Kaice Theodore Reilly

USA Experiment and RXTE Observations of a Variable Low-Frequency Quasi-periodic Oscillation in XTE J1118+480

The USA experiment on ARGOS and RXTE have exensively observed the X-ray transient XTE J1118+480 during its recent outburst in 2000 April--June. We present detailed monitoring of the evolution of a low frequency QPO which drifts from 0.07 Hz to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source, and compare this QPO to low frequency QPOs observed in other black hole candidates.Using the USA Experiment on the Advanced Research and Global Observation Satellite and the Rossi X-Ray Timing Explorer, we have extensively observed the X-ray transient XTE J1118+480 during its recent outburst in 2000 April-June. We present detailed monitoring of the evolution of a low-frequency quasi-periodic oscillation (QPO) that drifts from 0.07 to 0.15 Hz during the outburst. We examine possible correlations of the QPO frequency with the flux and spectral characteristics of the source, and we compare this QPO with low-frequency QPOs observed in other black hole candidates.

Long-Term X-Ray Variability of Circinus X-1

We present an analysis of long-term X-ray monitoring observations of Circinus X-1 (Cir X-1) made with four different instruments: Vela 5B, Ariel 5 all-sky monitor (ASM), Ginga ASM, and RXTE ASM, over the course of more than 30 yr. We use Lomb-Scargle periodograms to search for the ~16.5 day orbital period of Cir X-1 in each of these data sets. From this we derive a new orbital ephemeris based solely on X-ray measurements, which we compare with the previous ephemerides obtained from radio observations. We also use the phase dispersion minimization (PDM) technique, as well as fast Fourier transform analysis, to verify the periods obtained from periodograms. We obtain dynamic periodograms (both Lomb-Scargle and PDM) of Cir X-1 during the RXTE era, showing the period evolution of Cir X-1 and also displaying some unexplained discrete jumps in the location of the peak power.

OBSERVATION OF X-RAY VARIABILITY IN THE BL LACERTAE OBJECT 1ES 1959+65

This paper reports X-ray spectral observations of a relatively nearby (z = 0.048) BL Lac object 1ES 1959+65, which is a potential TeV emitter. The observations include 31 short pointings made by the Unconventional Stellar Aspect (USA) experiment on board the Advanced Research and Global Observation Satellite (ARGOS), and 17 pointings by the Proportional Counter Array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE). Most of these observations were spaced by less than 1 day. 1ES 1959+65 was detected by the ARGOS USA detector in the range 1-16 keV, and by the PCA in the 2-16 keV range but at different times. During the closely spaced RXTE observations beginning on 2000 July 28, the ending of one flare and the start of another are visible, associated with spectral changes, where the photon index Γ ranges between ~1.4 and ~1.7, and the spectrum is harder when the source is brighter. This implies that 1ES 1959+65 is an X-ray-selected BL Lac type (XBL) blazar, with the X-ray emission likely to originate via the synchrotron process. The USA observations reveal another flare that peaked on 2000 November 14 and doubled the flux within a few days, again associated with spectral changes of the same form. The spectral variability correlated with the flux and timing characteristics of this object that are similar to those of other nearby BL Lac objects and suggest relativistic beaming with a Doppler factor δ ≥ 1.6 and magnetic fields on the order of a few milligauss. We also suggest that the steady component of the X-ray emission—present in this object as well as in other XBLs—may be due to the large-scale relativistic jet (such as measured by Chandra in many radio-loud active galactic nuclei) but pointing very closely to our line of sight.

X-Ray Bursts in Neutron Star and Black Hole Binaries from Unconventional Stellar Aspect Experiment and Rossi X-Ray Timing Explorer Data: Detections and Upper Limits

Narayan & Heyl have developed a theoretical framework to convert suitable upper limits on type I X-ray bursts from accreting black hole candidates (BHCs) into evidence for an event horizon. However, no appropriate observational limit exists in the literature. In this paper we survey 2101.2 ks of data from the Unconventional Stellar Aspect (USA) X-ray timing experiment and 5142 ks of data from the Rossi X-Ray Timing Explorer (RXTE) experiment to obtain a formal constraint of this type. We find that 1122 ks of neutron star data yield a population-averaged mean burst rate of (1.7 ± 0.4) × 10-5 bursts s-1, while 6081 ks of BHC data yield a 95% confidence level upper limit of 4.9 × 10-7 bursts s-1. This is the first published limit of this type for BHCs. Applying the theoretical framework of Narayan & Heyl, we calculate regions of unstable luminosity, where the neutron stars are expected to burst and the BHCs would be expected to burst if they had a surface. In this unstable luminosity region, 464 ks of neutron star data yield an averaged mean burst rate of (4.1 ± 0.8) × 10-5 bursts s-1, and 1512 ks of BHC data yield a 95% confidence level upper limit of 2.0 × 10-6 bursts s-1 and a strong limit that BHCs do not burst with a rate similar to the rate of neutron stars in these unstable regions. This gives further evidence that BHCs do not have surfaces unless there is some new physics occurring on their surface.tournear@SLAC.stanford.edu Current address: Laboratoire Leprince-Ringuet, Ecole Polytechnique, Palaiseau F-91128 NRC/NRL Cooperative Research Associate Current address: Department of Astrophysics, Oxford University

USA experiment observation of spectral and timing evolution during the 2000 outburst of XTE J1550-564

We report on timing and spectral observations of the 2000 outburst of XTE J1550-564 made by the Unconventional Stellar Aspect (USA) experiment on board the Advanced Research and Global Observation Satellite (ARGOS). We observe a low-frequency quasi-periodic oscillation with a centroid frequency that tends to increase with increasing flux and a fractional rms amplitude that is correlated with the hardness ratio. Several high-frequency quasi-periodic oscillations were detected by Rossi X-Ray Timing Explorer during periods where the low-frequency quasi-periodic oscillation was seen to be weakening or not detectable at all. The evolution of the hardness ratio (4-16 keV/1-4 keV) with time and source flux is examined. The hardness-intensity diagram shows a cyclical movement in the counterclockwise direction and possibly indicates the presence of two independent accretion flows: a thin-disk flow and a hot sub-Keplerian flow.

The USA X-ray timing experiment

The USA Experiment is a new X-ray timing experiment with large collecting area and microsecond time resolution capable of conducting a broad program of studies of galactic X-ray binaries. USA is one of nine experiments aboard the Advanced Research and Global Observation Satellite which was launched February 23, 1999. USA is a collimated proportional counter X-ray telescope with about 1000 cm2 of effective area per detector with two detectors sensitive to photons in the energy range 1–15 keV. A unique feature of USA is that photon events are time tagged by reference to an onboard GPS receiver allowing precise absolute time and location determination. We will present an overview of the USA instrument, capabilities, and scientific observing plan as well as the current status of the instrument.

USA Observation of Spectral and Timing Evolution During the 2000 Outburst of XTE J1550--564

We report on timing and spectral observations of the 2000 outburst of XTE J1550--564 made by the Unconventional Stellar Aspect (USA) Experiment on board the Advanced Research and Global Observation Satellite (ARGOS). We observe a low-frequency quasi-periodic oscillation (LFQPO) with a centroid frequency that tends to increase with increasing flux and a fractional rms amplitude which is correlated with the hardness ratio. Several high-frequency quasi-periodic oscillations (HFQPO) were detected by RXTE, during periods where the LFQPO is seen to be weakening or not detectable at all. The evolution of the hardness ratio (4-16 keV/1-4 keV) with time and source flux is examined. The hardness-intensity diagram (HID) shows a cyclical movement in the counterclockwise direction and possibly indicates the presence of two independent accretion flows: a thin disk and a hot sub-Keplerian flow.

Shot noise in USA lightcurves of XTE J1118+480 and cygnus X-1

We investigate the behavior of the new transient black hole candidate XTE J1118+480 using shot noise models in the time domain. Using light curves from the Unconventional Stellar Aspect experiment (USA), we model a portion of the emission as a superposition of shots. Individual structures in the light curve are fit to a template function. The results are compared to a similar analysis performed on light curves from Cygnus X-1. Results indicate that the characteristic timescale for XTE J118+480 is around 3 times as long as for Cygnus X-1.

X-ray observations of Mkn 421 with USA

This paper presents the first AGN detections with the USA X-ray detector on board of ARGOS. We started observing the BL Lac object Mkn421 at the end of January 2000, when it was also observed by the CAT Cerenkov telescope (see Pierre Espigat, these proceedings), and optical telescopes (but the latter are not presented here). Mkn421 was at the beginning of a long period of high activity that lasted for many weeks. Some observations on Mkn421 were also carried out in this period by XTE (see Rita Sambruna, these proceedings).