Pulse Profiles, Accretion Column Dips and a Flare in GX 1+4 During a Faint State

The Rossi X-ray Timing Explorer (RXTE) spacecraft observed the X-ray pulsar GX 1+4 for a period of 34 hours on July 19/20 1996. The source faded from an intensity of ~20 mCrab to a minimum of <~0.7 mCrab and then partially recovered towards the end of the observation. This extended minimum lasted ~40,000 seconds. Phase folded light curves at a barycentric rotation period of 124.36568 +/- 0.00020 seconds show that near the center of the extended minimum the source stopped pulsing in the traditional sense but retained a weak dip feature at the rotation period. Away from the extended minimum the dips are progressively narrower at higher energies and may be interpreted as obscurations or eclipses of the hot spot by the accretion column. The pulse profile changed from leading-edge bright before the extended minimum to trailing-edge bright after it. Data from the Burst and Transient Source Experiment (BATSE) show that a torque reversal occurred <10 days after our observation. Our data indicate that the observed rotation departs from a constant period with a Pdot/P value of ~-1.5% per year at a 4.5 sigma significance. We infer that we may have serendipitously obtained data, with high sensitivity and temporal resolution about the time of an accretion disk spin reversal. We also observed a rapid flare which had some precursor activity, close to the center of the extended minimum.Comment: 19 pages, 6 figures, accepted for publication in Astrophysical Journal (tentatively scheduled for vol. 529 #1, 20 Jan 2000

Detection of hard X-ray pulsations and a strong iron K_beta emission line during an extended low state of GX 1+4

We present here results obtained from a detailed timing and spectral analysis of three BeppoSAX observations of the binary X-ray pulsar GX 1+4 carried out in August 1996, March 1997, and August 2000. In the middle of the August 2000 observation, the source was in a rare low intensity state that lasted for about 30 hours. Though the source does not show pulsations in the soft X-ray band (1.0-5.5 keV) during the extended low state, pulsations are detected in 5.5-10.0 keV energy band of the MECS detector and in hard X-ray energy bands (15-150 keV) of the PDS instrument. Comparing the 2-10 keV flux during this low state with the previously reported low states in GX 1+4, we suggest that the propeller regime in GX 1+4 occurs at a lower mass accretion rate than reported earlier. Broad-band (1.0-150 keV) pulse averaged spectroscopy reveals that the best-fit model comprises of a Comptonized continuum along with an iron K_alpha emission line. A strong iron K_beta emission line is detected for the first time in GX 1+4 during the extended low state of 2000 observation with equivalent width of ~550 eV. The optical depth and temperature of the Comptonizing plasma are found to be identical during the high and low intensity states whereas the hydrogen column density and the temperature of the seed photons are higher during the low state. We also present results from pulse phase resolved spectroscopy during the high and low flux episodes.Comment: Accepted for publication in The Astrophysical Journa

Accretion column disruption in GX 1+4

Daily observations of the binary X-ray pulsar GX 1+4 were made with the Rossi X-ray Timing Explorer (RXTE) satellite between 1997 May 16-20 as part of a four-month monitoring program. On May 17 the sharp dips normally observed in the lightcurve were all but absent, resulting in a pulse fraction f_p approx. 0.5 instead of the more typical value of approx. 0.8 measured before and after. Also observed was a dramatic hardening of the 2-40 keV phase-averaged spectrum. The power-law photon index was 1.16 +/- 0.02, whereas values of 1.6-2.0 are more typical. In terms of a Comptonization continuum component, the optical depth for scattering was tau approx 19, with 4-6 the usual range for RXTE spectra (Galloway 2000). Pulse-phase spectrosopy indicates that tau is decreased relative to the phase-averaged value around the primary minimum, where an increase is normally observed. The reduced depth of the dip is interpreted as disruption of the accretion column, and the accompanying spectral variation suggests a substantially different accretion regime than is usual for this source.Comment: 10 pages, 2 figures, minor abstract typo and wording of final paragraph correcte

A Puzzling Paucity of Double Peaked X-ray Pulsars

Accretion powered pulsars exhibit a variety of lightcurves. In this paper we propose to classify the observed lightcurves as single or double pulsed.We analyze the lightcurves of 86 accretion powered pulsars and assign them to these classes. We present three datasets: first in which the classification can be easily done, second for which the classification is more difficult and not certain, and third for which we were unable to classify the pulsar because of lack of published data. We analyze a simple model in which the angles between the magnetic and rotation axis $\beta$, and between the rotation axis and the line of sight $\theta$ are random, and show that it is inconsistent with the data. We also present a model in whichthe angle between the magnetic axis and rotation axis is restricted and compare it with the data. This leads to an upper limit on the angle $\beta < 40^\circ$. We conclude that there must be a mechanism that leads to alignment of the magnetic and spin axis in X-ray pulsars.Comment: submitted to A&

Large resistivity in numerical simulations of radially self-similar outflows

We investigate conditions in a radially self-similar outflow in the regime of large resistivity. Using the PLUTO code, we performed simulations with proper choice of boundary conditions, relaxed at the footpoints of critical surfaces in the flow. We investigate outflow propagation in a high-resistive disk corona, and compare it to the results with small or vanishing resistivity. © International Astronomical Union 2011