A new transient pulsar in the Small Magellanic Cloud with an unusual X-ray spectrum

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This article reports the discovery of a luminous (3.5 × 1037 ergs s-1 over the 0.2-2 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 PSPC (0.07-0.4 ke V) to about 50% in the high-energy band (1.0-2.4 ke V). The object, RX J0059.2 - 7138, also shows flickering variability in its X-ray emission on timescales of 50-100 s. 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 ∼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 ∼ 35 eV. This component is transient, but evidently unpulsed, 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 M 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 probable 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.

Original languageEnglish (US)
Pages (from-to)L25-L28
JournalAstrophysical Journal
Issue number1 PART 2
StatePublished - May 20 1994
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Binaries: general
  • Pulsars: individual (RX J0059.2-7138)
  • Stars: emission-line, Be
  • Stars: neutron
  • X-rays: stars

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