 ± 20 Hz, and rms amplitude 3.2% ± 0.8%. When both QPOs are visible simultaneously, the difference between their frequencies is 275 ± 8 Hz. When the source brightens beyond LX = 3.1 × 1037 ergs s-1 (~10% of the Eddington limit for a helium-rich envelope), neither QPO is detected. Neither the magnetospheric beat frequency model nor the sonic point model of HF QPOs provides a perfect explanation of the phenomenology we observe. These results represent the first detection of kilohertz QPO activity in a globular cluster X-ray binary, and provide a new method of directly comparing the properties of cluster and noncluster neutron star binaries. If the highest QPO frequency we observe is identified with the marginally stable orbit in the accretion disk, the neutron star mass may be ~2 M☉, 35%-50% more massive than usually assumed. This may have consequences for the current evolutionary scenarios for this source and also for the debate about the evolution of millisecond pulsars in globular clusters.