Astronomy and Astrophysics – Astronomy
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...427l..25h&link_type=abstract
Astrophysical Journal, Part 2 - Letters, vol. 427, no. 1, p. L25-L28
Astronomy and Astrophysics
Astronomy
43
Binary Stars, Magellanic Clouds, Neutron Stars, Pulsars, Stellar Luminosity, Stellar Models, X Ray Astronomy, X Ray Spectra, X Ray Stars, Calibrating, Position Sensing, Proportional Counters, Rosat Mission
Scientific paper
This article reports the discovery of a luminous (3.5 x 1037 ergs/sec over the 0.2 to 2 keV band) transient X-ray pulsar in the Small Magellanic Cloud (SMC) with an extremely soft component to its X-ray spectrum. This is the first time that a spectrum of this type has been seen in this class of X-ray source. The pulse period is 2.7632 s, and the pulse modulation appears to vary with energy from nearly unpulsed in the low-energy band of the ROSAT Position Sensitive Proportional Counter (PSPC) (0.07 to 0.4 keV) to about 50% in the high-energy band (1.0 to 2.4 keV). The object, RX J0059.2-7138, also shows flickering variability in its X-ray emission on timescales of 50 to 100s. The pulse-phase-averaged PSPC X-ray spectrum can be well described by a two-component source model seen through an absorbing column density of approximately 1021 atoms cm-2. One spectral component is a power law with photon index 2.4. The other component is significantly softer and can be described by either a steeply falling power law or a blackbody with a temperature KTBB approximately 35 eV. Ths component is transient, but evidently upulsed, and, for the blackbody model fits, requires a large bolometric luminosity: near, or even several times greater than, the Eddington luminosity for a 1.4 solar mass object. When these characteristics of its soft emission are considered, RX J0059.2-7138 appears quite similar to other X-ray sources in the magellanic Clouds, such as CAL 83, CAL 87, and RX J0527.8-6954, which show only extreme ultrasoft (EUS) X-ray spectra. The discovery of RX J0059.2-7138, a probably high-mass X-ray binary, clearly indicates that EUS spectra may arise from accretion-powered neutron-star X-ray sources. This result lends support to the idea that some of the 'pure' EUS sources may be shrouded low-mass X-ray binaries rather than accreting white dwarfs.
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