Astron. Astrophys. 334, L65-L68 (1998)

2. ROSAT observations

The PSPC (0.1-2.4 keV) and HRI (0.1-2.4 keV) detectors on board ROSAT observed the SMC field including 2E 0053.2-7242 several times. In the ROSAT public archive there are 13 observations performed between 1991 October and 1996 April: 9 were carried out with the PSPC and 4 with the HRI. The PSPC images, spectra and light curves were accumulated and corrected for the effective exposure map. This is particularly relevant to minimize the effects of the wobble in the pointing direction on count rate measurements of sources near the edge of the field of view or the ribs of the detector window support structure. The vignetting correction was also taken into account and the effective exposure time obtained for each PSPC pointing.

A sliding cell detection algorithm was used in order to characterise the physical parameters, such as position, count rate (90% confidence level), S/N ratio, etc., of 2E 0053.2-7242 when detected, and to obtain a count rate upper limit in case of non-detections. Table 1 summarises the results of this analysis.

Table 1. ROSAT observations of 2E 0053.2-7242

2E 0053.2-7242 was only detected on three occasions: twice with the PSPC (1991 October 8-9 and 1992 April 17-27) and once with the HRI (1996 April 26 - June 10).

2.1. PSPC data

The ROSAT event list and spectrum of 2E 0053.2-7242 were extracted from a circle of radius (corresponding to an encircled energy of 95%) around the X-ray position. On 1991 October 8-9 (sequence number 600195A00) the source count rate was 0.047 cts s^-1, while on 1992 April 17-27 (600195A01) it had decreased to about 0.01 cts s^-1. Of the 600 and 150 photons contained in the extraction circle of each of the two pointings, we estimated that about 45 and 20 photons derive from the background around the source, respectively.

The 1991 October observation is the only one that contains a sufficiently high number of photons to perform a detailed periodicity search. The photons arrival times were corrected to the barycenter of the solar system and a 1 s binned light curve accumulated. The corresponding power spectrum, calculated over the entire observation duration ( 1 day), is shown in Fig. 1, together with the preliminary peak detection threshold described by Israel & Stella (1996). The search was performed over a period interval around that detected by Beppo SAX and assuming a maximum P_ of 3 s yr^-1 (the highest ever observed from an X-ray pulsar: GX1+4). This translates into a search over an interval of 1100 Fourier frequencies centered on 0.017 Hz. Significant peaks were detected around a frequency of 0.0169 Hz. These are unique to 2E 0053.2-7242. The multiple peak structure is due to the sidelobes arising from the satellite orbital occultation. The highest of these peaks has a significance of over the explored frequency interval, and corresponds to a period of 59.072 0.003 s (90% uncertainties are used through this letter). The modulation is energy independent in the PSPC band (within the statistical uncertainties). The shape is fairly asymmetric with a pulse fraction of 40% (Fig. 1, inner panel). By using the period measured by Beppo SAX a period derivative of - 0.016 s yr^-1 was obtained.

Fig. 1. Oct 1991 power spectrum of the 0.1-2.4 keV ROSAT PSPC light curve of 2E 0053.2-7242. The preliminary detection threshold is also shown. A folded light curve at the best period of 59.072 s is shown as an insert. The peak structures around a frequency of 2.5 10-3 Hz are due to the wobble in the pointing direction

The PSPC Pulse Hight Analyser (PHA) rates were grouped so as to contain a minimum of 20 photons per energy bin. The spectrum of the 1991 October observation was well fit with an absorbed power-law model (see Fig. 2; upper curve). The best fit ( degree of freedom - dof = ) was obtained for a photon index of = 0.90 and a column density of = (1.3 ) 10²¹ cm^-2 (see Table 2). Note that the Galactic hydrogen column in the direction of the SMC is 6 10²⁰ cm^-2. The 0.1-2.4 keV luminosity at the source is 4.2 10³⁵ erg s^-1 for an assumed distance of 65 kpc (Wang & Wu 1992). We note that any further spectral component added to the power-law model in order to fit the data excess around 1.2 keV did not significantly improve the fit. For the 1992 April observation the number of photons (150) is too low to obtain an indipendent estimate of the spectral parameters. By keeping the photon index fixed to the best fit value of the 1991 October observation, an unabsorbed X-ray luminosity of 1.5 10³⁵ erg s^-1 was derived (see Table 2 and Fig. 2).

Fig. 2. ROSAT PSPC spectrum of 2E 0053.2-7242 during 1991 October and 1992 April. The best fit power-law is shown, together with the corresponding residuals. The 1992 Apr 17-27 data and residuals are marked with triangles

Table 2. ROSAT PSPC spectral results for 2E 0053.2-7242

2.2. HRI data

The ROSAT HRI observation during which 2E 0053.2-7242 was detected (sequence 600810; 1996 April-June) provided a significantely improved source position (Israel 1998). This was determined to be R.A. = and DEC = (equinox 2000; error radius of with a 90% confidence level) by using both a sliding cell and a Wavelet transform-based detection algorithm (Lazzati et al. 1998; Campana et al. 1998). However due to the unknown boresight correction the uncertainty radius increases up to .

A 0.1-2.4 keV source flux level of 1.8 10^-13 erg cm^-2 s^-1 was determined assuming the best fit spectral parameters of the 1991 October 8-9. This translates into an unabsorbed luminosity of 10³⁴ (Israel 1998).

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

helpdesk.link@springer.de