1994A&A...285..503M

The accreting binary pulsar 4U 1626-67 was detected in the soft X-rays by ROSAT in the course of the All-Sky survey at an average luminosity of 1.5 x 10³⁵ d²_5 kpc_ erg/s, the lowest observed so far. The ROSAT spectrum is better fitted by a bremsstrahlung model rather than a blackbody spectrum. The source spectrum appears to be absorbed by a column density consistent with the interstellar one. The pulse period determined from the survey data indicates a decrease in the spin-up rate. A previous pointed EXOSAT observation performed in 1983 is also analysed. Its light curve is dominated by a series of flares, characteristic to the source. Excess flux in the low energies is present in the EXOSAT GSPC spectrum of the source. There is no indication for significant photoelectric absorption in the hard X-ray spectrum. The pulse period established from the EXOSAT ME data fits nicely the historical spin-up trend while the ME pulse profiles are energy dependent, in agreement with other observations. Finally, evidence is presented suggesting a decrease in the luminosity of the soft component over the last several years. A reduction in the overall luminosity of the system is also supported by the pulse spin-down observed recently by the Gamma Ray Observatory.