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Astron. Astrophys. 361, 85-91 (2000)

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4. ASM observations

Given the NFI position, we have searched for signatures of SAX J2239.3+6116 in archival ASM data. The ASM is fully operational since March 1996 and monitors each position on the sky in 2 to 12 keV during 90 sec snapshots with a frequency of 5 to 10 times a day at a sensitivity of [FORMULA] mCrab per day of observervations on uncrowded fields (Levine et al. 1996). The lightcurve for SAX J2239.3+6116 at 4-day resolution is presented in Fig. 6a. There is the suggestion for a detection during 5 instances, at regular interval times of about 262 d. We tested this periodicity by first filtering out the data within 10 d of closest aproaches to the Sun and all data after MJD 51450 because there does not appear to be an outburst there, and then calculating the variance statistic [FORMULA] as defined by Stellingwerf (1978) for a range of test periods, see Fig. 6b. The resulting period is [FORMULA] d. The epoch for peak flux is MJD 50786. The predicted times of outbursts are indicated in again Fig. 6a. Only the last predicted outburst does not appear to have materialized. The two WFC detections synchronize with the 2nd and 5th outburst. A folded light curve (Fig. 6c) shows an average outburst profile which lasts [FORMULA]15 days. The average peak flux is 4 mCrab.

[FIGURE] Fig. 6. a ASM light curve at 4 d time resolution, the arrows point to the predicted times of outbursts for periodicity of 262 d; b  [FORMULA] statistic (Stellingwerf 1978) as a function of test period; c  folded light curve at the best fit period. The non-zero flux outside the outburst points to an instrumental bias level of about 0.5 mCrab.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000