Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...273..147m&link_type=abstract
Astronomy and Astrophysics, Vol.273, NO. 1/JUNI, P. 147, 1993
Astronomy and Astrophysics
Astrophysics
20
Stars: Individual (Her X-1), Stars: Neutron, X-Rays
Scientific paper
The first, spectrally resolved, soft X-ray observations of Her X-1 in the SHORT-ON state are presented. The data were taken during the ROSAT All-Sky Survey performed with the Position Sensitive Proportional Counter (PS PC) covering the energy range 0.1-2.4 keV.
The source was detected at an average count rate of 25.0±0.3 cts s-1 The light curve is asymmetric between eclipse ingress and egress, and intervals of low intensity are present at orbital phases φorb=O.15, 0.19, 0.50, 0.56. Timing analysis of the data revealed a highly significant signal at a period of 1.237749(1) s at JD 2448120±1. The period is consistent with the continuous spin up observed during the last years. No period derivative was detected. The pulsed fraction is 22%, roughly half the one observed during the MAIN-ON.
The source spectrum is best fitted by a single power-law model of photon index 2.7. Single black body or bremsstrahlung spectra give unacceptable fits. The mean luminosity in the ROSAT bandpass is 1.8 × 1036 erg s-1 The flux and the neutral hydrogen absorption appear to be constant throughout the orbit with the exception of at least 3h before eclipse ingress, the "dips" and of course the eclipse. Finally, evidence is presented to support the hypothesis that the reduction in intensity during the SHORT-ON state of the 35d cycle is caused by hot matter absorption.
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