R. K. Sood

Quenching of the Radio Jet during the X-Ray High State of GX 339−4

We have observed the black hole candidate X-ray binary GX 339-4 at radio wavelengths before, during, and after the 1998 high/soft X-ray state transition. We find that the radio emission from the system is strongly correlated with the hard X-ray emission and is reduced by a factor of ≥25 during the high/soft state compared with the more usual low/hard state. At the points of state transition, we note brief periods of unusually optically thin radio emission that may correspond to discrete ejection events. We propose that in the low/hard state, black hole X-ray binaries produce a quasi-continuous outflow, that in the high/soft state, this outflow is suppressed, and that state transitions often result in one or more discrete ejection events. Future models for low/hard states, such as advection-dominated solutions, need to take into account the strong outflow of relativistic electrons from the system. We propose that the inferred Comptonizing corona and the base of the jetlike outflow are the same thing, based on the strong correlation between radio and hard X-ray emission in GX 339-4 and other X-ray binaries and on the similarity in inferred location and composition of these two components.

Complex outburst behaviour from the black hole candidate 4U1630—47

We present data from different epochs in 1978, 1987-1991 and 1996 from the black-hole candidate 4U1630-47. For the first time we present almost complete coverage of the outbursts which started in 1987, 1988 and 1996. We find that the outburst behaviour of 4U1630-47 is more complex than previously realized. The source shows outbursts with durations on the order of ~100-200 days and sometimes intervals of long-term X-ray activity. The relatively short outbursts which occurred in 1987 and 1996 exhibited different colour behaviour: the outburst in 1987 showed softening of the X-ray spectrum, whereas the outburst in 1996 showed hardening of the X-ray spectrum, as the outbursts proceeded. The outburst which started in 1977 may have lasted for up to ~10 months, whereas the outburst which started in 1988 showed activity for up to ~2.4 years. Such long-term activity is reminiscent of that seen in GRS1716-249 and in the galactic superluminal sources GRS1915+105 and GROJ1655-40. We refine the outburst ephemeris of 4U\1630-47 and find that the outburst recurrence time scale may have changed from ~600 days to ~690 days between the 1984 and 1987 outbursts. If the recurrence interval of ~690 days continues, the next outburst of 4U1630-47 is predicted to occur in 1998 January.

A detailed study of 2S 0114+650 with the Rossi X‐ray Timing Explorer

We present the results of a detailed study of the high mass X-ray binary 2S 0114+650 made with the pointed instruments onboard the Rossi X-ray Timing Explorer. The spectral and temporal behaviour of this source was examined over the pulse, orbital, and super-orbital timescales, covering ∼2 cycles of the 30.7 d super-orbital modulation. Marginal evidence for variability of the power law photon index over the pulse period was identified, similar to that observed from other X-ray pulsars. If this variability is real it can be attributed to a varying viewing geometry of the accretion region with the spin of the neutron star. Variability of the neutral hydrogen column density over the orbital period was observed, which we attribute to the line of sight motion of the neutron star through the dense circumstellar environment. A reduction in the power law photon index was observed during the orbital maximum, which we speculate is due to absorption effects as the neutron star passes behind a heavily absorbing region near the base of the supergiant companion’s wind. No significant variability of the column density was observed over the super-orbital period, indicating that variable obscuration by a precessing warp in an accretion disc is not the mechanism behind the super-orbital modulation. In contrast, a significant increase in the power law photon index was observed during the super-orbital minimum. We conclude that the observed super-orbital modulation is tied to variability in the mass accretion rate due to some as yet unidentified mechanism.

Super-orbital period in the high-mass X-ray binary 2S 0114+650

We report the detection of a stable super-orbital period in the high-mass X-ray binary 2S 0114+650. Analyses of data from the Rossi X-ray Timing Explorer All-Sky Monitor from 1996 January 5 to 2004 August 25 reveal a super-orbital period of 30.7±0.1 d, in addition to confirming the previously reported neutron star spin period of 2.7 h and the binary orbital period of 11.6 d. It is unclear if the super-orbital period can be ascribed to the precession of a warped accretion disc in the system.

The X-ray fast-time variability of Sco X-2 (GX 349+2) with RXTE

Sco X-2 (GX 349+2) is a low-mass X-ray binary and Z source. We have analysed ∼156 ks of Rossi X-ray Timing Explorer data, obtained in 1998 January, on this source. We investigated the fast-time variability as a function of position on the Z track. During these observations, Sco X-2 traced out the most extensive Z track ever reported from this object, making this the most comprehensive study thus far. We found the broad peaked flaring branch noise that is typical of Sco X-2, with a centroid frequency in the range 3.3-5.8 Hz. We also discovered low-frequency noise, and a new peaked noise feature, with centroid frequencies in the range 5.4-7.6 and 11-54 Hz, respectively. We discuss the phenomenology of these features, their relationship with the power spectral components found in other low-mass X-ray binaries, and the implications for current models. In particular, the low-frequency noise we observed was strongest at intermediate energies, in contrast to the low-frequency noise seen in other Z sources. We also detected very low-frequency noise, and have calculated complex cross-spectra between intensity and hardness. We found that the very low-frequency noise is not entirely a result of motion along the Z track.

Gas amplification in high pressure proportional counters

Abstract We have made absolute gas gain measurements for proportional counters filled with argon/xenon at various pressure up to ∼ 30 atm (3000 kPa). Our data show that of the various empirical formulae which give gas gain as a function of electric field and gas pressure, the relation proposed by Charles [J. Phys. E5 (1972) 95] gives the best fit over a wide range of pressures for both argon and xenon. The measurements further reveal that gas gain data at low pressures cannot be extrapolated to higher pressures to explain the behaviour of proportional counters filled to such high pressures, because of the changing mobility of positive ions. The details of our measurements and the results obtained are described in this paper.

Energy Spectrum and Pulse Profiles of the Binary Pulsar GX 1+4

The X-ray pulsar GX 1+4 was observed during a balloon flight from Alice Springs, Australia on November 20, 1986. Significant flux was detected over the full energy range of the detector (15 – 114 keV). The spectrum was very hard with a photon power law index of 1.9 ± 0.2. The intensity at 30 keV was (5.1±0.2) × 10−4 cm−2s−1keV−1; a factor of ∼ 3 less than typical during the 1970s but orders of magnitude greater than during recent satellite measurements. The pulse profiles below ∼ 75 keV were very broad with indications of a notch at the peak. At higher energies there was a remarkable change with a narrow pulse of phase width ∼ 0.2 centred on the phase of the lower energy notch. We believe this to be the first observation of phase reversals in pulse profiles in the hard X-ray energy range. The observational evidence can be explained within the framework of a model based on two-photon emission as the dominant continuum source. This requires a magnetic field ∼ 2 × 1013 gauss — a value consistent with accretion models in which the system has a retrograde disc.

Relationship between the Jovian magnetospheric plasma density and Io torus emission

[1] Ground-based observations of [SII] 673.1 nm emissions from the lo plasma torus (IPT) showed a year-to-year decrease between 1997 and 2000. This phenomenon suggests that the electron and ion densities in the IPT decreased during the period, since an excitation source of the [SII] 673.1 nm emissions is an electron impact. Using Galileo plasma wave subsystem (PWS) data, local electron density in the magnetosphere (30-60 Rj) was investigated. The electron density in the middle and outer magnetosphere also showed a long-term decrease between 1997 and 2000. This concurrent long-term variation in the magnetospheric electron density with the IPT emissions verifies that the IPT gives influence to the magnetospheric plasma environment, as has been suggested since the Voyager era. Relative amplitudes of variations in the [SII] intensity and electron density were, however, different from each other.

Recurrent ~24 h Periods in RXTE ASM Data

Analysis of data from the Rossi X-ray Timing Explorer satellites All Sky Monitor (ASM) instrument for several X-ray binary sources has identified a recurrent ~24 h period. This period is sometimes highly significant, giving rise to the possibility of it being identified as an orbital or super-orbital period. Further analysis has revealed the same period in a number of other X-ray sources. As a result this period has been discounted as spurious, described variously as arising from daily variations in background levels, the scheduling of ASM observations and beating between the sampling period and long-term secular trends in the light curves. We present here an analysis of the spurious periods and show that the dominant mechanism is in fact spectral leakage of low-frequency power present in the light curves.

Ultra-high pressure proportional counters Part II. Xenon

Abstract Low-pressure argon/xenon-filled proportional counters have been used extensively for the detection of photons and charged particles in the fields of nuclear physics and X-ray astronomy. In general, gas counters have better energy resolution, lower background, simplicity of structure and lower cost compared to semiconductor and scintillation detectors but suffer from low detection efficiency at higher photon energies. To extend the use of gas detectors into the low-energy gamma-ray region, and examine their physical limitations, we have been pursuing a programme to develop and fabricate ultra-high-pressure argon/xenon-filled counting modules with pressures up to 2750 kPa (∼ 400 psi) for the detection of X-ray photons up to 1 MeV. This development is aimed at a new large-area balloon-borne detector system for low-energy gamma-ray astronomy. This paper concerns the detailed description of the gas multiplication behaviour and pulse properties for a detector filled with argon mixtures at high pressures.