1SAX J1324.4-6200: a possible low-mass X-ray binary pulsar

Abstract

We present results of a long ASCA observation made on 2000 February 2, of the X-ray pulsar 1SAX 1324.4-6200, in which the pulse period P s p i n was found to be 171.245 ′ 0.012 s, and a possible orbital period P o r b of27 ′ 1 h was discovered. P s p i n is significantly larger than the pulse period determined from the previous observations with BeppoSAX (1997 August) and ASCA (1994 August), and the three period values spread over a period of 5.5 yr agree well with a linear spin-down of the pulsar. The best-fitting rate of spin-down, P, is +5.2063 ′ 0.0005 x 10 - 9 s s - 1 , and P/P is +3.04 × 10 - 1 1 s - 1 , or +9.59 x 10 - 4 yr - 1 . However, we cannot rule out the possibility that the overall trend is of spin-up, and the observations represent fluctuations of spin-down. The ASCA X-ray spectrum of 1SAX 1324.4-6200 is rather featureless with an upper limit for a narrow iron line at 6.4 keV of 80 eV. The possible nature of the pulsar is discussed, the non-transient nature making it unlikely to be a Be system. It is possible that it is a wind-fed supergiant; however, this would require the overall trend in period evolution to be that of spin-up. If the tentative orbital period is real, the supergiant has to be lobe-filling. In fact, the observational evidence is more consistent with the object being a low-mass X-ray binary (LMXB) pulsar, including both spin and orbital periods, and the 5.5-yr apparent duration of spin-down. In this case, the object would join the small number of LMXB pulsars known.