Soft X-ray Transients Research Articles

The missing link: a low-mass X-ray binary in M31 seen as an ultraluminous X-ray source

A new, transient ultraluminous X-ray source (ULX) was recently discovered by Chandra in M31 with a luminosity at ~ 5 x 10^39 erg/s. Here we analyse a series of five subsequent XMM-Newton observations. The steady decline in X-ray luminosity over 1.5 months gives an observed e-fold timescale of ~40 days; similar to the decay timescales seen in soft X-ray transients in our own Galaxy. This supports the interpretation of this ULX as a stellar mass black hole in a low-mass X-ray binary (LMXB), accreting at super Eddington rates. This is further supported by the lack of detection of an O/B star in quiescence and the XMM-Newton spectral data being dominated by a disc-like component rather than the power-law expected from a sub-Eddington intermediate-mass black hole. These data give the best sequence of high Eddington fraction spectra ever assembled due to the combination of low absorption column to M31 and well calibrated bandpass down to 0.3 keV of XMM-Newton. The spectra can be roughly described by our best current disc model, BHSPEC, assuming a 10 M_solar black hole with best fit spin of ~0.4. However, the data are better described by a two component model, where the disc emission is significantly affected by advection, and with an additional low temperature Comptonisation component at high energies. This could simply indicate the limitations of current disc models, though the energy-dependent variability also weakly supports a two component interpretation of the data. Irrespective of the detailed interpretation of the spectral properties, these data support the presence of accretion onto a stellar mass black hole in a LMXB accreting in the Eddington-regime. This allows an unambiguous connection of this object, and, by extension, similar low luminosity ULXs, to `standard' X-ray binaries.

CXOM31 J004253.1+411422: the first ultraluminous X-ray transient in M 31

We seek clarification of the nature of X-ray sources detected in M 31. Here we focus on CXOM31 J004253.1+411422, the brightness of which suggests that it belongs to the class of ultraluminous X-ray sources. We determine the X-ray properties of sources detected in the XMM-Newton Chandra monitoring program. We investigate spectral properties and search for periodic or quasi-periodic oscillations. A multi-component model is applied to the spectra obtained from XMM-Newton data to evaluate the relative contributions from thermal and non-thermal emission. The time dependence of this ratio is evaluated over a period of forty days. We simultaneously fit data from XMM-Newton EPIC-pn, MOS1 and MOS2 detectors with (non-thermal) powerlaw and (thermal) multicolored blackbody. The X-ray spectrum is best fit by the combination of a thermal component with kT ~ 1 keV and a powerlaw component with photon index approximately 2.6. From combined analysis of Chandra, Swift and XMM-Newton data, the unabsorbed total luminosity of this source decreases from ~ 3.8 x 10^{39} erg s^{-1} in the first observation to ~ 0.5 x 10^{39} ergs s^{-1} over a period of three months. The decay closely follows an exponential decline with a time constant of 32 days. The source spectrum evolves significantly, exhibiting a faster decline of the thermal component. We do not find evidence of any significant temporal features in the power density spectrum. The presence of a thermal component at kT ~ 1 keV in conjunction with a non-thermal high energy tail, is also consistent with spectral properties of other ULXs in the "high state". Our analysis indicates that the underlying source of this first ULX in M~31 is a black hole of mass, M > 13 M_{sun}, accreting near the Eddington limit, that underwent a transient outburst followed by an exponential decay reminiscent of transients associated with galactic X-ray novae.

X-ray monitoring of classical novae in the central region of M 31

[Abridged] Classical novae (CNe) represent the major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M 31. We performed a dedicated monitoring of the M 31 central region with XMM-Newton and Chandra between Nov 2007 and Feb 2008 and between Nov 2008 and Feb 2009 respectively, in order to find SSS counterparts of CNe, determine the duration of their SSS phase and derive physical outburst parameters. We systematically searched our data for X-ray counterparts of CNe and determined their X-ray light curves and spectral properties. We detected in total 17 X-ray counterparts of CNe in M 31, only four of which were known previously. These latter sources are still active 12.5, 11.0, 7.4 and 4.8 years after the optical outburst. From the 17 X-ray counterparts 13 were classified as SSSs. Four novae displayed short SSS phases (< 100 d). Based on these results and previous studies we compiled a catalogue of all novae with SSS counterparts in M 31 known so far. We used this catalogue to derive correlations between the following X-ray and optical nova parameters: turn-on time, turn-off time, effective temperature (X-ray), t2 decay time and expansion velocity of the ejected envelope (optical). Furthermore, we found a first hint for the existence of a difference between SSS parameters of novae associated with the stellar populations of the M 31 bulge and disk. Additionally, we conducted a Monte Carlo Markov Chain simulation on the intrinsic fraction of novae with SSS phase. This simulation showed that the relatively high fraction of novae without detected SSS emission might be explained by the inevitably incomplete coverage with X-ray observations in combination with a large fraction of novae with short SSS states, as expected from the WD mass distribution. In order to verify our results with an increased sample further monitoring observations are needed.

On different types of instabilities in black hole accretion discs: implications for X-ray binaries and active galactic nuclei

We discuss two important instability mechanisms that may lead to the limit-cycle oscillations of the luminosity of the accretion disks around compact objects: ionization instability and radiation-pressure instability. Ionization instability is well established as a mechanism of X-ray novae eruptions in black hole binary systems but its applicability to AGN is still problematic. Radiation pressure theory has still very weak observational background in any of these sources. In the present paper we attempt to confront the parameter space of these instabilities with the observational data. At the basis of this simple survey of sources properties we argue that the radiation pressure instability is likely to be present in several Galactic sources with the Eddington ratios above 0.15, and in AGN with the Eddington ratio above 0.025. Our results favor the parameterization of the viscosity through the geometrical mean of the radiation and gas pressure both in Galactic sources and AGN. More examples of the quasi-regular outbursts in the timescales of 100 seconds in Galactic sources, and hundreds of years in AGN are needed to formulate firm conclusions. We also show that the disk sizes in the X-ray novae are consistent with the ionization instability. This instability may also considerably influence the lifetime cycle and overall complexity in the supermassive black hole environment.

Correlated optical/X-ray long-term variability in LMXB 4U 1636-536

We have conducted a 3-month program of simultaneous optical, soft and hard X-ray monitoring of the LMXB 4U1636-536/V801 Ara using the SMARTS 1.3m telescope and archival RXTE/ASM and Swift/XRT data. 4U1636-536 has been exhibiting a large amplitude, quasi-periodic variability since 2002 when its X-ray flux dramatically declined by roughly an order of magnitude. We confirmed that the anti-correlation between soft (2-12 keV) and hard (> 20 keV) X-rays, first investigated by Shih et al. (2005), is not an isolated event but a fundamental characteristic of this source's variability properties. However, the variability itself is neither strictly stable nor changing on an even longer characteristic timescale. We also demonstrate that the optical counterpart varies on the same timescale, and is correlated with the soft, and not the hard, X-rays. This clearly shows that X-ray reprocessing in LMXB discs is mainly driven by soft X-rays. The X-ray spectra in different epochs of the variability revealed a change of spectral characteristics which resemble the state change of black hole X-ray binaries. All the evidence suggests that 4U1636-536 is frequently (~monthly) undergoing X-ray state transitions, a characteristic feature of X-ray novae with their wide range of luminosities associated with outburst events. In its current behavioural mode, this makes 4U1636-536 an ideal target for investigating the details of state changes in luminous X-ray binaries.

KERR PARAMETERS FOR STELLAR MASS BLACK HOLES AND THEIR CONSEQUENCES FOR GAMMA-RAY BURSTS AND HYPERNOVAE

Recent measurements of the Kerr parameters a⋆ for two black hole binaries in our Galaxy, GRO J1655−40 and 4U 1543−47, of a⋆ = 0.65–0.75 and a⋆ = 0.75–0.85, respectively, fit the predictions of Lee et al. of a⋆ ≅ 0.8. They predicted a⋆>0.5 for 80% of the soft X-ray transient (SXT) sources. The maximum available energy in the Blandford–Znajek formalism for a⋆>0.5 gives E>3 × 1053 erg, orders of magnitude larger than the energy needed for the gamma-ray burst (GRB) and hypernova explosion. We interpret the SXTs to be relics of GRBs and hypernovae. We find that most galactic SXTs were subluminous given that they could use only a small part of the available rotational energy.

A search for dormant binaries with degenerate components inω Centauri and NGC 6397

We report on the first spectroscopic search for quiescent degenerate binaries in globular clusters. Our survey is based on a sample of short-period optical variables which are likely optical counterparts of quiescent X-ray sources in omega Centauri (NGC 5139) and NGC 6397. All of the studied candidates have nearly sinusoidal light curves with amplitudes of 0.1--0.35~mag in $V$ and periods of 0.1--1.3~days. This type of variability, most probably originating from the ellipsoidal effect, has been observed in several X-ray novae when they settled into quiescence after an outburst. We find that two of the surveyed systems harbor dim components with masses in excess of one solar masss, making them attractive targets for future investigations. We also suggest that there are two subpopulations of blue stragglers in omega Centauri, differing in mass-transfer history and/or helium content.

ORBITAL PERIOD AND OUTBURST LUMINOSITY OF TRANSIENT LOW MASS X-RAY BINARIES

In this paper we investigate the relation between the maximal luminosity of X-ray outburst and the orbital period in transient low mass X-ray binaries (or soft X-ray transients) observed by the Rossi X-ray Timing Explorer (RXTE) in the past decade. We find that the maximal luminosity (3-200 keV) in Eddington unit generally increases with increasing orbital period, which does not show a luminosity saturation but in general agrees with theoretical prediction. The peak luminosities in ultra-compact binaries might be higher than those with orbital period of 2-4 h, but more data are needed to make the claim. We also find that there is no significant difference in the 3-200 keV outburst peak luminosity between neutron star systems and black hole systems with orbital periods above 4 h; however, there might be significant difference at smaller orbital period where only neutron star systems are observed and radiatively ineffcient accretion flow is expected to work at the low luminosities for black hole accreters.

SPITZEROBSERVATIONS OF BLACK HOLE LOW-MASS X-RAY BINARIES: ASSESSING THE NON-STELLAR INFRARED COMPONENT

We have combined ground-based optical and near-infrared data with Spitzer Space Telescope mid-infrared data for five black hole (BH) soft X-ray transients (SXTs) in order to determine the levels of near- and mid-infrared emission from sources other than the secondary star. Mid-infrared emission from an accretion disk, circumbinary dust, and/or a jet could act as sources of near-infrared contamination, thereby diluting ellipsoidal variations of the secondary star and affecting determined BH mass estimates. Based on optical to mid-infrared spectral energy distribution modeling of the five SXTs along with the prototype, V616 Mon, we detected mid-infrared excesses in half of the systems, and suggest that the excesses detected from these systems arise from non-thermal synchrotron jets rather than circumbinary dust disks.

Echo outbursts in KS 1731–260

Context. The thermal-viscous instability of the accretion disk is thought to play an important role in the activity of low-mass X-ray binaries (LMXBs). It should also appear when a (quasi)persistent LMXB goes into its low state. Aims. We analyze the long-term X-ray activity of the neutron star LMXB KS 1731–260 during a part of its long main outburst and a complicated transition back to the low state (quiescence). We pay special attention to the variations during the state transition. Methods. We make use of the ASM/RXTE observations for a timeseries analysis of the longterm variations and investigation of the X-ray color changes in the 1.5–12 keV passband. Results. In our interpretation, the mass transfer rate u mtr between the donor and neutron star significantly increased during the main outburst of KS 1731–260, which prolonged this event by keeping the disk ionized to its outer rim. Later on, a considerable fraction of the outer part of the disk was rapidly brought out of steady-state when u mtr decreased again. Only a slow decrease of u mtr continued even after the end of the main outburst. This lead to the formation of the central disk region, which was kept thermally stable, ionized by irradiation, and was surrounded by the outer, thermally unstable disk region. This configuration triggered the series of the echo outbursts. These outbursts are outside-in, caused by the still continuing u mtr via the mass stream impinging the outer rim of the disk without overflow. We argue that introducing any strong bursts of matter from the donor to the disk as the cause of the echo outbursts is superfluous. The spectral evolution suggests that the disk returns to steady-state during the maxima of the echo outbursts. The X-ray emission between the echo outbursts is much higher than in the true quiescence and can be due to the ongoing u mtr. We place the peculiar long-term activity of KS 1731–260 in the context of the systems with the echo outbursts, both soft X-ray transients and dwarf novae.

Characterizing black hole variability with nonlinear methods: the case of the X-ray Nova 4U 1543–47

We investigate the possible nonlinear variability properties of the black hole X-ray nova 4U1543-47 to complement the temporal studies based on linear techniques, and to search for signs of nonlinearity in Galactic black hole (GBH) light curves. First, we apply the weighted scaling index method (WSIM) to characterize the X-ray variability properties of 4U1543-47 in different spectral states during the 2002 outburst. Second, we use surrogate data to investigate whether the variability is nonlinear in any of the different spectral states. The main findings can be summarized as follows. The mean weighted scaling index appears to be able to parametrize uniquely the temporal variability properties of this GBH: the 3 different spectral states of the 2002 outburst of 4U1543-47 are characterized by different and well constrained values. The search for nonlinearity reveals that the variability is linear in all light curves with the notable exception of the very high state. Our results imply that we can use the WSIM to assign a single number, namely the mean weighted scaling index, to a light curve, and in this way discriminate among the different spectral states of a source. The detection of nonlinearity in the VHS, that is characterized by the presence of most prominent QPOs, suggests that intrinsically linear models which have been proposed to account for the low frequency QPOs in GBHs may be ruled out (abridged).

The correlation timescale of the X-ray flux during the outbursts of soft X-ray transients

Recent studies of black hole and neutron star low mass X-ray binaries (LMXBs) show a positive correlation between the X-ray flux at which the low/hard(LH)-to-high/soft(HS) state transition occurs and the peak flux of the following HS state. By analyzing the data from the All Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE), we show that the HS state flux after the source reaches its HS flux peak still correlates with the transition flux during soft X-ray transient (SXT) outbursts. By studying large outbursts or flares of GX 339-4, Aql X-1 and 4U 1705-44, we have found that the correlation holds up to 250, 40, and 50 d after the LH-to-HS state transition, respectively. These time scales correspond to the viscous time scale in a standard accretion disk around a stellar mass black hole or a neutron star at a radius of ~104-5 Rg, indicating that the mass accretion rates in the accretion flow either correlate over a large range of radii at a given time or correlate over a long period of time at a given radius. If the accretion geometry is a two-flow geometry composed of a sub-Keplerian inflow or outflow and a disk flow in the LH state, the disk flow with a radius up to ~105 Rg would have contributed to the nearly instantaneous non-thermal radiation directly or indirectly, and therefore affects the time when the state transition occurs.

The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images

Abstract The Monitor of All-sky X-ray Image (MAXI) mission is the first astronomical payload to be installed on the Japanese Experiment Module — Exposed Facility (JEM-EF or Kibo-EF) on the International Space Station. It has two types of X-ray slit cameras with wide FOVs and two kinds of X-ray detectors consisting of gas proportional counters covering the energy range of 2 to 30 keV and X-ray CCDs covering the energy range of 0.5 to 12 keV. MAXI will be more powerful than any previous X-ray All Sky Monitor payloads, being able to monitor hundreds of Active Galactic Nuclei. A realistic simulation under optimal observation conditions suggests that MAXI will provide all-sky images of X-ray sources of $\sim $20 mCrab ($\sim $7 $\times$ 10$^{-10} $erg cm$^{-2} $s$^{-1}$ in the energy band of 2–30 keV) from observations during one ISS orbit (90 min), $\sim $4.5 mCrab for one day, and $\sim $2 mCrab for one week. The final detectability of MAXI could be $\sim $0.2 mCrab for two years, which is comparable to the source confusion limit of the MAXI field of view (FOV). The MAXI objectives are: (1) to alert the community to X-ray novae and transient X-ray sources, (2) to monitor long-term variabilities of X-ray sources, (3) to stimulate multi-wavelength observations of variable objects, (4) to create unbiased X-ray source cataloges, and (5) to observe diffuse cosmic X-ray emissions, especially with better energy resolution for soft X-rays down to 0.5 keV.

Discovery of long-term superorbital periodicities in the pseudo-transient LMXBs: IGR J17098−3628 and EXO 0748−676

Long-term monitoring of the recently discovered X-ray transient, IGR J17098−3628, by the All-Sky Monitor on-board the Rossi X-Ray Timing Explorer, has shown that it displays a long-term (≈163 d) quasi-periodic modulation in the data spanning its ‘active’ state (i.e. approximately MJD 53450–54200). Furthermore, this light curve is not typical of ‘classical’ soft X-ray transients, in that J17098−3628 has remained active since its initial discovery, and may be more akin to the pseudo-transient EXO 0748−676, which is now classified as a persistent low-mass X-ray binary (LMXB). However, EXO 0748−676 recently entered a more active phase (since approximately MJD 53050), and since then we find that it too displays a quasi-periodic modulation (≈181 d) in its light curve. This must be a ‘superorbital’ modulation, as the orbital period of EXO 0748−676 is well established (3.8 h), and hence we interpret both objects' long periods as representing some intrinsic properties of the accretion disc (such as coupled precessional and warping effects). By analogy, we therefore suggest that IGR J17098−3628 is another member of this class of pseudo-transient LMXBs and is likely to have a <1 d orbital period.

Doppler and modulation tomography of XTE J1118+480 in quiescence

We present Doppler and modulation tomography of the X-ray nova XTE J1118+480 with data obtained during quiescence using the 10-m Keck II telescope. The hot spot where the gas stream hits the accretion disc is seen in H-Alpha, H-Beta, He I Lambda-5876, and Ca II Lambda-8662, thus verifying the presence of continued mass transfer within the system. The disc is clearly seen in H-Alpha and Ca II Lambda-8662. We image the mass-donor star in narrow absorption lines of Na I Lambda-Lambda-5890, 5896, 8183, 8195 and Ca II Lambda-8662, implying an origin from the secondary itself rather than the interstellar medium. We also detect deviations in the centroid of the double peak of H-Alpha akin to those found by Zurita et al. 2002 suggesting disc eccentricity.

Detailed radio to softγ-ray studies of the 2005 outburst of the new X-ray transient XTE J1818-245

XTE J1818-245 is an X-ray nova that experienced an outburst in 2005, first seen by the RXTE satellite. The source was observed simultaneously at various wavelengths up to soft gamma-rays with the INTEGRAL satellite, from 2005 February to September. X-ray novae are extreme systems that often harbor a black hole, and are known to emit throughout the electromagnetic spectrum when in outburst. We analyzed radio, (N)IR, optical, X-ray and soft gamma-ray observations and constructed simultaneous broad-band X-ray spectra. Analyzing both the light curves in various energy ranges and the hardness-intensity diagram enabled us to study the long-term behavior of the source. Spectral parameters were typical of the Soft Intermediate States and the High Soft States of a black hole candidate. The source showed relatively small spectral variations in X-rays with considerable flux variation in radio. Spectral studies showed that the accretion disc cooled down from 0.64 to 0.27 keV in 100 days and that the total flux decreased while the relative flux of the hot medium increased. Radio emission was detected several times, and, interestingly, five days after entering the HSS. Modeling the spectral energy distribution from the radio to the soft gamma-rays reveals that the radio flares arise from several ejection events. XTE J1818-245 is likely a black hole candidate transient source that might be closer than the Galactic Bulge. The results from the data analysis trace the physical changes that took place in the system at a maximum bolometric luminosity of (0.4-0.9)e38 erg/s (assuming a distance between 2.8-4.3 kpc) and they are discussed within the context of disc and jet models.

Early x-ray burst sighting

We were intrigued by the story “X-ray Outburst Reveals a Supernova Before It Explodes” ( Physics Today 0031-9228 61 8 2008 21 https://doi.org/10.1063/1.2970204. http://dx.doi.org/10.1063/1.2970204 ), which describes the likely discovery of a core-collapse supernova by Alicia Soderberg and colleagues. 1 1. A. M. Soderberg, et al. , Nature 453, 469 (2008). https://doi.org/10.1038/nature06997 The story’s figure 2 resembles a similar x-ray light curve, reported by collaborators at Los Alamos National Laboratory, 2 2. R. D. Belian, J. P. Conner, W. D. Evans, Astrophys. J. Lett. 171, L87 (1972). https://doi.org/10.1086/180874 from an x-ray outburst that occurred on 7 July 1969 and preceded by two days the x-ray nova Centaurus XR-4. 3 3. W. D. Evans, R. D. Belian, J. P. Conner, Astrophys. J. Lett. 159, L57 (1970). https://doi.org/10.1086/180477 The spin periods of the Vela satellites that recorded the 1969 event were roughly 1 minute, and any location within the instruments’ field of view would be sampled for 2 or 3 seconds out of that period, followed by subsequent samplings every 60 seconds or so. When first observed, the precursor to the Cen XR-4 nova was already at its highest level, but the subsequent decline is almost identical to that of SN 2008D.The outburst was discernable above background for seven minutes; 2 2. R. D. Belian, J. P. Conner, W. D. Evans, Astrophys. J. Lett. 171, L87 (1972). https://doi.org/10.1086/180874 the Physics Today item indicates a similar duration for the outburst of SN 2008D. The x-ray nova part of the transient Cen XR-4 was observed two days later on 9 July 1969, the next time the satellites’ detector scanned that part of the sky.An article about the original discovery of Cen XR-4 was published right around the time the nova phase was rapidly declining. By 24 September 1969, the source was no longer visible above background. In a second article covering the known life of the Cen XR-4 x-ray nova, 3 3. W. D. Evans, R. D. Belian, J. P. Conner, Astrophys. J. Lett. 159, L57 (1970). https://doi.org/10.1086/180477 we stated that there was no definite optical identification of Cen XR-4; a nova outburst had not been reported at the location of the source. It is not clear whether Cen XR-4 was a core-collapse supernova as the similarities between it and SN 2008D suggest. But it is certainly clear that the occurrence of x-ray precursors to energetic cosmic processes was documented in the 1972 event and again in 2008.REFERENCESSection:ChooseTop of pageREFERENCES <<1. A. M. Soderberg, et al. , Nature 453, 469 (2008). https://doi.org/10.1038/nature06997 , Google ScholarCrossref, ISI2. R. D. Belian, J. P. Conner, W. D. Evans, Astrophys. J. Lett. 171, L87 (1972). https://doi.org/10.1086/180874 , Google ScholarCrossref, ISI3. W. D. Evans, R. D. Belian, J. P. Conner, Astrophys. J. Lett. 159, L57 (1970). https://doi.org/10.1086/180477 , Google ScholarCrossref, ISI© 2008 American Institute of Physics.

Outburst Morphology in the Soft X‐Ray Transient Aquila X‐1

We present optical and near-IR (OIR) observations of the major outbursts of the neutron star soft X-ray transient binary system Aquila X-1, from 1998 summer to 2007 fall. The major outbursts of the source over the observed timespan seem to exhibit two main types of light-curve morphologies, (1) the classical fast rise and exponential decay (FRED) type outburst seen in many soft X-ray transients and (2) the low-intensity state (LIS), where the optical-to-soft X-ray flux ratio is much higher than that seen during a FRED. Thus there is no single correlation between the optical (R band) and soft X-ray (1.5-12 keV, as seen by the ASM on board RXTE) fluxes even within the hard state for Aquila X-1, suggesting that LISs and FREDs have fundamentally different accretion flow properties. Time evolution of the OIR fluxes during the major LIS and FRED outbursts is compatible with thermal heating of the irradiated outer accretion disk. No signature of X-ray spectral state changes or any compact jet are seen in the OIR, showing that the OIR color-magnitude diagram (CMD) can be used as a diagnostic tool to separate thermal and nonthermal radiation from X-ray binaries where orbital and physical parameters of the system are reasonably well known. We suggest that the LIS may be caused by truncation of the inner disk in a relatively high M state, possibly due to matter being diverted into a weak outflow.

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.

Modeling of non-stationary accretion disks in X-ray novae A 0620–00 and GRS 1124–68 during outburst

Aims. We address the task of modeling soft X-ray and optical light curves of X-ray novae in the high/soft state. Methods. The analytic model of viscous evolution of an externally truncated accretion α-disk is used. Relativistic effects near a Kerr black hole and self-irradiation of an accretion disk are taken into account. Results. The model is applied to the outbursts of X-ray nova Monocerotis 1975 (A 0620−00) and X-ray nova Muscae 1991 (GRS 1124−68). Comparison of observational data with the model yields constraints on the angular momentum (the Kerr parameter) of the black holes in A 0620−00 and GRS 1124−68: 0.3−0. 6a nd≤0.4, and on the viscosity parameter α of the disks: 0.7−0.95 and 0.55−0.75. We also conclude that the accretion disks should have an effective geometrical thickness 1.5−2 times greater than the theoretical value of the distance between the photometric layers.