Title: Monitoring the latest stages of neutron star soft X-ray transients with INTEGRAL
Proposal ID: 0120210
Subject category: Compact Object
Principal investigator: Campana
Institute: Osservatorio astronomico di Brera
Abstract
Soft X-ray transients (SXRTs) contain fastly rotating (a few millisecond) weaky magnetic (~10^8 G) neutron
stars.
At the outburst peak these sources show soft spectral with equivalent bremstrahlung temperature of
about 1-5 keV. As first suggested by Sigma data, at luminosities ~0.1 the Eddington value a hard tail power
law develops (with photon index 2-3) extending up to 100-200 keV. Transient sources provide the unique
opportunity of observing in the same source the soft and hard spectra (even if the transition has never been
monitored). Studiying these hard tails have clear impact on the physics of the emission processes (e.g.
temperature, optical depth) as well as on the possibility of finding a signature for separating neutron star
from black hole transients.
Given the short spin period and existing magnetic field the neutron star of
SXRTs undergoes a transition to the propeller regime around luminosities of ~10^36 erg/s. A hardness ratio
increase has been observed as marking the transition and a bright hard tail has been later observed up to
100 keV in the 1997 outburst of Aql X-1. INTEGRAL observations will allow to shed light on the physics of
the propeller. In the most favourable cases nearby SXRTs can be detected up to 1 MeV with SPI. This provides
a unique opportunity to search for spectral features in their spectra,
e.g. looking for the presence of a
jet through 511 keV emission.
We propose to follow with INTERGAL the outburst from a nearby SXRT (i.e. not
in the Galactic bulge) from the outburst peak to deep in the propeller regime. The total observing time
request amounts to 300 ks.