Outbursts, State Transitions, and Periodicities Observed with the RXTE All-Sky Monitor

Results from the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer are reviewed. A number of recurrent transient sources have been detected, while only a few previously unreported sources have been discovered. The ASM light curves show a wide variety of phenomena in general, and, in particular, those of transient sources show a wide range of properties. Examples are used to illustrate that the distinction between persistent and transient sources may be very unclear. The results of searches for periodicities in the ASM light curves are summarized, and other astrophysical investigations using ASM light curves are suggested. The latter include investigations of the possible causes of long-term quasiperiodic and chaotic variability, and comparative studies on the basis of the observed variability.Comment: 8 pages, 4 figures, in proceedings of the conference "The Active X-ray Sky: Results from BeppoSAX and Rossi-XTE, Rome, Italy, 21-24 October, 1997. Nuclear Physics B Proceedings Supplement

Discovery of a new Transient X-ray Pulsar in the Small Magellanic Cloud

Rossi X-Ray Timing Explorer observations of the Small Magellanic Cloud have revealed a previously unknown transient X-ray pulsar with a pulse period of 95s. Provisionally designated XTE SMC95, the pulsar was detected in three Proportional Counter Array observations during an outburst spanning 4 weeks in March/April 1999. The pulse profile is double peaked reaching a pulse fraction \~0.8. The source is proposed as a Be/neutron star system on the basis of its pulsations, transient nature and characteristically hard X-ray spectrum. The 2-10 keV X-ray luminosity implied by our observations is > 2x10^37 erg/s which is consistent with that of normal outbursts seen in Galactic systems. This discovery adds to the emerging picture of the SMC as containing an extremely dense population of transient high mass X-ray binaries.Comment: Accepted by A&A. 7 pages, 6 figure

An Extended and More Sensitive Search for Periodicities in RXTE/ASM X-ray Light Curves

We present the results of a systematic search in approximately 14 years of Rossi X-ray Timing Explorer All-Sky Monitor data for evidence of periodicities not reported by Wen et al. (2006). Two variations of the commonly used Fourier analysis search method have been employed to achieve significant improvements in sensitivity. The use of these methods and the accumulation of additional data have resulted in the detection of the signatures of the orbital periods of eight low-mass X-ray binary systems and of ten high-mass X-ray binaries not listed in the tables of Wen et al.Comment: 20 pages, 22 figures, in emulateapj format; submitted to ApJ

4U2206+54 - an Unusual High Mass X-ray Binary with a 9.6 Day Orbital Period but No Strong Pulsations

Rossi X-ray Timing Explorer All-Sky Monitor observations of the X-ray source 4U2206+54, previously proposed to be a Be star system, show the X-ray flux to be modulated with a period of approximately 9.6 days. If the modulation is due to orbital variability then this would be one of the shortest orbital periods known for a Be star X-ray source. However, the X-ray luminosity is relatively modest whereas a high luminosity would be predicted if the system contains a neutron star accreting from the denser inner regions of a Be star envelope. Although a 392s pulse period was previously reported from EXOSAT observations, a reexamination of the EXOSAT light curves does not show this or any other periodicity. An analysis of archival RXTE Proportional Counter Array observations also fails to show any X-ray pulsations. We consider possible models that may explain the properties of this source including a neutron star with accretion halted at the magnetosphere and an accreting white dwarf.Comment: Accepted for publication in the Astrophysical Journa

INTEGRAL and Swift observations of IGRJ19294+1816 in outburst

IGRJ19294+1816 was discovered by INTEGRAL in 2009 during a bright X-ray outburst and was classified as a possible Be X-ray binary or supergiant fast X-ray transient. On 2010 October 28, the source displayed a second X-ray outburst and a 2 months-long monitoring with Swift was carried out to follow the evolution of the source X-ray flux during the event. We report on the INTEGRAL and Swift observations of the second X-ray outburst observed from IGRJ19294+1816. We detected pulsations in the X-ray emission from the source at \sim12.5 s up to 50 keV. The source X-ray flux decreased smoothly during the two months of observation displaying only marginal spectral changes. Due to the relatively rapid decay of the source X-ray flux, no significant variations of the source spin period across the event could be measured. This prevented a firm confirmation of the previously suggested orbital period of the source at 117 d. This periodicity was also searched by using archival Swift /BAT data. We detected a marginally significant peak in the periodogram and determined the best period at 116.2\pm0.6 days (estimated chance probability of a spurious detection 1%). The smooth decline of the source X-ray flux across the two months of observations after the onset of the second outburst, together with its relatively low value of the spin period and the absence of remarkable changes in the spectral parameters (i.e., the absorption column density), suggests that IGRJ19294+1816 is most likely another member of the Be X-ray binaries discovered by INTEGRAL and not a supergiant fast X-ray transient.Comment: Accepted for publication in A&A. 7 pages, 10 figure

The X-ray Properties of M101 ULX-1 = CXOKM101 J140332.74+542102

We report our analysis of X-ray data on M101 ULX-1, concentrating on high state Chandra and XMM-Newton observations. We find that the high state of M101 ULX-1 may have a preferred recurrence timescale. If so, the underlying clock may have periods around 160 or 190 days, or possibly around 45 days. Its short-term variations resemble those of X-ray binaries at high accretion rate. If this analogy is correct, we infer that the accretor is a 20-40 Msun object. This is consistent with our spectral analysis of the high state spectra of M101 ULX-1, from which we find no evidence for an extreme (> 10^40 ergs/s) luminosity. We present our interpretation in the framework of a high mass X-ray binary system consisting of a B supergiant mass donor and a large stellar-mass black hole.Comment: 23 pages, 7 figures, accepted for publication in the Astrophysical Journa

Orbital Period Determinations for Four SMC Be/X-ray Binaries

We present an optical and X-ray study of four Be/X-ray binaries located in the Small Magellanic Cloud (SMC). OGLE I-band data of up to 11 years of semi-continuous monitoring has been analysed for SMC X-2, SXP172 and SXP202B, providing both a measurement of the orbital period (Porb = 18.62, 68.90, and 229.9 days for the pulsars respectively) and a detailed optical orbital profile for each pulsar. For SXP172 this has allowed a direct comparison of the optical and X-ray emission seen through regular RXTE monitoring, revealing that the X-ray outbursts precede the optical by around 7 days. Recent X-ray studies by XMM-Newton have identified a new source in the vicinity of SXP15.3 raising doubt on the identification of the optical counterpart to this X-ray pulsar. Here we present a discussion of the observations that led to the proposal of the original counterpart and a detailed optical analysis of the counterpart to the new X-ray source, identifying a 21.7 d periodicity in the OGLE I-band data. The optical characteristics of this star are consistent with that of a SMC Be/X-ray binary. However, this star was rejected as the counterpart to SXP15.3 in previous studies due to the lack of H{\alpha} emission.Comment: Accepted for publication in MNRAS, 11 pages, 17 figure

Monitoring and Discovering X-ray Pulsars in the Small Magellanic Cloud

Regular monitoring of the SMC with RXTE has revealed a huge number of X-ray pulsars. Together with discoveries from other satellites at least 45 SMC pulsars are now known. One of these sources, a pulsar with a period of approximately 7.8 seconds, was first detected in early 2002 and since discovery it has been found to be in outburst nine times. The outburst pattern clearly shows a period of 45.1 +/- 0.4 d which is thought to be the orbital period of this system. Candidate outburst periods have also been obtained for nine other pulsars and continued monitoring will enable us to confirm these. This large number of pulsars, all located at approximately the same distance, enables a wealth of comparative studies. In addition, the large number of pulsars found (which vastly exceeds the number expected simply by scaling the relative mass of the SMC and the Galaxy) reveals the recent star formation history of the SMC which has been influenced by encounters with both the LMC and the Galaxy.Comment: 5 pages, 4 figures, AIP conference proceedings format. Contribution to "X-ray Timing 2003: Rossi and Beyond." meeting held in Cambridge, MA, November, 200

X-ray Pulsars in the Small Magellanic Cloud

XMM-Newton archival data for the Small Magellanic Cloud have been examined for the presence of previously undetected X-ray pulsars. One such pulsar, with a period of 202 s, is detected. Its position is consistent with an early B star in the SMC and we identify it as a high mass X-ray binary (HMXB). In the course of this study we determined the pulse period of the possible AXP CXOU J010043.1-721134 to be 8.0 s, correcting an earlier report (Lamb et al 2002b) of a 5.4 s period for this object. Pulse profiles and spectra for each of these objects are presented as well as for a recently discovered (Haberl & Pietsch 2004) 263 s X-ray pulsar. Properties of an ensemble of 24 optically identified HMXB pulsars from the SMC are investigated. The locations of the pulsars and an additional 22 X-ray pulsars not yet identified as having high mass companions are located predominately in the young (ages $\le 3 \times 10^{7}$ years) star forming regions of the SMC as expected on the basis of binary evolution models. We find no significant difference between the distribution of spin periods for the HMXB pulsars of the SMC compared with that of the Milky Way. For those HMXB pulsars which have Be companions we note an inverse correlation between spin period and maximum X-ray flux density. (This anti-correlation has been previously noted for all X-ray binary pulsars by Stella, White & Rosner 1986). The anti-correlation for the Be binaries may be a reflection of the fact that the spin periods and orbital periods of Be HMXBs are correlated. We note a similar correlation between X-ray luminosity and spin period for the Be HMXB pulsars of the Milky Way and speculate that exploitation of the correlation could serve as a distance indicator.Comment: final version accepted in The Astrophysical Journa