Astron. Astrophys. 354, 938-942 (2000)

1. Introduction

X-ray transients constitute an important class of binary X-ray sources. Although a majority of transient sources are low mass X-ray binaries (LMXBs), a large number (about 40%) are high mass X-ray binaries (HMXBs). Most of the HMXB transients have a X-ray pulsar with a Be star optical companion. The LMXB transients may contain either a neutron star or a black hole as X-ray source. Of the 33 LMXBs classified as transients by Tanaka & Shibazaki (1996), 18 are listed to be black-hole binaries. Thus almost all the LMXBs with a black-hole X-ray source are transients. The neutron star transients, on the other hand, comprise only a small fraction (15%) of the LMXBs with a neutron star. For a review of the LMXB transients refer to Tanaka & Shibazaki (1996).

A new transient X-ray source XTE J1748-288 was first detected with the All Sky Monitor (ASM) onboard the ROSSI X-ray Timing Explorer (RXTE) on 1998 June 4.69 UT at a flux level of about 860 mCrab in 1.5 - 12 keV band (Smith, Lewin & Wood (1998)). The source had a hard spectrum as indicated by BATSE detection of a bright object with a flux of about 310 mCrab in 20-70 keV interval on June 3 which increased to 730 mCrab on June 4 (Harmon et al. 1998). Following the discovery with the ASM, the X-ray transient was scanned with the Proportional Counter Array (PCA) on board the RXTE on June 5 to measure its position more precisely (Strohmayer & Marshall 1998). A radio candidate was detected within one arc minute of the center of the error box of the X-ray transient at a flux level of 28 2 mJy at 4.86 GHz by Hjellming & Rupen (1998a). Further radio observations revealed that the radio source became brighter and was highly variable with a flux of about 110 mJy at 4.86 GHz on June 10 which increased to 410 mJy on June 14 (Hjellming & Rupen 1998a; Fender & Stappers 1998). The VLA observations revealed the radio source to be a relativistic jet source. It was unresolved with a size of 0".1 on June 10.19 UT which became extended along the East-West with a size of 0".25 on June 14.31 UT (Rupen, Hjellming & Mioduszewski 1998). The one-sided jet was found to be linearly polarised and moving at a rate of 20 mas year^-1. This corresponds to a jet velocity of 0.93c for a distance of 8 kpc obtained from measurement of 21 cm absorption when the source was bright (0.5Jy) (Hjellming, Rupen & Mioduszewski 1998b) There is no report so far of any optical or infrared identification of the source.

Following the announcement of the discovery of XTE J1748-288, we made observations of the X-ray transient with the Pointed-mode Proportional Counters (PPCs) of the Indian X-ray Astronomy Experiment (IXAE) during 1998 June 14 - June 25. From a detailed timing analysis of the IXAE data we did not find any QPOs in the frequency range of 0.002 Hz 5 Hz in the power density spectra of XTE J1748-288. In this paper we report these results and discuss their implications.

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
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