Rossi X-Ray Timing Explorer All-Sky Monitor Research Articles

EXTREME ULTRAVIOLET EXPLOREROBSERVATIONS OF HERCULES X-1 OVER A 35 DAY CYCLE

Observations of Hercules X-1 by the Extreme Ultraviolet Explorer covering most of the 35 day cycle are reported here. This is the only long extreme ultraviolet (EUV) observation of Her X-1. Simultaneous X-ray observations with the Rossi X-ray Timing Explorer All-Sky Monitor (RXTE/ASM) X-ray show that Her X-1 is in an X-ray anomalous low state. The first 4 days are also observed with the RXTE proportional counter array (PCA), which shows that the X-ray properties are nearly the same as for normal low states in Her X-1 with flux reduced by a factor of 2. In contrast, the EUV emission from Her X-1 is reduced by a factor of ∼4 compared to normal low states. The twisted-tilted accretion disk responsible for the normal 35 day X-ray cycle can be modified to explain this behavior. An increased disk twist reduces the X-ray illumination of HZ Her by a factor of ∼2 and of the disk surface by a somewhat larger factor, leading to a larger reduction in EUV flux compared to X-ray flux.

Discovery of a short orbital period in the Supergiant Fast X-ray Transient IGR J16479–4514

Abstract We report here the discovery of a 3.32 d orbital period in the Supergiant Fast X-ray Transient (SFXT) source IGR J16479–4514. Using the long-term light curve of this source obtained with Swift Burst Alert Telescope (BAT) in the energy range of 15–50 keV, we have clearly detected an orbital modulation including a full eclipse of duration ∼0.6 d. In the hard X-ray band of the BAT instrument, the eclipse ingress and egress are rapid. We have also used the long-term light curve obtained with the Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) in the energy range of 1.5–12 keV. Taken independently, the detection of orbital modulation in the RXTE–ASM light curve is not significant. However, considering a clear detection of orbital modulation in the BAT light curve, we have used the ASM light curve for a more precise determination of the orbital period. IGR J16479–4514 has the shortest orbital period among the three SFXTs with measured/known orbital period. We discuss the implication of a short orbital period with the various mechanisms proposed to explain the transient nature of this class of sources.

Integral and RXTE/ASM Observations on IGR J17098−3628

Abstract To further probe the possible nature of the unidentified source IGR J17098$-$3628, we have carried out a detailed analysis of its long-term time variability, as monitored by Rossi X-ray Timing Explorer/All Sky Monitor (RXTE/ASM), and of its hard X-ray properties, as observed by the International Gamma-Ray Astrophysics Laboratory (INTEGRAL). INTEGRAL has monitored this sky region for years, and significantly detected IGR J17098$-$3628 only when the source was in its dubbed active state. In particular, at $\ge$20 keV, Imager on Board of the INTEGRAL Satellite/INTEGRAL Soft Gamma-Ray Imager (IBIS/ISGRI) caught an outburst in 2005 March, lasting for $\sim$5 d with a detection significance of 73$\sigma$ (20-40 keV) and with emission at $/lt$200 keV. ASM observations have revealed that the outburst of the soft X-ray lightcurve is similar to that detected by INTEGRAL; however the peak of the soft X-ray lightcurve lags behind, or is preceded by, the hard X-ray ($/gt$20 keV) outburst by $\sim$2 d. This resembles the behavior of X-ray novae like XN 1124$-$683, and hence further suggests a Low-Mass X-ray Binary (LMXB) nature for IGR J17098$-$3628. While the quality of the ASM data prevents us from drawing any definite conclusions, these discoveries are important clues that, coupled with future observations, will help us to resolve the nature of IGR J17098$-$3628 that is unknown so far.

A Comparison of the Variability of the Symbiotic X‐Ray Binaries GX 1+4, 4U 1954+31, and 4U 1700+24 fromSwiftBAT andRXTEASM Observations

We present an analysis of the X-ray variability of three symbiotic X-ray binaries, GX 1+4, 4U 1700+24, and 4U 1954+31, using observations made with the Swift Burst Alert Telescope (BAT) and the Rossi X-ray Timing Explorer (RXTE) All-Sky Monitor (ASM). Observations of 4U 1954+31 with the Swift BAT show modulation at a period near 5 hours. Models to explain this modulation are discussed including the presence of an exceptionally slow X-ray pulsar in the system and accretion instabilities. We conclude that the most likely interpretation is that 4U 1954+31 contains one of the slowest known X-ray pulsars. Unlike 4U 1954+31, neither GX 1+4 nor 4U 1700+24 show any evidence for modulation on a timescale of hours. An analysis of the RXTE ASM light curves of GX 1+4, 4U 1700+24, and 4U 1954+31 does not show the presence of periodic modulation in any source, although there is considerable variability on long timescales for all three sources. There is no modulation in GX 1+4 on either the optical 1161 day orbital period or a previously reported 304 day X-ray period. For 4U 1700+24 we do not confirm the 404 day X-ray period previously proposed for this source from a shorter duration ASM light curve. We conclude that all three sources have substantial low-frequency noise in their power spectra that may give the appearance of periodic modulation if this noise is not properly accounted for, particularly if short duration light curves are examined.

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 4U1636-53 with the Rossi X-ray Timing Explorer (RXTE) for more than 1.5yr. Our campaign consisted of short (~2ks) pointings separated by 2d, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ~30-40d, 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.

SwiftBAT andRXTEObservations of the Peculiar X‐Ray Binary 4U 2206+54: Disappearance of the 9.6 Day Modulation

Observations of the high-mass X-ray binary 4U 2206+54 with the Swift Burst Alert Telescope (BAT) do not show modulation at the previously reported period of 9.6 days found from observations made with the Rossi X-ray Timing Explorer (RXTE) All-Sky Monitor (ASM). Instead, the strongest peak in the power spectrum of the BAT light curve occurs at a period of 19.25 +/- 0.08 days, twice the period found with the RXTE ASM. The maximum of the folded BAT light curve is also delayed compared to the maximum of the folded ASM light curve. The most recent ASM data folded on twice the 9.6 day period show similar morphology to the folded BAT light curve. This suggests that the apparent period doubling is a recent secular change rather than an energy-dependent effect. The 9.6 day period is thus not a permanent strong feature of the light curve. We suggest that the orbital period of 4U 2206+54 may be twice the previously proposed value.

X-ray spectral transitions of black holes from RXTE All-Sky Monitor

We have analysed X-ray outbursts from several Galactic black hole (GBH) transients, as seen by the All-Sky Monitor (ASM) on board Rossi X-ray Timing Explorer (RXTE). We have used the best estimates of distance and black hole mass to find their luminosity (scaled to the Eddington limit), which allowed for direct comparison of many sources. We have found two distinct hard-to-soft state transitions in the initial part of the outburst. The distinction is made on the basis of the transition luminosity, its duration, the shape of the track in the hardness-luminosity diagram and evolution of the hardness ratio. The bright/slow transition occurs at ∼30 per cent of Eddington (estimated bolometric) luminosity and takes ≥30 d, during which the source quickly reaches the intermediate/very high state and then proceeds to the soft state at much slower pace. The dark/slow transition is less luminous (≤10 per cent of Eddington), shorter (≤15 d) and the source does not slow its transition rate before reaching the soft state. We speculate that the distinction is due to irradiation and evaporation of the disc, which sustains the Comptonizing corona in the bright intermediate/very high state.

X-ray outburst of 4U 0115+634 and ROTSE observations of its optical counterpart V635 Cas

ROTSE IIId (The Robotic Optical Transient Experiment) observations of X-ray binary system 4U 0115+634/V635 Cas obtained during 2004 June and 2005 January make possible, for the first time, to study the correlation between optical and type II X-ray outbursts. The X-ray outburst sharply enhanced after periastron passage where the optical brightness was reduced by 0.3 magnitude for a few days. We interpret the sharp reduction of optical brightness as a sign of mass ejection from the outer parts of the disc of the Be star. After this sharp decrease, the optical brightness healed and reached the pre X-ray outburst level. Afterwards, gradual decrease of the optical brightness followed a minimum then a gradual increase started again. Qualitatively, change of optical lightcurve suggests a precession of the Be star disc around a few hundred days. We also investigate the periodic signatures from the archival RXTE-ASM (Rossi X-ray Timing Explorer - All Sky Monitor) light curve covering a time span of $\sim 9$ years. We find significant orbital modulation in the ASM light curve during the type I X-ray outburst.

Periodic variability during the X-ray decline of 4U 1636-53

We report the onset of a large amplitude, statistically significant periodicity (∼46 d) in the Rossi X-ray Timing Explorer/All Sky Monitor (RXTE/ASM) data of the prototype X-ray burster 4U 1636-53, the X-ray flux of which has been gradually declining over the last 4 yr. This behaviour is remarkably similar to that observed in the neutron star low mass X-ray binary (LMXB), KS 1731-260, which is a long-term transient. We also report on an INTEGRAL/IBIS observation of 4U 1636-53 during its decline phase, and find that the hard X-ray flux (20-100 keV) indicates an apparent anticorrelation with soft X-rays (2-12 keV). We argue that 4U 1636-53 is transiting from activity to quiescence, as occurred in KS 1731-260. We also suggest that the variability during the X-ray decline is the result of an accretion rate variability related to the X-ray irradiation of the disc.

RXTE observations of the dipping low-mass X-ray binary 4U 1624–49

We analyse ~ 360 ks of archival data from the Rossi X-Ray Timing Explorer (RXTE) of the 21 hr orbital period dipping low-mass X-ray binary 4U 1624-49. We find that outside the dips the tracks in the colour-colour and hardness-intensity diagrams (CDs and HIDs) are reminiscent of those of atoll sources in the middle and upper parts of the banana branch. The tracks show secular shifts up to ~ 10%. We study the power spectrum of 4U 1624-49 as a function of the position in the CD. This is the first time power spectra of this source are presented. No quasi-periodic oscillations (QPOs) are found. The power spectra are dominated by very low frequency noise (VLFN), characteristic for atoll sources in the banana state, and band limited noise (BLN) which is not reliably detected but may, uncharacteristically, strengthen and increase in frequency with spectral hardness. The VLFN fits to a power law, which becomes steeper when the source moves to the harder part of the CD. We conclude that 4U 1624-49 is an atoll source which in our observations is in the upper banana branch. Combining this with the high (0.5-0.7 L_Edd) luminosity, the long-term flux stability of the source as seen with the RXTE All-Sky Monitor (ASM), and with the fact that it is an X-ray dip source, we conclude that 4U 1624-49 is most likely a GX atoll source such as GX 3+1 and GX 9+9, but seen edge on.

Discovery of a Transition to Global Spin-up in EXO 2030+375

EXO 2030+375, a 42 s transient X-ray pulsar with a Be star companion, has been observed to undergo an outburst at nearly every periastron passage for the last 13.5 years. From 1994 through 2002, the global trend in the pulsar spin frequency was spin-down. Using Rossi X-Ray Timing Explorer (RXTE) data from 2003 September, we have observed a transition to global spin-up in EXO 2030+375. Although the spin-frequency observations are sparse, the relative spin-up between 2002 June and 2003 September observations, along with an overall brightening of the outbursts since mid-2002 observed with the RXTE All-Sky Monitor, accompanied by an increase in density of the Be disk, indicated by infrared magnitudes, suggest that the pattern observed with BATSE of a roughly constant spin frequency, followed by spin-up, followed by spin-down is repeating. If so, this pattern has approximately an 11 yr period, similar to the 15 +/- 3 yr period derived by Wilson et al. for the precession period of a one-armed oscillation in the Be disk. If this pattern is indeed repeating, we predict a transition from spin-up to spin-down in 2005.

Long-term periodicities in the flux from low mass X-ray binaries

Using data from the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE), we have searched for long term periodicities in the X-ray flux of GX 1+4, Sco X-2 (GX 349+2), and GX 339−4. For GX 1+4, we also used data from BATSE and Galactic Centre scans performed by RXTE. We find no evidence for X-ray modulations at the suggested ∼304 d orbital period of GX 1+4. However, we find tentative evidence for a periodicity at 420–460 d. An upper limit of 15% peak-to-peak is set on any sinusoidal modulation in the 1.5–3.0 keV flux of Sco X-2 for periods in the 30–100 d range. For GX 339−4, we confirm the Low State modulation and report the detection of significant low-frequency modulations in both the High State and Very High State. We fail to detect this modulation in the Off State. We show that if the reported orbital period of GX 339−4 lies in the range 0.5–1.7 d, then it is not present in the RXTE ASM light curve.

The X‐Ray Afterglow of Dark GRB 970815: A Common Origin for Gamma‐Ray Bursts and X‐Ray Flashes?

GRB 970815 is a well-localized gamma-ray burst (GRB) detected by the All-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) for which no afterglow was identified despite follow-up ASCA and ROSAT pointings and optical imaging to limiting magnitude R > 23. Although an X-ray source, AX/RX J1606.8+8130, was detected just outside the ASM error box, it was never associated with the GRB because it was not clearly fading and because no optical afterglow was ever found. We recently obtained an upper limit for this source with Chandra that is at least a factor of 100 fainter than the ASCA detection. We also made deep optical observations of the AX/RX J1606.8+8130 position, which is blank to limits V > 25.2 and I > 24.0. In view of these extreme limits, we conclude that AX/RX J1606.8+8130 is indeed the afterglow of GRB 970815, which corresponds to an optically dark GRB. AX/RX J1606.8+8130 can therefore be ruled out as the counterpart of the persistent EGRET source 3EG J1621+8203. The early light curves from BATSE and the RXTE ASM show spectral softening between multiple peaks of prompt emission. We propose that GRB 970815 might be a case in which the properties of an X-ray flash and a normal GRB coincide in a single event.

An Unveiling Event in the Type 2 Active Galactic Nucleus NGC 4388:A Challenge for a Parsec-Scale Absorber

We present two Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array (PCA) observations of the type 2 Seyfert galaxy NGC 4388 caught in an unusual low X-ray absorption state. The observations were triggered by a detection in the 1.5-3 keV band of the RXTE all-sky monitor. NGC 4388 was found at a somewhat high continuum level [f(2-10 keV) = 8 × 10-11 ergs cm-2 s-1] and with a column density NH ~ 3 × 1022 cm-2, a factor of ~10 lower than normal. The second PCA observation, 4 hr later, gave NH < 2 × 1021 cm-2 indicating, at the 3.1 σ level, variability so rapid it puts the absorber on a few 100 Schwarzschild radii scale, similar to the broad emission line region or smaller. This small scale creates difficulties for the parsec-scale obscuring torus paradigm of unified schemes for type 1 and type 2 active galactic nuclei.

X1908+075: A Pulsar Orbiting in the Stellar Wind of a Massive Companion

We have observed the persistent but optically unidentified X-ray source X1908+075 with the Proportional Counter Array and High-Energy X-Ray Timing Experiment instruments on the Rossi X-Ray Timing Explorer (RXTE). The binary nature of this source was recently established by Wen et al., who found a 4.4 day orbital period in results from the RXTE All-Sky Monitor. We report the discovery of 605 s pulsations in the X-ray flux. The Doppler delay curve is measured and provides a mass function of 6.1 M☉, which is a lower limit to the mass of the binary companion of the neutron star. The degree of attenuation of the low-energy end of the spectrum is found to be a strong function of orbital phase. A simple model of absorption in a stellar wind from the companion star fits the orbital phase dependence reasonably well and limits the orbital inclination angle to the range 38°-72°. These measured parameters lead to an orbital separation of ~60-80 lt-s, a mass for the companion star in the range 9-31 M☉, and an upper limit to the size of the companion of ~22 R☉. From our analysis, we also infer a wind mass-loss rate from the companion star of ≳1.3 × 10-6 M☉ yr-1 and, when the properties of the companion star and the effects of photoionization are considered, likely ≳4 × 10-6 M☉ yr-1. Such a high rate is inconsistent with the allowed masses and radii that we find for a main-sequence or modestly evolved star unless the mass-loss rate is enhanced in the binary system relative to that of an isolated star. We discuss the possibility that the companion might be a Wolf-Rayet star that could evolve to become a black hole in 104 to 105 yr. If so, this would be the first identified progenitor of a neutron star-black hole binary.

X-ray behaviour of Circinus X-1 - I. X-ray dips as a diagnostic of periodic behaviour

We examine the periodic nature of detailed structure (particularly dips) in the Rossi X-Ray Timing Explorer/All Sky Monitor (RXTE/ASM) light-curve of Circinus X-1. The significant phase wandering of the X-ray maxima suggests their identification with the response on a viscous time-scale of the accretion disc to perturbation. We find that the X-ray dips provide a more accurate system clock than the maxima, and thus use these as indicators of the times of periastron passage. We fit a quadratic ephemeris to these dips, and find its predictive power for the X-ray light-curve to be superior to ephemerides based on the radio flares and the full archival X-ray light-curve. Under the hypothesis that the dips are tracers of the mass transfer rate from the donor, we use their occurrence rate as a function of orbital phase to explore the (as yet unconstrained) nature of the donor. The high P term in the ephemeris provides another piece of evidence which shows that Cir X-1 is in a state of dynamical evolution, and thus is a very young post-supernova system. We further suggest that the radio 'synchrotron nebula' immediately surrounding Cir X- 1 is, in fact, the remnant of the event that created the compact object, and discuss briefly the evidence for and against such an interpretation.

Hysteresis in the light curves of soft X-ray transients

Using Proportional Counter Array (PCA) data from the Rossi X-Ray Timing Explorer (RXTE), we track the spectral states of the neutron star transient system Aql X-1 through a complete outburst cycle. We find a hard-to-soft state transition during the very early, rising phase of the outburst, and show that there is a hysteresis effect such that the transition back to the hard state occurs at a luminosity ∼5 times lower than the hard-to-soft transition. This hysteresis effect rules out the propeller mechanism as the sole cause of state transitions in Aql X-1. Assuming the propeller mechanism only operates at a luminosity equal to or below that of the observed soft-to-hard transition requires that the magnetic field of Aql X-1 be less than 7 × 107 G, the lowest neutron star field known to date. To compare the state transition behaviour of Aql X-1 with that found in transient black hole systems, we use RXTE All-Sky Monitor (ASM) data to compute hardness—intensity diagrams for four black hole candidate transients where the ASM data should also give us state information throughout much of the outburst cycles. In all four systems, we find evidence for a hard-to-soft state transition during the rising outburst phase and for the source staying in a soft state down to much lower luminosities during the declining phase, i.e. a hysteresis effect. This similarity suggests a common origin for state transitions in low magnetic field neutron star and black hole systems, and the hysteresis effect rules out the ‘strong ADAF (advection-dominated accretion flow) principle’ for determining the state of an accretion disc. We discuss the general implications of these observations for current models of state transitions. We note the contrast to previous observations of the non-transient systems Cygnus X-1 and X-3, which do not show a hysteresis effect.

Modelling RXTE/ASM observations of the 35-d cycle in Her X-1

Her X-1 exhibits a 35-d cycle in X-ray intensity: ‘main high state–low state–short high state–low state’, which is caused by a varying obscuration of the neutron star by a tilted–twisted precessing accretion disc. The 35-d X-ray light curve in Hercules X-1 has been well characterized by the Rossi X-ray Timing Explorer All-Sky Monitor (RXTE/ASM). Here, different models of the accretion disc and of the neutron star source region are constructed and applied to fit the RXTE/ASM observations. Such detailed modelling of the shape of the 35-d light curve has not been carried out previously. The results show that a detailed model can fit the details of the light curve, and yield numerical values for the parameters of the accretion disc, its atmosphere and the inner edge of the disc. The best model here has a two-layer outer disc, a thick inner disc edge and an extended central source.

Serendipitous Detections of XTE J1906+09 with theRossi X‐Ray Timing Explorer

The 89 s X-ray pulsar XTE J1906+09 was discovered during Rossi X-Ray Timing Explorer (RXTE) observations of SGR 1900+14 in 1996. Because of monitoring campaigns of SGR 1900+14, XTE J1906+09 was also monitored regularly in 1996 September, 1998 May-June, 1998 August-1999 July, and 2000 March-2001 January. A search for pulsations resulted in detections of only the two previously reported outbursts in 1996 September and 1998 August-September. Pulsed flux upper limits for the rest of the observations show that XTE J1906+09 is a transient X-ray pulsar and likely has a Be star companion. The RXTE all-sky monitor did not reveal XTE J1906+09. Pulse-timing analysis of the second outburst discovered a sinusoidal signature in the pulse frequencies that is likely produced by an orbital periastron passage. Fits to pulse phases using an orbital model and quadratic phase model have chi(exp 2) minima at orbital periods of 26-30 days for fixed mass functions of 5, 10, 15, and 20 solar masses. The pulse shape showed energy- and intensity-dependent variations. Pulse-phase spectroscopy quantified the energy-dependent variations. The phase-averaged spectrum used the pulse minimum spectrum as the background spectrum to eliminate effects from SGR 1900+14 and the Galactic ridge and was well fitted by an absorbed power law with a high-energy cutoff with column density N(sub H) = 6 +/- 1 x 10(exp 22)/sq cm, a photon index of 1.01 +/- 0.08, cutoff energy E(sub cut) = 11 +/- 1 keV, and e-folding energy E(sub fold) = 19 +/- 4 keV. Estimated 2-10 keV peak fluxes, corrected for contributions from the Galactic ridge and SGR 1900+14, are 6 x l0(exp -12) and 1.1 x 10(exp -10) ergs/sq cm/s for the 1996 and 1998 outbursts, respectively. XTE J1906+09 may be part of an unusual class of Be/X-ray binaries that do not lie on the general spin period versus orbital period correlation with the majority of Be/X-ray binaries.

Binary and Long‐Term (Triple?) Modulations of 4U 1820−30 in NGC 6624

We present timing analysis results for Rossi X-Ray Timing Explorer (RXTE) observations of X-ray binary source 4U 1820-30 located in the globular cluster NGC 6624. The light curves of observations made between 1996 October and 1997 September show that the maximum of the 685 s binary period modulation folded by the linear ephemeris from previous observations has a phase shift of -0.20 ± 0.06. Combined with historical results (1976-1997), the binary period derivative is measured to be /P = (-3.47 ± 1.48) × 10-8 yr-1. The previous known (~176 day) long-term modulation is significant in the X-ray light curve obtained by analysis of the RXTE All-Sky Monitor (ASM) during the years 1996-2000. The RXTE ASM ephemeris is extended by analysis of all historical data (Vela 5B and Ginga) to yield a period of 171.033 ± 0.326 days with no evidence for period change (|/P| < 2.20 × 10-4 yr-1). All reported X-ray burst activity is confined to within ±23 days of the predicted minima. This stable long-term modulation is consistent with 4U 1820-30 being a hierarchical triple system with a ~1.1 day period companion.