Archival Rossi X-ray Timing Explorer Research Articles

Long-term Coherent Timing of the Accreting Millisecond Pulsar IGR J17062–6143

Abstract We report on a coherent timing analysis of the 163 Hz accreting millisecond X-ray pulsar IGR J17062–6143. Using data collected with the Neutron Star Interior Composition Explorer and XMM-Newton, we investigated the pulsar evolution over a time span of four years. We obtained a unique phase-coherent timing solution for the stellar spin, finding the source to be spinning up at a rate of (3.77 ± 0.09) × 10−15 Hz s−1. We further find that the 0.4–6 keV pulse fraction varies gradually between 0.5% and 2.5% following a sinusoidal oscillation with a 1210 ± 40 day period. Finally, we supplemented this analysis with an archival Rossi X-ray Timing Explorer observation and obtained a phase-coherent model for the binary orbit spanning 12 yr, yielding an orbital period-derivative measurement of (8.4 ± 2.0) × 10−12 s s−1. This large orbital period derivative is inconsistent with a binary evolution that is dominated by gravitational wave emission and is suggestive of highly nonconservative mass transfer in the binary system.

A minimal timescale for the continuum in 4U 1608-52 and Aql X-1

ABSTRACT Similar to black hole X-ray binary transients, hysteresis-like state transitions are also seen in some neutron-star X-ray binaries. Using a method based on wavelets and light curves constructed from archival Rossi X-ray Timing Explorer observations, we extract a minimal timescale over the complete range of transitions for 4U 1608-52 during the 2002 and 2007 outbursts and the 1999 and 2000 outbursts for Aql X-1. We present evidence for a strong positive correlation between this minimal timescale and a similar timescale extracted from the corresponding power spectra of these sources.

Search for intermittent X-ray pulsations from neutron stars in low-mass X-ray binaries

Abstract We present the results of our extensive binary orbital motion corrected pulsation search for 13 low-mass X-ray binaries. These selected sources exhibit burst oscillations in X-rays with frequencies ranging from 45 to 1 122 Hz and have a binary orbital period varying from 2.1 to 18.9 h. We first determined episodes that contain weak pulsations around the burst oscillation frequency by searching all archival Rossi X-ray Timing Explorer data of these sources. Then, we applied Doppler corrections to these pulsation episodes to discard the smearing effect of the binary orbital motion and searched for recovered pulsations at the second stage. Here we report 75 pulsation episodes that contain weak but coherent pulsations around the burst oscillation frequency. Furthermore, we report eight new episodes that show relatively strong pulsations in the binary orbital motion corrected data.

A Machine Learning Approach For Classifying Low-mass X-ray Binaries Based On Their Compact Object Nature

Abstract Low Mass X-ray binaries (LMXBs) are binary systems where one of the components is either a black hole or a neutron star and the other is a less massive star. It is challenging to unambiguously determine whether a LMXB hosts a black hole or a neutron star. In the last few decades, multiple observational works have tried, with different levels of success, to address this problem. In this paper, we explore the use of machine learning to tackle this observational challenge. We train a random forest classifier to identify the type of compact object using the energy spectrum in the energy range 5-25 keV obtained from the Rossi X-ray Timing Explorer archive. We report an average accuracy of 87±13% in classifying the spectra of LMXB sources. We further use the trained model for predicting the classes for LMXB systems with unknown or ambiguous classification. With the ever-increasing volume of astronomical data in the X-ray domain from present and upcoming missions (e.g., SWIFT, XMM-Newton, XARM, ATHENA, NICER), such methods can be extremely useful for faster and robust classification of X-ray sources and can also be deployed as part of the data reduction pipeline.

A QPO in NGC 4945 from Archival RXTE Data

Abstract We report the discovery of an ∼6 week quasi-periodic oscillation (QPO) in archival NGC 4945 data observed by the Rossi X-ray Timing Explorer (RXTE) satellite. QPOs are important observables in accretion disks and have been studied extensively in both neutron star (NS) and black hole (BH) X-ray binaries (XRB). QPOs should be present in active galactic nuclei (AGN) if galactic BHs and supermassive black holes are governed by a common set of physical processes. The search for QPOs in AGN has proven difficult because the time scales would be much longer, due to their higher mass. RXTE AGN light curves spanning 1996 to 2011 provide an excellent and perhaps unique opportunity to search for low-frequency QPOs. We investigated the 533 RXTE observations made of the Seyfert-2 AGN, NGC 4945. During a large cluster of observations in 2006–2007 (194 observations, spanning 396 days), the Lomb–Scargle periodogram shows a candidate QPO at 0.274 μHz (period ≈42.2 days). We estimate the uncertainties using the false alarm probability. We discuss the possible identification of this feature with the Lense–Thirring precession period.

A minimal time-scale for the spectral states of GX 339−4

ABSTRACT Black hole transients are known to undergo spectral transitions that form q-shaped tracks on a hardness intensity diagram. In this work, we use the archival Rossi X-ray Timing Explorer data to extract a characteristic minimal time-scale for the spectral states in GX 339−4 for the 2002–2003 and 2010 outbursts. We use the extracted time-scale to construct an intensity variability diagram for each outburst. This new diagram is comparable to the traditional hardness intensity diagram and offers the potential for probing the underlying dynamics associated with the evolution of the relevant emission regions in black hole transients. We confirm this possibility by connecting the minimal time-scale with the inner disc radius, Rin (estimated from spectral fits), and demonstrate a positive correlation between these variables as the system evolves through its spectral transitions. Furthermore, we probe the relation between the minimal time-scale and the break frequencies extracted from the power spectral densities. Lastly, we examine a possible link between the extracted time-scale and a traditional measure of variability, i.e. the root mean square, determined directly from the power spectra.

Modeling the Upper kHz QPOs of 4U 1728-34 with X-Ray Reverberation

Abstract While kilohertz quasi-periodic oscillations (kHz QPOs) have been well studied for decades since their initial discovery, the cause of these signals remains unknown, as no model has been able to accurately predict all of their spectral and timing properties. Separately, X-ray reverberation lags have been detected in active galactic nuclei and stellar-mass black hole binaries, and reverberation may be expected to occur in neutron star systems as well, producing lags of the same amplitude as the lags measured of the kHz QPOs. Furthermore, the detection of a relativistically reflected Fe K line in the time-averaged spectra of many neutron star systems provides an additional motivation for testing reverberation. While it has been shown that the lag-energy properties of the lower kHz QPOs are unlikely to be produced by X-ray reverberation, the upper kHz QPOs have not yet been explored. We therefore model the upper kHz QPO lag-energy spectra using relativistic ray-tracing functions and apply them to archival Rossi X-Ray Timing Explorer data on 4U 1728–34 where upper kHz QPOs have been detected. By modeling the time-averaged spectra in which upper kHz QPOs had been significantly detected, we determine the reflected flux fraction across all energies and produce a model for the lag-energy spectra from X-ray reverberation. We explore the dependence of the modeled lag properties on several different types of reflection models but are unable to successfully reproduce the measured lags of 4U 1728–34. We conclude that reverberation alone does not explain the measured time lags detected in upper kHz QPOs.

A systematic study of the phase difference between QPO harmonics in black hole X-ray binaries

We perform a systematic study of the evolution of the waveform of black hole X-ray binary low-frequency QPOs, by measuring the phase difference between their fundamental and harmonic features. This phase difference has been studied previously for small number of QPO frequencies in individual sources. Here, we present a sample study spanning fourteen sources and a wide range of QPO frequencies. With an automated pipeline, we systematically fit power spectra and calculate phase differences from archival Rossi X-ray Timing Explorer (RXTE) observations. We measure well-defined phase differences over a large range of QPO frequencies for most sources, demonstrating that a QPO for a given source and frequency has a persistent underlying waveform. This confirms the validity of recently developed spectral-timing methods performing phase resolved spectroscopy of the QPO. Furthermore, we evaluate the phase difference as a function of QPO frequency. For Type-B QPOs, we find that the phase difference stays constant with frequency for most sources. We propose a simple jet precession model to explain these constant Type-B QPO phase differences. The phase difference of the Type-C QPO is not constant but systematically evolves with QPO frequency, with the resulting relation being similar for a number of high inclination sources, but more variable for low-inclination sources. We discuss how the evolving phase difference can naturally arise in the framework of precession models for the Type-C QPO, by considering the contributions of a direct and reflected component to the QPO waveform.

LAXPC/AstroSat Study of ∼1 and ∼2 mHz Quasi-periodic Oscillations in the Be/X-Ray Binary 4U 0115+63 during Its 2015 Outburst

The Be X-ray Binary 4U 0115+63 was observed by Large Area X-ray Proportional Counter (LAXPC) instrument on AstroSat on 2015 October 24 during the peak of a giant Type II outburst. Prominent intensity oscillations at ~ 1 and ~ 2 mHz frequency were detected during the outburst. Nuclear Spectroscopic Telescope Array (NuSTAR) observations made during the same outburst also show mHz quasi periodic oscillations (QPOs). Details of the oscillations and their characteristics deduced from LAXPC/AstroSat and NuSTAR observations are reported in this paper. Analysis of the archival Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array (PCA) data during 2001-11 also show presence of mHz QPOs during some of the outbursts and details of these QPOs are also reported. Possible models to explain the origin of the mHz oscillations are examined. Similar QPOs, albeit at higher frequencies, have been reported from other neutron star and black hole sources and both may have a common origin. Current models to explain the instability in the inner accretion disk causing the intense oscillations are discussed.

Low Frequency Quasi-periodic Oscillations in the High-eccentric LMXB Cir X-1: Extending the WK Correlation for Z Sources

Abstract Using archival Rossi X-ray Timing Explorer (RXTE) data, we studied the low-frequency quasi-periodic oscillations (LFQPOs) in the neutron star low-mass X-ray binary (LMXB) Cir X-1 and examined their contribution to frequency–frequency correlations for Z sources. We also studied the orbital phase effects on the LFQPO properties and found them to be phase independent. Comparing LFQPO frequencies in different classes of LMXBs, we found that systems that show both Z and atoll states form a common track with atoll/BH sources in the so-called WK correlation, while persistent Z systems are offset by a factor of about two. We found that neither source luminosity nor mass accretion rate is related to the shift of persistent Z systems. We discuss the possibility of a misidentification of fundamental frequency for horizontal branch oscillations from persistent Z systems and interpreted the oscillations in terms of models based on relativistic precession.

EVIDENCE FOR SIMULTANEOUS JETS AND DISK WINDS IN LUMINOUS LOW-MASS X-RAY BINARIES

ABSTRACT Recent work on jets and disk winds in low-mass X-ray binaries (LMXBs) suggests that they are to a large extent mutually exclusive, with jets observed in spectrally hard states and disk winds observed in spectrally soft states. In this paper we use existing literature on jets and disk winds in the luminous neutron star (NS) LMXB GX 13+1, in combination with archival Rossi X-ray Timing Explorer data, to show that this source is likely able to produce jets and disk winds simultaneously. We find that jets and disk winds occur in the same location on the source’s track in its X-ray color–color diagram. A further study of literature on other luminous LMXBs reveals that this behavior is more common, with indications for simultaneous jets and disk winds in the black hole LMXBs V404 Cyg and GRS 1915+105 and the NS LMXBs Sco X-1 and Cir X-1. For the three sources for which we have the necessary spectral information, we find that simultaneous jets/winds all occur in their spectrally hardest states. Our findings indicate that in LMXBs with luminosities above a few tens of percent of the Eddington luminosity, jets and disk winds are not mutually exclusive, and the presence of disk winds does not necessarily result in jet suppression.

A possible 55-d X-ray period of the ultraluminous accreting pulsar M82 X–2

We report a possible detection of a 55-day X-ray modulation for the ultraluminous accreting pulsar M82 X-2 from archival Chandra observations. Because M82 X-2 is known to have a 2.5-day orbital period, if the 55-day period is real, it will be the superorbital period of the system. We also investigated variabilities of other three nearby ultraluminous X-ray sources in the central region of M82 with the Chandra data and did not find any evidence of periodicities. Furthermore, we re-examined the previously reported 62-day periodicity near the central region of M82 by performing a systematic timing study with all the archival Rossi X-Ray Timing Explorer and Swift data. Using various dynamic timing analysis methods, we confirmed that the 62-day period is not stable, suggesting that it is not the orbital period of M82 X-1 in agreement with previous work.

THE SPECTRAL-TIMING PROPERTIES OF UPPER AND LOWER kHz QPOs

Soft lags from the emission of the lower kilohertz quasi-periodic oscillations (kHz QPOs) of neutron star low mass X-ray binaries have been reported from 4U1608-522 and 4U1636-536. Those lags hold prospects for constraining the origin of the QPO emission. In this paper, we investigate the spectral-timing properties of both the lower and upper kHz QPOs from the neutron star binary 4U1728-34, using the entire Rossi X-ray Timing Explorer archive on this source. We show that the lag-energy spectra of the two QPOs are systematically different: while the lower kHz QPO shows soft lags, the upper kHz QPO shows either a flat lag-energy spectrum or hard variations lagging softer variations. This suggests two different QPO-generation mechanisms. We also performed the first spectral deconvolution of the covariance spectra of both kHz QPOs. The QPO spectra are consistent with Comptonized blackbody emission, similar to the one found in the time-averaged spectrum, but with a higher seed-photon temperature, suggesting that a more compact inner region of the Comptonization layer (boundary/spreading layer, corona) is responsible for the QPO emission. Considering our results together with other recent findings, this leads us to the hypothesis that the lower kHz QPO signal is generated by coherent oscillations of the compact boundary layer region itself. The upper kHz QPO signal may then be linked to less-coherent accretion-rate variations produced in the inner accretion disk, being detected when they reach the boundary layer.

The magnetic-field strengths of accreting millisecond pulsars

In this work we have estimated upper and lower limits to the strength of the magnetic dipole moment of all 14 accreting millisecond X-ray pulsars observed with the Rossi X-ray Timing Explorer (RXTE). For each source we searched the archival RXTE data for the highest and lowest flux levels with a significant detection of pulsations. We assume these flux levels to correspond to the closest and farthest location of the inner edge of the accretion disc at which channelled accretion takes place. By estimating the accretion rate from the observed luminosity at these two flux levels, we place upper and lower limits on the magnetic dipole moment of the neutron star, using assumptions from standard magnetospheric accretion theory. Finally, we discuss how our field strength estimates can be further improved as more information on these pulsars is obtained.

High-frequency quasi-periodic oscillations from GRS 1915+105

We report the results of a systematic timing analysis of all archival Rossi X-Ray Timing Explorer (RXTE) observations of the bright black-hole binary GRS 1915+105 in order to detect high-frequency quasi-periodic oscillations (HFQPO). We produced power-density spectra in two energy bands and limited the analysis to the frequency range 30-1000 Hz. We found 51 peaks with a single trial significance larger than 3 sigma. As all but three have centroid frequencies that are distributed between 63 and 71 Hz, we consider most of them significant regardless of the number of trials involved. The average centroid frequency and FWHM are 67.3 +/- 2.0 Hz and 4.4 +/- 2.4 Hz respectively. Their fractional rms varies between 0.4% and 2% (total band detections) and between 0.5% and 3% (hard ban detections). As GRS 1915+105 shows large variability on time scales longer than 1s, we analysed the data in 16s intervals and found that the detections are limited to a specific region in the colour-colour diagram, corresponding to state B of the source, when the energy spectrum is dominated by a bright accretion disk component. However, the rms spectrum of the HFQPO is very hard and does not show a flattening up to 40 keV, where the fractional rms reaches 11%. We discuss our findings in terms of current proposed models and compare them with the results on other black-hole binaries and neutron-star binaries.

MAXIMUM LIKELIHOOD FITTING OF X-RAY POWER DENSITY SPECTRA: APPLICATION TO HIGH-FREQUENCY QUASI-PERIODIC OSCILLATIONS FROM THE NEUTRON STAR X-RAY BINARY 4U1608-522

High frequency quasi-periodic oscillations (QPOs) from weakly magnetized neutron stars display rapid frequency variability and high coherence with quality factors up to at least 200 at frequencies around 850 Hz. Their parameters have been estimated so far from standard min(chi2) fitting techniques, after combining a large number of Power Density Spectra (PDS), as to have the powers normally distributed. Accounting for the statistical properties of PDS, we apply a maximum likelihood method to derive the QPO parameters in the non Gaussian regime. The method presented is general, easy to implement and can be applied to fitting individual PDS, several PDS simultaneously or their average, and is obviously not specific to the analysis of kHz QPO data. It applies to the analysis of any PDS optimized in frequency resolution and for low frequency variability or PDS containing features whose parameters vary on short timescales, as is the case for kHz QPOs. It is equivalent to the standard chi^2 minimization fitting when the number of PDS fitted is large. The accuracy, reliability and superiority of the method is demonstrated with simulations of synthetic PDS. We show that the maximum likelihood estimates of the QPO parameters are asymptotically unbiased, and have negligible bias when the QPO is reasonably well detected. By contrast, we show that the standard min(chi2) fitting method gives biased parameters with larger uncertainties. The maximum likelihood fitting method is applied to a subset of archival Rossi X-ray Timing Explorer (RXTE) data of the neutron star X-ray binary 4U1608-522. We show that the kHz QPO parameters can be measured on 8 second timescales and that the time evolution of the frequency is consistent with a random walk. This enables us to estimate the intrinsic quality factor of the QPO to be around 260, whereas previous analysis indicated a maximum value around 200 (abridged).

THE NATURE AND CAUSE OF SPECTRAL VARIABILITY IN LMC X-1

We present the results of a long-term observation campaign of the extragalactic wind-accreting black-hole X-ray binary LMC X-1, using the Proportional Counter Array on the Rossi X-Ray Timing Explorer (RXTE). The observations show that LMC X-1's accretion disk exhibits an anomalous temperature-luminosity relation. We use deep archival RXTE observations to show that large movements across the temperature-luminosity space occupied by the system can take place on time scales as short as half an hour. These changes cannot be adequately explained by perturbations that propagate from the outer disk on a viscous timescale. We propose instead that the apparent disk variations reflect rapid fluctuations within the Compton up-scattering coronal material, which occults the inner parts of the disk. The expected relationship between the observed disk luminosity and apparent disk temperature derived from the variable occultation model is quantitatively shown to be in good agreement with the observations. Two other observations support this picture: an inverse correlation between the flux in the power-law spectral component and the fitted inner disk temperature, and a near-constant total photon flux, suggesting that the inner disk is not ejected when a lower temperature is observed.

SPITZEROBSERVATIONS OF GX17+2: CONFIRMATION OF A PERIODIC SYNCHROTRON SOURCE

GX17+2 is a low-mass X-ray binary (LMXB) that is also a member of a small family of LMXBs known as Z-sources that are believed to have persistent X-ray luminosities that are very close to the Eddington limit. GX17+2 is highly variable at both radio and X-ray frequencies, a feature common to Z-sources. What sets GX17+2 apart is its dramatic variability in the near-infrared, where it changes by ΔK ~ 3 mag. Previous investigations have shown that these brightenings are periodic, recurring every 3.01 days. Given its high extinction (A_V≥9 mag), it has not been possible to ascertain the nature of these events with ground-based observations. We report mid-infrared Spitzer observations of GX17+2 which indicate a synchrotron spectrum for the infrared brightenings. In addition, GX17+2 is highly variable in the mid-infrared during these events. The combination of the large-scale outbursts, the presence of a synchrotron spectrum, and the dramatic variability in the mid-infrared suggest that the infrared brightening events are due to the periodic transit of a synchrotron jet across our line of sight. An analysis of both new, and archival, infrared observations has led us to revise the period for these events to 3.0367 days. We also present new Rossi X-Ray Timing Explorer (RXTE) data for GX17+2 obtained during two predicted infrared brightening events. Analysis of these new data, and data from the RXTE archive, indicates that there is no correlation between the X-ray behavior of this source and the observed infrared brightenings. We examine various scenarios that might produce periodic jet emission.

The hardness-intensity diagram of Cygnus X-3: revisiting the radio/X-ray states

Cygnus X-3 is one of the brightest X-ray and radio sources in the Galaxy, and is well known for its erratic behaviour in X-rays as well as in the radio, occasionally producing major radio flares associated with relativistic ejections. However, even after many years of observations in various wavelength bands Cyg X-3 still eludes clear physical understanding. Studying different emission bands simultaneously in microquasars has proved to be a fruitful approach towards understanding these systems, especially by shedding light on the accretion disc/jet connection. We continue this legacy by constructing a hardness-intensity diagram (HID) from archival Rossi X-ray Timing Explorer data and linking simultaneous radio observations to it. We find that surprisingly Cyg X-3 sketches a similar shape in the HID to that seen in other transient black hole X-ray binaries during outburst but with distinct differences. Together with the results of this analysis and previous studies of Cyg X-3 we conclude that the X-ray states can be assigned to six distinct states. This categorization relies heavily on the simultaneous radio observations and we identify one new X-ray state, the hypersoft state, similar to the ultrasoft state, which is associated to the quenched radio state during which there is no or very faint radio emission. Recent observations of GeV flux observed from Cyg X-3 (Tavani et al. 2009; Fermi LAT Collaboration et al. 2009) during a soft X-ray and/or radio quenched state at the onset of a major radio flare hint that a very energetic process is at work during this time, which is also when the hypersoft X-ray state is observed. In addition, Cyg X-3 shows flaring with a wide range of hardness.

The disc–jet coupling in the neutron star X-ray binary Aquila X-1

We study the accretion/ejection processes (i.e. disc/jet coupling) in the neutron star X-ray binary Aquila X-1 via a multiwavelength approach. We use in the radio band the publicly available Very Large Array archive containing observations of the object between 1986 and 2005, in the X-ray band the archival Rossi X-ray Timing Explorer data (Proportional Counter Array and High Energy X-ray Timing Experiment) between 1997 and 2008, and in optical (R band) observations with the Small and Moderate Aperture Research Telescope System recorded between 1998 and 2007. In the combined data set, we find three outbursts for which quasi-simultaneous radio, optical (R band) and X-ray data exist and focus on them to some extent. We provide evidence that the disc/jet coupling in Aquila X-1 is similar to what has been observed in black hole X-ray binaries, at least from the point of view of the behaviour in the hardness-intensity diagrams (the hysteresis effect included), when the phenomenology of the jet is taken into account. Although based on a very small number of observations, a radio/X-ray correlation seems to exist for this system, with a slope of α= 0.40 ± 0.07 (Fradio∝FαX), which is different than the slope of α= 1.40 ± 0.25 found for another atoll source, 4U 1728−34, but interestingly enough is relatively close to the values obtained for several black hole X-ray binaries. No significant correlation is found between the radio and optical (R-band) emissions. We also report a significant drop in the radio flux from Aquila X-1 above an X-ray flux of ∼5 × 10−9 erg cm−2 s−1. This behaviour, also reported in the neutron star X-ray binary 4U 1728−34, may be analogous to the suppression of radio emission in black hole X-ray binaries in bright, soft X-ray states. It suggests that from this point of view neutron star X-ray binaries can mimic the behaviour of black hole X-ray binaries in suppressing the jet in soft/disc-dominated X-ray states.