Astron. Astrophys. 339, L41-L44 (1998)

1. Observations

1.1. BeppoSAX

BeppoSAX (Boella et al. 1997) observed XB 1323-619, a low-mass X-ray binary (Parmar et al. 1998) between 1997 August 22 17:06 and August 24 02:02 UTC. The Narrow Field Instruments on BeppoSAX include the LECS (0.1-10 keV) and three MECS (1.8-10 keV) detectors, each at the focus of imaging telescopes. The fields of view (FOV) of the LECS and MECS are and , respectively. The exposures in the LECS and MECS are 15 and 70 ks, respectively. Data from the MECS 2 and 3 (MECS1 failed in 1997 May) are summed. The MECS image, shown in Fig. 1, reveals in addition to XB 1323-619, the presence of 3 serendipitous sources. Sources A and J are 2E 1322.2-6157 and 2E 1325.5-6138, previously detected with the Einstein IPC (Parmar et al. 1989, P89). Of interest here is the new source located from XB 1323-619, with a count rate of s^-1. The J2000 coordinates, derived from the MECS data, are R.A.=, Dec= (galactic ()= (306.793, 0.609)) with an uncertainty radius of (limited by the current uncertainty in the BeppoSAX position reconstruction for sources off-axis). We designate the source 1SAX J1324.4-6200. In the LECS the source is 16:07 off-axis. Because of the smaller FOV a large fraction of the photons (75%) are lost on the detector wall. This, together with the reduced LECS exposure due to observational constraints, prevent the use of these data for spectral and timing analysis.

Fig. 1. The XB 1323-619 field MECS image (equinox J2000) smoothed with a Gaussian filter with a of . The two "cut-outs" are due to the removal of calibration source events

A total of 1940 MECS events within a radius of of 1SAX J1324.4-6200 were extracted. The arrival times were corrected to the solar system barycenter and binned with an integration time of 5 s. A single power spectrum (16384 frequencies) was calculated for the entire observation and is shown in Fig. 2. A strong peak is detected at  Hz (170.85 s) with a significance of 9. No other peaks exceed the 3 detection threshold. The period was refined by cross-correlating pulse profiles each obtained by folding data from 12 consecutive intervals. This yields a pulse period of s (at 90% confidence). The 1.8-10 keV pulse profile (Fig. 3) is approximately sinusoidal with a semi-amplitude (half of the peak to peak modulation divided by the mean count rate) of %. The pulse shape and semi-amplidute do not show a strong energy dependence. The 4.5-10 keV/2.0-4.5 keV hardness ratio is constant, except for a slight hardening at = 0.6-0.7 (where = 0.0 is the intensity minimum). The lightcurve does not show eclipses, dips or strong variability with an upper limit of 14% rms at a binning of 400 s.

Fig. 2. The MECS 1.8-10 keV 1SAX J1324.4-6200 power spectrum. The dashed line indicates the 3 threshold

Fig. 3. Folded lightcurves at the best period for the BeppoSAX MECS (upper panel, 1.8-10 keV) and ASCA GIS (lower panel, 1.0-10 keV) data. Two cycles are shown

The MECS spectrum was rebinned to a minimum of 20 counts per bin, and was analyzed using an appropriate response matrix for the source position in the FOV. Due to the close proximity of the galactic ridge emission, a background spectrum was obtained from the same data using a source free region and the same extraction radius. A power-law model represents the data well with a of 32 for 44 degrees of freedom (dof), with a photon index, , of and absorption, N, of  atom cm^-2 (Fig. 4). The 1-10 keV flux is  erg cm ^{- 2} s^-1. No iron K line is detected with a 90% confidence upper limit of 98 eV to the equivalent width of a narrow line at 6.4 keV. Blackbody and bremsstrahlung models also fit the data. For a blackbody model, the temperature, kT, is keV and N atom cm^-2 ( = 35/44). For a bremsstrahlung model, the temperature and absorption cannot be simultaneously constrained. The 90% confidence limit for kT is 10 keV and  N_H  atom cm^-2. with a . Althought 1SAX J1324.4-6200 is in the FOV of the non-imaging HPGSPC (5-120 keV) and PDS (15-300 keV) detectors, no useful spectral or timing information could be extracted from these data. The folded lightcurves are consistent with a constant with a semi-amplitude of 62% (HPGSPC) and 10% (PDS) respectively. The observed counts are dominated by XB 1323-619 which is predicted to give 10 and 4 times more than 1SAX J1324.4-6200 in the HPGSPC and PDS respectively.

Fig. 4. MECS and GIS unfolded photon spectra. The solid line represents the best fit power-law model. Energy is in keV

1.2. ASCA

ASCA (Tanaka et al. 1994) observed XB 1323-619 in 1994 August for a total of 20 ks. 1SAX J1324.4-6200 is detected in the GIS2 and GIS3 (0.8-10 keV) instruments (FOV ) at 12:09 and 10:03 off-axis, respectively. It is outside the (1 CCD mode) FOV of the SIS. The J2000 ASCA position is R.A.=, Dec= (with an uncertainty radius of 1:05), consistent with the MECS position. Photon event lists and spectra extracted from GIS2 and GIS3 were combined. The events were binned with an integration time of 5 s after barycentric correction. Power spectra of 4096 frequencies do not reveal significant peaks at the frequency of interest, with a limiting semi-amplitude 45%. A peak with a significance of is found at 170 s by searching with a folding technique in a small range of periods (155-185 s). Cross-correlating the mean pulses obtained from 5 intervals of the ASCA data gives a period of s. The 1.0-10 keV GIS pulse profile is similar to that obtained with the MECS (Fig. 3). A lower limit of the s s^-1 is obtained combining the ASCA and BeppoSAX period measurements (using the lowest value allowed for the ASCA measurement). The lightcurve does not show significant variations with an upper limit of 15% rms at a binning of 400 s.

The combined GIS2 and GIS3 source spectrum was rebinned to have a minimum of 20 counts per bin. A power-law fit, using the appropriate off-axis response gives and N_H =  atom cm^-2 with a of 27 for 34 dof, similar to the values obtained with the MECS. The 1-10 keV flux is  erg cm ^{- 2} s^-1, 30% less than observed by BeppoSAX.

1.3. Earlier observations

Table 1 lists the 1SAX J1324.4-6200 count rates or upper limits for all available observations. The source was within the FOV of observations made with the Einstein IPC (0.4-4.5 keV) in 1983 and with the EXOSAT CMA (0.04-2 keV) in 1984 and 1985 (P89). It is not detected in any of these observations. The high absorption measured in the BeppoSAX and ASCA spectra means that the EXOSAT CMA non-detections are almost certainly due to the instrument's lower band pass and sensitivity. The predicted on-axis count rate of s^-1 is well below the upper limits given in Table 1. In the Einstein IPC observation, the pulsar was located at large off-axis angle, very close to a detector rib, at reduced detector efficency. Assuming the BeppoSAX spectral parameters, the expected on-axis count rate is s^-1, consistent with the upper limit. The source is not included in the ROSAT all sky survey catalog (Voges et al. 1996), but the detection limit of 0.05 count s^-1 is above the expected count rate of s^-1. No counterpart was found by searching in the SIMBAD and HEASARC catalogs. 1SAX J1324.4-6200 lies in a very crowded region of the galactic plane. The digitized sky survey image contains more than 20 stars in the 1SAX J1324.4-6200 1:05 radius position uncertainty circle, where the 10 brightest have a Vmag 13. These are unlikely to be the optical counterpart because of the high N inferred from the X-ray spectrum.

Table 1. Observations of 1SAX J1324.4-6200. C is count rate and off-axis angle. Upper limits are quoted at 3 confidence. The MECS and GIS count rates are uncorrected for factor 2 vignetting, and is the average for each pair of units. The LECS count rate is affected by severe vignetting (see text)

© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998
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