Accretion column eclipses in the X-ray pulsars GX 1+4 and RX J0812.4−3114

Sharp dips observed in the pulse profiles of three X-ray pulsars (GX 1+4, RX J0812.4−3114 and A 0535+26) have previously been suggested to arise from partial eclipses of the emission region by the accretion column occurring once each rotation period. We present pulse-phase spectroscopy from Rossi X-ray Timing Explorer satellite observations of GX 1+4 and RX J0812.4−3114, which for the first time confirms this interpretation. The dip phase corresponds to the closest approach of the column axis to the line of sight, and the additional optical depth of photons escaping from the column in this direction gives rise to both the decrease in flux and increase in the fitted optical depth measured at this phase. Analysis of the arrival time of individual dips in GX 1+4 provides the first measurement of azimuthal wandering of a neutron star accretion column. The column longitude varies stochastically with a standard deviation ranging between 2° and 6° depending on the source luminosity. Measurements of the phase width of the dip both from mean pulse profiles and from the individual eclipses demonstrate that the dip width is proportional to the flux. The variation is consistent with that expected if the azimuthal extent of the accretion column depends only upon the Keplerian velocity at the inner disc radius, which varies as a consequence of the accretion rate M˙ .     

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.     

A ten-year-long peak of the X-ray flux from the burster 4U 1724-307 in the globular cluster Terzan 2: Evolution of the donor star or the influence of a third star?

We analyze observations of the burster 4U 1724-307 (1E 1724-3045) in the globular cluster Terzan 2 from 1971 until 2001. Uhuru, OSO-8, Einstein, and EXOSAT observations are used. In addition, we analyze data from the TTM telescope onboard the Mir/Kvant observatory and from the ASM and PCA instruments onboard the RXTE observatory. Based on PCA/RXTE scanning observations, we have mapped the sky in the Galactic-center region. The derived light curve of the burster 4U 1724-307 shows the source to be variable on a time scale of 30 years: over the first 20 years of observations (1970–1990), the source flux was approximately constant, while in the 1990s, it rose over ～5 years and declined over approximately the same period, approaching its original value. We discuss several scenarios to explain the behavior of the light curve, including the evolution of the mass outflow rate from the donor-star surface, the episodic appearance of a second source in the globular cluster, and gravitational microlensing. As one of the scenarios, we also consider the possibility that the passage of a third star affects the accretion rate in the binary 4U 1724-307. We show that if this event is responsible for the 10-year-long rise in the luminosity of the burster 4U 1724-307, then the size and period of this binary can be estimated.     

Photometric observations of three high mass X-ray binaries and a search for variations induced by orbital motion

We searched for long period variation in V-band,IC-band and RXTE X-ray light curves of the High Mass X-ray Binaries(HMXBs) LS 1698 /RX J1037.5-5647,HD 110432 /1H 1249-637 and HD 161103 /RX J1744.7-2713 in an attempt to discover orbitally induced variation. Data were obtained primarily from the ASAS database and were supplemented by shorter term observations made with the 24-and 40-inch ANU telescopes and one of the robotic PROMPT telescopes. Fourier periodograms suggested the existence of long period variat...     

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.     

Observational manifestations of the change in the tilt of the accretion disk to the orbital plane in her X-1/HZ her with phase of its 35-day period

We analyze and interpret the RXTE/ASM X-ray light curves for the close binary system Her X-1/HZ Her obtained from February 1996 to September 2004. Some of the features found previously in the averaged X-ray light curves are confirmed by the new RXTE/ASM data. In particular, the anomalous dips and post-eclipse recoveries in two successive orbits in the short-on state are clearly distinguishable and are stable features of the X-ray light curves. We argue that to account for these features, the tilt of the accretion disk to the orbital plane must be assumed to change with phase of the 35-day period. We present a numerical model that can reproduce the observed features of the light curves.     

Orbital evolution measurement of the accreting millisecond X-ray pulsar SAX J1808.4-3658

We present results from a pulse timing analysis of the accretion-powered millisecond X-ray pulsar SAX J1808.4-3658 using X-ray data obtained during four outbursts of this source. Extensive observations were made with the proportional counter array of the Rossi X-ray Timing Explorer (RXTE) during the four outbursts that occurred in 1998, 2000, 2002 and 2005. Instead of measuring the arrival times of individual pulses or the pulse arrival time delay measurement that is commonly used to determine the orbital parameters of binary pulsars, we have determined the orbital ephemeris during each observation by optimizing the pulse detection against a range of trial ephemeris values. The source exhibits a significant pulse shape variability during the outbursts. The technique used by us does not depend on the pulse profile evolution, and is therefore, different from the standard pulse timing analysis. Using 27 measurements of orbital ephemerides during the four outbursts spread over more than 7 years and more than 31,000 binary orbits, we have derived an accurate value of the orbital period of 7249.156862(5) s (MJD = 50915) and detected an orbital period derivative of (3.14 ± 0.21) × 10⁻¹² s s⁻¹. We have included a table of the 27 mid-eclipse time measurements of this source that will be valuable for further studies of the orbital evolution of the source, especially with ASTROSAT. We point out that the measured rate of orbital period evolution is considerably faster than the most commonly discussed mechanisms of orbital period evolution like mass transfer, mass loss from the companion star and gravitational wave radiation. The present time scale of orbital period change, 73 Myr is therefore likely to be a transient high value of period evolution and similar measurements during subsequent outbursts of SAX J1808.4-3658 will help us to resolve this.     

Variability of the extreme z = 4.72 blazar, GB 1428+4217

On 1998 May 9–11, the highly variable, low-luminosity Seyfert 1 galaxy NGC 4051 was observed in an unusual low-flux state by BeppoSAX , RXTE and EUVE . We present fits of the 4–15 keV RXTE spectrum and BeppoSAX MECS spectrum obtained during this observation, which are consistent with the interpretation that the source had switched off, leaving only the spectrum of pure reflection from distant cold matter. We place this result in context by showing the X-ray light curve of NGC 4051 obtained by our RXTE monitoring campaign over the past two and a half years, which shows that the low state lasted for ∼150 d before the May observations (implying that the reflecting material is >10¹⁷ cm from the continuum source) and forms part of a light curve showing distinct variations in long-term average flux over time-scales > months. We show that the long-time-scale component to X-ray variability is intrinsic to the primary continuum and is probably distinct from the variability at shorter time-scales. The long-time-scale component to variability maybe associated with variations in the accretion flow of matter on to the central black hole . As the source approaches the low state, the variability process becomes non-linear. NGC 4051 may represent a microcosm of all X-ray variability in radio-quiet active galactic nuclei (AGNs), displaying in a few years a variety of flux states and variability properties which more luminous AGNs may pass through on time-scales of decades to thousands of years.     

Study of the fast X-ray transient XTE J1901+014 based on INTEGRAL,RXTE, and ROSAT data

The source XTE J1901+014 discovered by the RXTE observatory during an intense outburst of hard radiation and classified as a fast X-ray transient is studied. The source’s spectral characteristics in the quiescent state have been investigated for the first time both in the soft X-ray energy range (3–20 keV) based on ROSAT and RXTE data and in the hard energy range (>20 keV) based on INTEGRAL data. A timing analysis of the source’s properties has revealed weak nonperiodic bursts of activity on time scales of several tens of seconds and two intense (～0.5–1 Crab) outbursts more than several hundred seconds in duration. Certain assumptions about the nature of the object under study are made.     

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...     

Optical studies of two Large Magellanic Cloud X-ray transients: RX J0544.1–7100 and RX J0520.5–6932

We report observations that confirm the identities of the optical counterparts to the transient sources RX J0544.1–7100 and RX J0520.5–6932 . The counterparts are suggested to be B-type stars. Optical data from the observations carried out at ESO and SAAO, together with results from the OGLE data base, are presented. In addition, X-ray data from the RXTE All-Sky Monitor are investigated for long-term periodicities. A strong suggestion for a binary period of 24.4 d is seen in RX J0520.5–6932 from the OGLE data.     

RXTE observations of 4U 1630−47 during the peak of its 1998 outburst

We present an analysis of the RXTE observations of 4U 1630−47 during its 1998 outburst. The light curve and the spectral evolution of the outburst were significantly different from the outbursts of the same source in 1996 and 1999. Special emphasis was placed on observations taken during the initial rise and during the maximum of the outburst. The maximum of the outburst was divided into three plateaux, with almost constant flux within each plateau, and fast jumps between them. The spectral and timing parameters are stable for each individual plateau, but significantly different between the plateaux. The variability detected on the first plateau is of special interest. During these observations the source exhibits quasi-regular oscillations with a period of ∼ 10–20 s. Our analysis revealed a difference in temporal behaviour of the source at high and low fluxes during this period of time. The source behaviour can be generally explained in the framework of the two-phase model of accretion flow, involving a hot inner Comptonization region and surrounding optically thick disc.
The variability and spectral evolution of the source were similar to what was observed earlier for other X-ray novae. We show that 4U 1630−47 resembles, in several aspects, other transient and persistent black hole binaries.     

Detection of the optical counterpart of the proposed double degenerate polar RX J1914+24

We have detected the optical counterpart of the proposed double degenerate polar RX J1914+24. The I -band light curve is modulated on the 9.5-min period seen in X-rays. There is no evidence for any other periods. No significant modulation is seen in J . The infrared colours of RX J1914+24 are not consistent with a main-sequence dwarf secondary star. Our ASCA spectrum of RX J1914+24 is typical of a heavily absorbed polar and our ASCA light curve also shows only the 9.5-min period. We find that the folded I band and X-ray light curves are out of phase. We attribute the I -band flux to the irradiated face of the donor star. The long-term X-ray light curve shows a variation in the observed flux of up to an order of magnitude. These observations strengthen the view that RX J1914+24 is indeed the first double degenerate polar to be detected. In this light, we discuss the synchronizing mechanisms in such a close binary and other system parameters.     

RXTE observations of the strongly absorbed sources IGR J16318-4848 and IGR J16358-4726

We analyzed the RXTE observations of two strongly absorbed sources, IGR J16318-4848 and IGR J16358-4726. We were able to obtain the 3–25 keV spectra of the sources by taking into account the contribution of the Galactic diffuse background to the X-ray flux recorded with the RXTE/PCA spectrometer. The spectra of the sources are well described by a power-law decrease of the photon flux with energy with a photon index of ～1 and strong photoabsorption. The photoabsorption column density n_HL for IGR J16318-4848 derived from the RXTE observation on March 14.1, 2003, is shown to be much higher than its value obtained by the XMM observatory on February 10.7, 2003. This result may suggest that the source has variable absorption that may depend on the orbital phase of the system. We point out that all of the three X-ray sources discovered by the INTEGRAL observatory in the region (l,b)～(336,0) (IGR J16318-4848, IGR J16320-4751, and IGR J16358-4726) have strong intrinsic photoabsorption and may be high-mass binaries. Their proximity to the region where the tangent to the Galactic spiral arm passes, i.e., to the region of enhanced concentration of young high-mass stars, can serve as an indirect confirmation of this assumption. If our assumption about the positions of the sources in the Norma spiral arm is valid, then we can roughly estimate their heliocentric distances: ～6–8 kpc.     

The 150d Modulation of the X-ray Emission of CYG X-1

The X-ray observations of Cyg X-1, covering a period of 3.8 years, by the All Sky Monitor (ASM) on board the Rossi X-ray Timing Explorer (RXTE) were analysed to search for long periodicities. The periodicities in the original data were seen to be distorted by the high/soft state and the failed state counts of the system, and thus the power spectra showed two unrealistic peaks at 1608and 406 days. The ∼ 300 day modulation reported previously(e.g. Priedhorsky et al. 1983) is not confirmed by the present ASM data. Truncation of the high counts above 30 cnts/s partly eliminates the distortion and reveals the true period of 150^d in the power spectra. It is interesting that the high/soft state (HS) and the failed state transitions (FST) always occur on the maximum phase of the150^d cycle. This revised version was published online in July 2006 with corrections to the Cover Date.     

The 0.1–100 keV spectrum and variability of Mrk 421 in a high state

The results of a BeppoSAX target of opportunity (TOO) observation of the BL Lac object Mrk 421 during a high-intensity state are reported and compared with monitoring X-ray data collected with the BeppoSAX Wide Field Cameras (WFC) and the RXTE All Sky Monitor (ASM). The 0.1–100 keV spectrum of Mrk 421 shows continuous convex curvature that can be interpreted as the high-energy end of the synchrotron emission. The source shows significant short-term temporal and spectral variability, which can be interpreted in terms of synchrotron cooling. The comparison of our results with those of previous observations when the source was a factor 3–5 fainter shows evidence for strong spectral variability, with the maximum of the synchrotron power shifting to higher energy during high states. This behaviour suggests an increase in the number of energetic electrons during high states.     

Observations of the X-ray binaries 4U 1700-37 and GRO J1655-40 during the Grif experiment onboard the mir orbiting station

Various areas of the sky, including the Galactic-center region and the region with the X-ray binaries 4U 1700-37 and GRO J1655-40, were observed in the hard (10–300 keV) energy range during the Grif experiment onboard the Mir orbiting station. An epoch-folding analysis of the data has revealed periodicities with periods of 82 and 62 h, which are equal to the orbital periods of 4U 1700-37 and GRO J1655-40. Previously, these periodicities were observed during the Prognoz-9 X-ray experiment. Periodicities with periods in the range of days, 98 and 152 h, which were also observed during the Prognoz-9 experiment, were not revealed by the Grif data. We obtained upper limits on the intensities of these periodicities in various energy ranges. For the 62-h periodicity, we constructed an average 25–50-keV light curve and estimated the spectral flux density, which characterizes the intensity of the periodic component at different energies in different observing intervals during 1995–1997. The Prognoz-9 and Grif observations of GRO J1655-40 are compared with its CGRO, RXTE, and BeppoSax observations. The orbital periodicity is shown to manifest itself in the hard emission from the extremely bright X-ray transient GRO J1655-40, a likely black-hole candidate, even at the epochs between its X-ray outbursts.     

On timing and spectral characteristics of the X-ray pulsar 4U 0115+63: Evolution of the pulsation period and the cyclotron line energy

An overview of the results of observations for the transient X-ray pulsar 4U 0115+63, amember of a binary system with a Be star, since its discovery to the present day (～40 years) based on data from more than dozen observatories and instruments is presented. An overall light curve and the history of change in the spin frequency of the neutron star over the entire history of its observations, which also includes the results of recent measurements made by the INTEGRAL observatory during the 2004, 2008, and 2011 outbursts, are provided. The source’s energy spectra have also been constructed from the INTEGRAL data obtained during the 2011 outburst for a dynamic range of its luminosities 10³⁷?7 × 10³⁷ erg s^?1. We show that apart from the fundamental harmonic of the cyclotron absorption line at energy～11 keV, its four higher harmonics at energies ?24, 35.6, 48.8, and 60.7 keV are detected in the spectrum. We have performed a detailed analysis of the source’s spectra in the 4–28 keV energy band based on all of the available RXTE archival data obtained during bright outbursts in 1995–2011. We have confirmed that modifying the source’s continuum model can lead to the disappearance of the observed anticorrelation between the energy of the fundamental harmonic of the cyclotron absorption line and the source’s luminosity. Thus, the question about the evolution of the cyclotron absorption line energy with the luminosity of the X-ray pulsar 4U 0115+63 remains open and a physically justified radiation model for X-ray pulsars is needed to answer it.     

A search for periodicities in the radio emission from XTE J1118+480

A search for modulation of the 15-GHz radio flux density at the orbital period from the X-ray nova XTE J1118+480 during its outburst in 2000 April–June found no signal ≳ a few per cent of the mean. No orbital modulation was found in the RXTE ASM data over the same interval.     

A search for electron cyclotron maser emission from compact binaries

Unipolar induction (UI) is a fundamental physical process, which occurs when a conducting body transverses a magnetic field. It has been suggested that UI is operating in RX J0806+15 and RX J1914+24, which are believed to be ultracompact binaries with orbital periods of 5.4 and 9.6 min, respectively. The UI model predicts that those two sources may be electron cyclotron maser sources at radio wavelengths. Other systems in which UI has been predicted to occur are short period extrasolar terrestrial planets with conducting cores. If UI is present, circularly polarized radio emission is predicted to be emitted. We have searched for this predicted radio emission from short period binaries using the Very Large Array (VLA) and Australian Telescope Compact Array (ATCA). In one epoch, we find evidence for a radio source, coincident in position with the optical position of RX J0806+15. Although we cannot completely exclude that this is a chance alignment between the position of RX J0806+15 and an artefact in the data reduction process, the fact that it was detected at a significance level of 5.8σ and found to be transient suggests that it is more likely that RX J0806+15 is a transient radio source. We find an upper limit on the degree of circular polarization to be ∼50 per cent. The inferred brightness temperature exceeds 10¹⁸ K, which is too high for any known incoherent process, but is consistent with maser emission and UI being the driving mechanism. We did not detect radio emission from ES Cet, RX J1914+24 or Gliese 876.