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

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5. BATSE observations

Due to the hardness of the spectrum measured with the WFC a search of the BATSE data was made to see if SAX J2239.3+6116 was detectable in the hard x-rays. The BATSE experiment onboard the Compton Gamma-Ray Observatory (Fishman et al. 1989) using the Large Area Detectors (LADs) could monitor the whole sky almost continuously in the energy range of 20 keV to 2 MeV with a typical daily 3[FORMULA] sensitivity of better than 100 mCrab. Detector counting rates with 16 energy channel energy resolution and a timing resolution of 2.048 seconds (CONT data) are used for our data analysis.

To produce the SAX J2239.3+6116 light curve, single step occultation data were taken using a standard Earth occultation analysis technique used for monitoring hard X-ray sources (Harmon et al. 1992). Interference from known bright sources was removed. The single occultation step data were then fit with a power law with a -2.0 photon index to determine daily flux measurements in the 20-100 keV band. A time period of 30 days centered on each outburst was examined for emission. Various time averages of the data were performed to search for a possible signal in the data.

During the time of the March 1997 outburst we find evidence of a weak two day outburst in the BATSE data (see Fig. 7). The 20-100 flux for highest daily average corresponds to 0.080 times that of the Crab in the same bandpass. It appears that there is a [FORMULA]3 day delay between the peak in the BATSE data and that detected in the WFC. For any of the other outbursts predicted throughout the BATSE operational period we do not find evidence for hard x-ray emission, but we can not rule out the presence of a source with a flux 0.050 times that of the Crab or less being present in the data.

[FIGURE] Fig. 7. BATSE light curve of the outburst of SAX J2239.3+6116 in March, 1997, for 20-100 keV with a one day time resolution.

We have extracted spectra from the BATSE data for the March 1997 outburst. During this time period the source was measured in three detectors. Spectra and responses for each detector were created and a joint fit was made with a simple power law. The fit was satisfactory, with a reduced chi-squared = 0.90 (40 dof). The photon index is [FORMULA]. In Fig. 8 is plot the spectrum in which the three data sets have been grouped for plotting purposes. The 20-100 keV flux is [FORMULA] erg cm-2 s-1 which is 0.080 times that of the Crab in the same bandpass. Very little flux is detected above 100 keV.

[FIGURE] Fig. 8. Top panel: BATSE-measured spectrum of SAX J2239.3+6116 (crosses) and the fitted power-law model (histogram). Bottom panel: residuals between data and model in units of sigma.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000