Wan Chen

The 1996 outburst of GRO J1655-40: the challenge of interpreting the multiwavelength spectra

We report on the results of a multiwavelength campaign to observe the soft X-ray transient (SXT) and superluminal jet source GRO J1655-40 in outburst using HSTRXTECGRO together with ground-based facilities. This outburst was qualitatively quite different from other SXT outbursts and from previous outbursts of this source. The onset of hard X-ray activity occurred very slowly, over several months, and was delayed relative to the soft X-ray rise. During this period, the optical fluxes declined steadily. This apparent anticorrelation is not consistent with the standard disc instability model of SXT outbursts, nor is it expected if the optical output is dominated by reprocessed X-rays, as in persistent low-mass X-ray binaries. Based on the strength of the 2175-Angstroms interstellar absorption feature we constrain the reddening to be E(B-V)=1.2+/-0.1, a result which is consistent with the known properties of the source and with the strength of interstellar absorption lines. Using this result we find that our dereddened spectra are dominated by a component peaking in the optical, with the expected nu^1/3 disc spectrum seen only in the ultraviolet. We consider possible interpretations of this spectrum in terms of thermal emission from the outer accretion disc and/or secondary star, both with and without X-ray irradiation, and also as non-thermal optical synchrotron emission from a compact self-absorbed central source. In addition to the prominent Heii 4686-Angstroms line, we see Bowen fluorescence lines of Niii and Oiii, and possible P Cygni profiles in the ultraviolet resonance lines, which can be interpreted in terms of an accretion disc wind. The X-ray spectra broadly resemble the high-soft state commonly seen in black hole candidates, but evolve through two substates. Taken as a whole, the outburst data set cannot readily be interpreted by any standard model for SXT outbursts. We suggest that many of the characteristics could be interpreted in the context of a model combining X-ray irradiation with the limit-cycle disc instability, but with the added ingredient of a very large disc in this long-period system.

The timing evolution of 4U 1630-47 during its 1998 outburst

We report on the evolution of the timing of 4U 1630(47 during its 1998 outburst using data obtained with the Rossi X-ray T iming Explorer (RXT E). The count rate and position in hardness inten- sity, color-color diagrams, and simple spectral —ts are used to track the concurrent spectral changes. The source showed seven distinct types of timing behavior, most of which show diUerences with the canonical black hole spectral/timing states. In marked contrast to previous outbursts, we —nd quasi-periodic oscil- lation (QPO) signals during nearly all stages of the outburst with frequencies between 0.06 and 14 Hz and a remarkable variety of other characteristics. In particular, we —nd large (up to 23% rms) amplitude QPOs on the early rise. Later, slow 0.1 Hz semiregular short (D5 s), 9%¨16% deep dips dominate the light curve. At this time there are two QPOs, one stable near 13.5 Hz and the other whose frequency drops from the 6¨8 Hz range to D4.5 Hz during the dips. BeppoSAX observations during the very late declining phase show 4U 1630(47 in a low state. Subject headings: accretion, accretion disksbinaries: closestars: individual (4U 1630(47) ¨ X-rays: stars 0.03 and 0.3 Hz, and the energy spectrum is well —t with a power law with index of ! \( 1.5 to (2.5 up to at least 200 keV (Gilfanov et al. 1994). In the high state (HS), the PDS shows weak (few percent rms) power-law noise. The energy spectrum is dominated by a soft component that can be satisfactorily —t by a multitemperature disk blackbody. The —tted temperature (kT D 1 keV) and innermost radius are comparable to values expected for a stellar-mass black hole. In the very high state (VHS), the soft component increases in —ux by a further factor of 2¨8 over that in the high state. The power-law component strengthens and is visible in the energy spectra. The power density spectrum shows a variable broadband component and 3¨10 Hz quasi-periodic oscillation (QPO). The noise component is either band-limited with a break between 1 and 20 Hz or a power law similar to that in the high state. Occasionally at times between an HS and an LS, an intermediate state (IS) is observed, e.g., GX 339(4

The secondary maxima in black hole X-ray nova light curves : clues toward a complete picture

We study the secondary maxima observed commonly in the X-ray/optical light curves of black hole X-ray novae and show that they can play an important role in our understanding of the X-ray nova phenomenon. We discuss the observational characteristics of the secondary maxima and possible mechanisms to produce them. We propose a complete scenario for black hole X-ray nova events. The main outburst is caused by a disk instability. The second maximum is caused by X-ray evaporation of the matter near the inner Lagrangian (L1) region when the disk becomes optically thin. The third maximum (or the final minioutburst) is due to a mass transfer instability caused by hard X-ray heating of the subphotospheric layers of the secondary during the outburst. We predict that the newly discovered X-ray nova GRO J0422 + 32 may develop a final minioutburst in early 1993 and that its binary orbital period is less than 7 hr.

Understanding COMPTEL Al-26 1.8 MeV map features

The COMPTEL sky map of the 1.8 MeV line emission from Al-26 shows an extended diffuse distribution along the Galactic plane with a peculiar large scale asymmetry about the Galactic center (GC) and a clumpy structure with several noticeable hot spots. The most prominent hot spot at the GC appears shifted to positive longitude by about 2 deg. Nearby supernova remnants or Wolf-Rayet stars are plausible explanations for individual hot spots such as in the Vela region. We show that the global asymmetry and most hot spots can be understood by a more general model: the Al-26 sources are confined in the spiral arms of our Galaxy, and coagent star formation is responsible for additional clumpiness. The off-centered GC hot spot is probably due to the observed asymmetrically distributed circumnuclear molecular gas interacting with a central stellar bar, whose existence has been inferred from IR observations.

ON THE OPTICAL COUNTERPARTS, LONG-TERM VARIABILITIES, RADIO JETS, AND ACCRETION SOURCES IN 1E 1740.7-2942 AND GRS 1758-258

In this paper we discuss a variety of issues concerning the exciting and mysterious Galactic center gamma-ray sources 1E 1740.7-2942 and GRS 1758-258. We discuss the problem associated with the highly uncertain X-ray absorption column toward 1E 1740.7-2942 and use the recent Roentgen Satellite (ROSAT) results to narrow its range to 0.5-1 x 10(exp 23)/sq cm. Then the current upper limits from deep optical and near-IR searches of stellar objects at these source locations are plotted on an H-R diagram, from which we find the mass of a potential companion star of the (supposed) black hole in GRS 1758-258 to be less than 4 solar mass and in 1E 1740.7-2942 to be less than 9 solar mass. The observed well-collimated radio jets in 1E 1740.7-2942 require the existence of a stable accretion disk (presumably from binary accretion). The apparent association of 1E 1740.7-2942 with a high-density molecular cloud, on the other hand, points to possible accretion directly from the interstellar medium (ISM). We present an analysis of the energetics and kinematics of the radio jets in 1E 1740.7-2942. We present the long-term X-ray light curves of the two sources which include both the Granat/SIGMAs 3 yr monitoring data and all the data from previous imaging balloon and satellite observations over the last decade. The possible physical mechanisms responsible for producing both the long-term X-ray variations and the radio jets are postulated. We also consider Roche lobe-overflowing, low-mass X-ray binaries and Bondi-Hoyle accretion directly from a high-density surrounding medium. We propose a plausible scenario in which both sources are binary systems with a black hole primary and a low-mass companion and they are accreting mainly from the ISM at a rate self-regulated by the interaction between the accretion flow and the emerging hard X-ray flux.

An interpretation of the narrow positron annihilation feature from X-ray Nova Muscae 1991

We study the physical mechanisms responsible for producing the narrow, redshifted positron annihilation γ-ray line from X-ray Nova Muscae 1991 observed by GRANAT/SIGMA. We argue that the line centroid redshift is most probably of gravitational origin, and that the annihilation very likely takes place in the inner region of an accretion disk in the system. The narrow line width can then be naturally explained by a small inclination angle of the accretion disk to our line of sight. The recently measured photometric binary mass function of Remillard, McClintock, & Bailyn (1992) gives the black hole mass for this system as a function of orbital inclination angle which we find it to be 26°±25°