Preliminary results on the Hard Xray signals from Abell 3376

By generating a spectra and subtracting the background model provided by the HXD team, we obtained the hard X-ray spectra from this object. The signals thus detected were about 5-10% of the detector background. Therefore, the hard X-ray flux is smaller than 1 mCrab. With this small flux, it is not sufficient to only consider the statistical errors, but we also need to estimate the systematic errors in the current response and background modeling of the HXD. From the very preliminary study briefly presented in the appendix, we found that with current background modeling and uncertainties, analyzing only the data obtained in orbits with no SAA passage ("SAA-free analysis") provides smaller errors, sacrificing a large loss of exposure.

Spectra obtained with the "SAA-free" method are presented in Fig.3. The center/east observation resulted in a 31 ks good exposure, while that of the west relic in 21 ks. With this spectra, the count rate of both observations were generally consistent with the expected CXB. Thus, from the systematic

A3376CEN T_SAA 5700s (80ks-->31ks) A3376WR T_SAA 5700s (106ks->21 ks)

- ae20051006_1429_o3_t5700DS.Di - ae20051107_1402_cor8_a3_t5700DS.Dl

A3376CEN T_SAA 5700s (80ks-->31ks) A3376WR T_SAA 5700s (106ks->21 ks)

- ae20051006_1429_o3_t5700DS.Di - ae20051107_1402_cor8_a3_t5700DS.Dl

channel energy (keV) channel energy (keV)

Fig. 3. Center/east relic (left) and west relic (right) spectra after "SAA-free" screening. Lower dashed lines represents the best-fit r = 2 power law model, while the upper dashed lines the CXB model.

channel energy (keV) channel energy (keV)

Fig. 3. Center/east relic (left) and west relic (right) spectra after "SAA-free" screening. Lower dashed lines represents the best-fit r = 2 power law model, while the upper dashed lines the CXB model.

error analysis presented in the appendix, the preliminary upper limit flux in the 20-80 keV band is evaluated to be ~ 7 x 10"12 erg s"1 cm"2 when the hard X-rays are modeled with r = 2.0 fixed power-law component. Since the systematic error is evaluated using only 27 observations, we conservatively quote 1 x 10"11 erg s"1 cm"2 as a current ~ 90% upper-limits for both observations.

Note that this is a very rough trial and there is a big room to improve both the selection criteria to obtain the highest sensitive results and the background modeling accuracy itself. Currently, with much improved background modeling, we are re-analyzing the data in detail. The final upper-limit quoted above is about ~ 7% of the detector background, while our goal is to achieve ~ 3% accuracy or better. Thus, we think there is a room even for detecting the excess hard X-rays in future, through intense studies.

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