RXTE /ASM and Swift /BAT observations of spectral transitions in bright X-ray binaries in 2005-2010

We have studied X-ray spectral state transitions that can be seen in the longterm monitoring light curves of bright X-ray binaries from the All-Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE) and the Burst Alert Telescope (BAT) onboard Swift during a period of five years from 2005 to 2010.We have applied a program to automatically identify the hard-to-soft (H-S) spectral state transitions in the bright X-ray binaries monitored by the ASM and the BAT.In total,we identified 128 hard-to-soft t...     

Echoes from the companion star in Sco X-1

We present simultaneous X-ray ( RXTE ) and optical (ULTRACAM) narrow-band (Bowen blend/He  ii and nearby continuum) observations of Sco X-1 at 2–10 Hz time resolution. We find that the Bowen/He  ii emission lags the X-ray light curves with a light traveltime of     s which is consistent with reprocessing in the companion star. The echo from the donor is detected at orbital phase ∼0.5 when Sco X-1 is at the top of the flaring branch (FB). Evidence of echoes is also seen at the bottom of the FB but with time-lags of 5–10 s which are consistent with reprocessing in an accretion disc with a radial temperature profile. We discuss the implication of our results for the orbital parameters of Sco X-1.     

Where are the X-ray quasi-periodic oscillations in active galaxies?

In this paper, we address the question of whether existing X-ray observations of Seyfert galaxies are sufficiently sensitive to detect quasi-periodic oscillations (QPOs) similar to those observed in the X-ray variations of Galactic black holes (GBHs). We use data from XMM–Newton and simulated data based on the best Rossi X-ray Timing Explorer ( RXTE ) long-term monitoring light curves to show that if X-ray QPOs are present in Seyfert X-ray light curves – with similar shapes and strengths to those observed in GBHs, but at lower frequencies commensurate with their larger black hole masses – they would be exceedingly difficult to detect. Our results offer a simple explanation for the present lack of QPO detections in Seyferts. We discuss the improvements in telescope size and monitoring patterns needed to make QPO detections feasible. The most efficient type of future observatory for searching for X-ray QPOs in active Galactic nuclei (AGN) is an X-ray All-Sky Monitor (ASM). A sufficiently sensitive ASM would be ideally suited to detect low-frequency QPOs in nearby AGN. The detection of AGN QPOs would strengthen the AGN–GBH connection, and could serve as powerful diagnostics of the black hole mass and the structure of the X-ray emitting region in AGN.     

Rossi XTE monitoring of 4U 1636–53 – I. Long-term evolution and kHz quasi-periodic oscillations

We have monitored the atoll-type neutron star low-mass X-ray binary 4U 1636−53 with the Rossi X-ray Timing Explorer ( RXTE ) for more than 1.5 yr. Our campaign consisted of short (∼2 ks) pointings separated by 2 d, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ∼30–40 d, already seen in the light curves from the RXTE All-Sky Monitor, which corresponded to regular transitions between the hard (island) and soft (banana) states. We detected kilohertz (kHz) quasi-periodic oscillations (QPOs) in about a third of the observations, most of which were in the soft (banana) state. The distribution of the frequencies of the peak identified as the lower kHz QPO is found to be different from that previously observed in an independent data set. This suggests that the kHz QPOs in the system shows no intrinsically preferred frequency.     

Long-term Mir-Kvant observations of the transient X-ray burster KS 1731-260

We present the observations of the X-ray burster KS 1731-260 from 1988 until 1999 with the Kvant/TTM telescope supplemented with published data from the ASM and PCA instruments of the RXTE observatory for 1996–2001. We constructed the light curve of the source and confirmed the dependence of spectral variations on its X-ray luminosity.     

On the neutron star–disc interaction in Be/X-ray binaries

We have investigated the long-term X-ray variability, defined as the root-mean-square (rms) of the All Sky Monitor Rossi X-ray Timing Explorer (ASM RXTE ) light curves, of a set of galactic Be/X-ray binaries and searched for correlations with system parameters, such as the spin period of the neutron star and the orbital period and eccentricity of the binary. We find that systems with larger rms are those harbouring fast-rotating neutron stars, low eccentric and narrow orbits. These relationships can be explained as the result of the truncation of the circumstellar disc. We also present an updated version of the Hα equivalent width–orbital period diagram, including sources in the Small Magellanic Cloud (SMC). This diagram provides strong observational evidence of the interaction of neutron star with the circumstellar envelope of its massive companion.     

Simultaneous X-ray/optical observations of GX 9+9 (4U 1728−16)

We report on the results of the first simultaneous X-ray ( RXTE ) and optical [South African Astronomical Observatory (SAAO)] observations of the luminous low-mass X-ray binary (LMXB) GX 9+9 in 1999 August. The high-speed optical photometry revealed an orbital period of 4.1958 h and confirmed previous observations, but with greater precision. No X-ray modulation was found at the orbital period. On shorter time-scales, a possible 1.4-h variability was found in the optical light curves which might be related to the MHz quasi-periodic oscillations seen in other LMXBs. We do not find any significant X-ray/optical correlation in the light curves. In X-rays, the colour–colour and hardness-intensity diagrams indicate that the source shows characteristics of an atoll source in the upper banana state, with a correlation between intensity and spectral hardness. Time-resolved X-ray spectroscopy suggests that two-component spectral models give a reasonable fit to the X-ray emission. Such models consist of a blackbody component which can be interpreted as the emission from an optically thick accretion disc or an optically thick boundary layer, and a hard Comptonized component for an extended corona.     

The Orbital Period of the Be/Neutron Star Binary RX J0812.4-3114

We present the results of Rossi X-Ray Timing Explorer (RXTE) observations of the Be star X-ray binary system RX J0812.4-3114. A light curve obtained with the RXTE All-Sky Monitor (ASM) shows that the source is currently in an active state with outbursts occurring at approximately 80 day intervals. The source underwent a transition from an inactive state to this regular outburst state early in 1998. An observation of RX J0812.4-3114 was obtained with the RXTE Proportional Counter Array (PCA) close to the time of a predicted maximum in 1999 March, and strong pulsations were detected at a period of 31.88 s. This confirms the result of an earlier PCA observation by Reig & Roche that was also obtained serendipitously near the predicted maximum flux of the 80 day period and also near the start of the current active state. We interpret the periodicity in the ASM light curve as indicating the orbital period of RX J0812.4-3114 with outbursts occurring around periastron passage.     

Multiwavelength monitoring of GRS 1915+105

Since its discovery in 1992, the superluminal X-ray transient GRS 1915+105 has been extensively observed in an attempt to understand its behaviour. We present here first results from a multiwavelength campaign undertaken from 1996 July to September. This study includes X-ray data from the RXTE All Sky Monitor and BATSE , two-frequency data from the Nançay radio telescope, and infrared photometry from the 1.8-m Perkins telescope at Lowell Observatory. The first long-term, well-sampled infrared light curve of GRS 1915+105 is presented herein; it is consistent with the interpretation of this source as a long-period binary. We compare the various light curves, searching for correlations in the behaviour of the source at differing wavelengths and for possible periodicities.     

Periodicities in the high-mass X-ray binary system RX J0146.9+6121/LS I+61°235

The high-mass X-ray binary RX J0146.9+6121, with optical counterpart LS I+61°235 (V831 Cas), is an intriguing system on the outskirts of the open cluster NGC 663. It contains the slowest Be type X-ray pulsar known with a pulse period of around 1400 s and, primarily from the study of variation in the emission line profile of Hα, it is known to have a Be decretion disc with a one-armed density wave period of approximately 1240 d. Here we present the results of an extensive photometric campaign, supplemented with optical spectroscopy, aimed at measuring short time-scale periodicities. We find three significant periodicities in the photometric data at, in order of statistical significance, 0.34, 0.67 and 0.10 d. We give arguments to support the interpretation that the 0.34 and 0.10 d periods could be due to stellar oscillations of the B-type primary star and that the 0.67 d period is the spin period of the Be star with a spin axis inclination of  23⁺¹⁰₋₈  degrees. We measured a systemic velocity of  −37.0 ± 4.3 km s⁻¹  confirming that LS I+61°235 has a high probability of membership in the young cluster NGC 663 from which the system's age can be estimated as 20–25 Myr. From archival RXTE All Sky Monitor (ASM) data we further find 'super' X-ray outbursts roughly every 450 d. If these super outbursts are caused by the alignment of the compact star with the one-armed decretion disc enhancement, then the orbital period is approximately 330 d.     

Research on Multiband Observations of X-ray Millisecond Pulsar J0437-4715

A study on the two-band observations of the millisecond pulsar J0437- 4715 is carried out based on the radio data of the Parkes Observatory of Australia and the X-ray data obtained with the ASM (All Sky Monitor) in the RXTE (Rossi X-ray Timing Explorer). For the radio observations of J0437-4715 the software TEMPO2 is applied to the calibration of the timing data of different terminal systems, thereby improving the accuracy of the timing model. By taking advantage of the all-weather observations of the ASM in the RXTE, the study of the medium- and long-term light variations of the pulsar J0437-4715 at the X-ray wavelengths is carried out by means of structure function, and it is found that this pulsar has a 620 d light period in the X-ray waveband.     

Keck II spectroscopy of mHz quasi-periodic oscillations in Hercules X-1

We present Keck II spectroscopy of optical mHz quasi-periodic oscillations (QPOs) in the light curve of the X-ray pulsar binary Hercules X-1. In the power spectrum it appears as 'peaked noise', with a coherency ∼2, a central frequency of 35 mHz and a peak-to-peak amplitude of 5 per cent. However, the dynamic power spectrum shows it to be an intermittent QPO, with a lifetime of ∼100 s, as expected if the lifetime of the orbiting material is equal to the thermal time-scale of the inner disc. We have decomposed the spectral time series into constant and variable components and used blackbody fits to the resulting spectra to characterize the spectrum of the QPO variability and constrain possible production sites. We find that the spectrum of the QPO is best fitted by a small hot region, possibly the inner regions of the accretion disc, where the ballistic accretion stream impacts on to the disc. The lack of any excess power around the QPO frequency in the X-ray power spectrum, created using simultaneous light curves from RXTE , implies that the QPO is not simply reprocessed X-ray variability.     

Mapping the shape of the accretion disk of Hercules X‐1

Hercules X‐1 is an x‐ray binary with a 1.7 day orbit and which exhibits a regular 35‐day intensity cycle, which comes in two types: 0.2 orbital phase turn‐on and 0.7 phase turn‐on. The cycle is well measured by the RXTE/ASM and is caused by a sequence of occultations by the inner and outer edges of the accretion disk. In addition to the 35‐day x‐ray cycle, the accretion disk shadows the companion star HZ Her to give the regular and well known optical modulation, and gives a modulation of the EUV emission from the system. The x‐ray modulation is most precisely measured and best for determination of the disk shape. Here disk and emission region models are used to derived the disk shape from the 0.2 turn‐on cycles and compared to a previous derivation based on 0.7 turn‐on cycles. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Is the 35 day cycle of her x-1 caused by the free precession of the neutron star?

It has been suggested that the 35 day clock mechanism of Her X-1 is caused by the precession of the neutron star. We made calculations for this model. Comparing our calculated results with observations we find:

1. (1) The theoretical light curve over the the 35 day cycle does not agree with the observed curve by Jones and Forman (1976).

2. (2) The change of pulse width and intensity with the 35 day phase is large in the model, seemingly disagreeing with the observations (e.g., Gruber et al. 1980).

3. (3) Parmer et al. (1985) reported that they did not see the expected 35 day variation in X-ray intensity expected in the model.

Pravdo et al. (1977) suggested that the observed X-ray pulses were formed by Thomson scattering of symmetric radiating beams generated near the surface of the neutron star. Our analysis based on the theory of Thomson scattering in strong magnetic fields shows that this model agrees well with the observations.     

Fast transition between high-soft and low-soft states in GRS 1915 + 105: Evidence for a critically viscous accretion flow

We present the results of a detailed analysis of RXTE observations of classω (Klein-Woltet al. 2002) which show an unusual state transition between high-soft and low-soft states in the Galactic microquasar GRS 1915 + 105. Out of about 600 pointed RXTE observations, the source was found to exhibit such state transition only on 16 occasions. An examination of the RXTE/ASM data in conjunction with the pointed observations reveals that these events appeared as a series of quasi-regular dips in two stretches of long duration (about 20 days during each occasion) when hard X-ray and radio flux were very low. The X-ray light curve and colour-colour diagram of the source during these observations are found to be different from any reported so far. The duration of these dips is found to be of the order of a few tens of seconds with a repetition time of a few hundred seconds. The transition between these dips and non-dips which differ in intensity by a factor of ∼ 3.5, is observed to be very fast (∼ a few seconds). It is observed that the low-frequency narrow QPOs are absent in the power density spectrum (PDS) of the dip and non-dip regions of classω and the PDS is a power law in the 0.1–10 Hz frequency range. There is a remarkable similarity in the spectral and timing properties of the source during the dip and non-dip regions in this set of observations. These properties of the source are distinctly different from those seen in the observations of other classes. This indicates that the basic accretion disk structure during both dip and non-dip regions of classω is similar, but differ only in intensity. To explain these observations, we invoke a model in which the viscosity is very close to critical viscosity and the shock wave is weak or absent.     

The accretion flow in the discless intermediate polar V2400 Ophiuchi

RXTE observations confirm that the X-ray light curve of V2400 Oph is pulsed at the beat cycle, as expected in a discless intermediate polar. There are no X-ray modulations at the orbital or spin cycles, but optical line profiles vary with all three cycles. We construct a model for line-profile variations in a discless accretor, based on the idea that the accretion stream flips from one magnetic pole to the other, and show that this accounts for the observed behaviour over the spin and beat cycles. The minimal variability over the orbital cycle implies that (1) V2400 Oph is at an inclination of only ≈10°, and (2) much of the accretion flow is not in a coherent stream, but is circling the white dwarf, possibly as a ring of denser, diamagnetic blobs. We discuss the light that this sheds on disc formation in intermediate polars.     

On the correlation between the very-high-energy gamma-ray and X-ray fluxes from the blazar 3C 66A

We consider the very-high-energy (VHE) gamma-ray observations of the blazar 3C 66A with the GT-48 Cherenkov telescope in the period 2002–2004 in comparison with the quasi-simultaneous ASM/RXTE observations in the energy range 2–10 keV. We show that there are positive correlations between the VHE gamma-ray and X-ray fluxes from this object recorded in the observing periods of 2002–2004.     

Differences between the two anomalous X-ray pulsars: variations in the spin-down rate of 1E 1048.1−5937 and an extended interval of quiet spin-down in 1E 2259+586

We analysed the Rossi X-ray Timing Explorer ( RXTE ) archival data of 1E 1048.1−5937 covering a time-span of more than one year. The spin-down rate of this source decreases by ∼30 per cent during the observation. We could not resolve the X-ray flux variations because of contamination by eta Carinae. We find that the level of pulse frequency fluctuations of 1E 1048.1−5937 is consistent with typical noise levels of accretion-powered pulsars . Recent RXTE observations of 1E 2259+586 have shown a constant spin-down with a very low upper limit on timing noise. We used the RXTE archival X-ray observations of 1E 2259+586 to show that the intrinsic X-ray luminosity times-series is also stable, with an rms fractional variation of less than 15 per cent. The source could have been in a quiet phase of accretion with a constant X-ray luminosity and spin-down rate.     

X-ray and UV variability in the core of M15: the nature of X2127+119/AC211

The prototypical post-core-collapse globular cluster M15 provides an ideal environment for the formation of exotic binaries, it being already known to contain a luminous LMXB (X2127+119, optically identified with the 17.1-h eclipsing binary AC211) and 6 millisecond-pulsars in the core. However, the X-ray properties of X2127+119 are strange in that it appears to be a high inclination accretion disc corona source (ADC) from which we only see scattered X-radiation, and yet it has produced extremely luminous type I X-ray bursts, which may or may not have come from AC211. We have therefore examined the ∼5-year RXTE/ASM light curve of this object in order to search for long-term modulations which might shed light on this unusual behaviour, and which led to our discovery of further X-ray bursts. Furthermore, we have used archival HST UV images of the M15 core to search for other variable objects which might indicate the presence of a second LMXB. From these we have found one highly variable (>5 mag) object which we interpret as a dwarf nova.     

XMM–Newton observations of the complex spin pulse of the intermediate polar PQ Geminorum

The intermediate polar PQ Geminorum shows a complex pulsation, caused by a spinning white dwarf, which varies markedly with wavelength. We report XMM–Newton observations, including the soft and hard X-ray bands and the first ultraviolet light curves of this star. We update the ephemeris for PQ Gem allowing us to align these data with a compilation of light curves from the optical to the X-ray. Building on work by previous authors, we show how a model in which accretion flows along skewed field lines, viewed at the correct inclination, can explain the major features of the light curves in all bands. We discuss how the skew of the field lines relates to the spinning down of the white dwarf rotation.