B. Alan Harmon

GX 339-4: The Distance, state transitions, hysteresis and spectral correlations

We study X-ray and variability and distance of GX 339–4. We derive the distance of > 7 kpc, based on recent determination of the binary parameters. We study data from the ASM aboard Ginga, the BATSE aboard CGRO, and the ASM, PCA and HEXTE aboard RXTE. From 1987 to 2004, GX 339–4 underwent �15 outbursts and went through all known states of black-hole binaries. For the first time, we present the PCA data from the initial hard state of the outburst of 2004. We then study colourcolour and colour-flux correlations. In the hard state, there is a strong anticorrelation between the 1.5–5 and 3–12 keV spectral slopes, which we explain by thermal Comptonization of disc photons. There is also a softening of the spectrum above 3 keV with the increasing flux that becomes stronger with increasing energy up to �200 keV. This indicates an anticorrelation between the electron temperature and luminosity, explained by hot accretion models. In addition, we see a variable broad-band slope with a pivot at �200 keV. In the soft state, there is a high energy tail with varying amplitude beyond a strong and variable blackbody component. We confirm the presence of pronounced hysteresis, with the hard-to-soft state transitions occurring at much higher (and variable) luminosities than the soft-to-hard transitions. We fit the RXTE/ASM data with a model consisting of an outer accretion disc and a hot inner flow. State transitions are associated then with variations in the disc truncation radius, which we fit as � 6GM/c 2 in the soft state and several times that in the hard state. The disappearence of the inner disc takes place at a lower accretion rate than its initial appearance due to the dependence of the transitions on the source history. We provide further evidence against the X-ray emission in the hard state being nonthermal synchrotron, and explain the observed radio-X-ray correlation by the jet power being correlated with the accretion power.

Observations of the X-ray Nova GRO J0422+32. 1: Outburst and the decay to quiescence

We present optical photometry and spectroscopy and Burst and Transient Source Experiment (BATSE) observations of the X-ray nova GRO J0422+32, obtained during outburst and its subsequent decay to quiescence. Although the X-ray and optical properties of GRO J0422+32 are broadly similar to those of other X-ray novae, it is unique in several respects. The unusually protracted decay to quiescence of the optical light curve has been punctuated by at least two minioutbursts of approximately 4 mag. The BATSE and optical outbursts are each separated by approximately 120 days. We find that the optical luminosity of GRO J0422+32 during the primary outburst is dominated by reprocessing of E greater than 10 keV X-rays. In contrast, the optical minioutbursts are most likely generated by an intrinsically bright disk rather than X-ray reprocessing: they do not appear to have any X-ray counterparts. Extremely broad (up to 6000 km/s FWZI) absorption lines have also been observed during both primary outbursts and minioutbursts. During the second minioutburst, H-alpha and H-beta emission was observed superposed on redshifted absorption features. We find that the interoutburst light curve of GRO J0422+32 may be inconsistent with an accretion disk instability as the origin of the minioutbursts. Finally, a transient 5.1/10.2 hr modulation, which may be related to the orbital period, has been observed during roughly half of our observations. However, confirmation of the orbital period must await observations in quiescence.

A Sequence of Outbursts from the Transient X-Ray Pulsar GS 0834–430

GS 0834-430, a 12.3 s accretion-powered pulsar, has been observed in seven outbursts with the BATSE large-area detectors on the Compton Gamma Ray Observatory. The first five outbursts observed by BATSE occurred at intervals of about 107 days, while the final two outbursts were separated by about 140 days. The photon energy spectrum, measured by Earth occultation in the 20-100 keV band, can be fitted by a power law with photon index α ≈ -3.7 or by an exponential spectrum with temperature kT ≈ 15 keV, with some variations within outbursts. The source has a low pulse fraction, ≾ 0.15 in the 20-50 keV band. We have observed significant temporal and energy-dependent variations in epoch folded pulse profiles. Because the intrinsic torque effects for this system are at least comparable to orbital effects, pulse timing analysis did not produce a unique orbital solution. However, confidence regions for the orbital elements yielded the following 1 σ limits: orbital period P_(orb) = 105.8 ± 0.4 days and eccentricity 0.10 ≾ e ≾ 0.17. GS 0834-430 is most likely a Be/X-ray binary.

Hard x‐ray observations of A 0535+262

Three outbursts of the x‐ray binary system A 0535+262 have been detected in the 20–120 keV energy range by the BATSE instrument onboard the Compton Gamma Ray Observatory. The system orbital parameters have been determined by a pulse timing analysis during data from these outbursts.

Discovery of the hard x‐ray pulsar GRO J1008‐57 by BATSE

BATSE detection of a new hard x‐ray pulsar is reported. The source was first observed on 14 Jul 1993, reached a maximum intensity on 25 Jul 1993, then declined smoothly until it became undetectable on 16 Aug 1993. The BATSE location was adequate to permit OSSE, ASCA, and ROSAT observations, leading to an improved source location. The observed period at the solar system barycenter was 93.548±0.002s, 93.5665±0.0005s, and 93.541±0.004 on 15 Jul 1993, 23 Jul 1993, and 10 Aug 1993, respectively. The source is detected between ∼20 keV and 160 keV, with a spectrum fit by an optically thin thermal bremsstrahlung form, having a kT of 25.4±2.1 keV during the rise to maximum intensity, decreasing monotonically during the remainder of the outburst to 17.1±2.5 keV on 5 Aug 1993. The pulse profile in this energy range has a single broad peak, with the maximum occurring later at higher energies. The pulse profile, intensity history, and spectral behavior observed by BATSE are reported.

EREBUS: An experiment to REveal the BUrster sites

There have been several suggestions that GRBs, if galactic, might be detected from M31. With the current crisis in determining the burster distance scale, this experiment has become crucial. EREBUS has been designed to detect a significant burst signal from M31 for any reasonable galactic model that is consistent with the BATSE isotropy and inhomogeneity constraints. The experiment uses 16 broadly collimated 2000 cm2 NaI detectors, obtaining a factor of ∼10 improvement in peak sensitivity over BATSE. The similarity to the BATSE detectors eliminates spectral and temporal free parameters in model fits.