AstroSat observation of the Be/X-ray binary Pulsar 3A 0726-260 (4U 0728-25)

Abstract

Results on timing and spectral properties of the Be/X-ray binary pulsar 3A 0726–260 (4U 0728–25) are presented. The binary was observed on 2016 May 6–7 with the Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) instruments onboard the AstroSat satellite. During this observation, the source was in non-flaring persistent state at a flux level of ∼ 8.6 ± 0.3 ×10−11 erg cm−2 s−1 in 0.4–20 keV. Strong X-ray pulsations with a period of 103.144±0.001 s are detected in 0.3–7 keV with the SXT and in 3–40 keV with the LAXPC. The pulse profile is energy dependent, and there is an indication that the pulse shape changes from a broad single pulse to a double pulse at higher energy. At energies above 20 keV, we report the first time detection of pulsation period 103.145±0.001 s and the double peaked pulse profile from the source. The energy spectrum of the source is derived from the combined analysis of the SXT and LAXPC spectral data in 0.4–20 keV. The best spectral fit is obtained by a power law model with a photon index (1.7±0.03) with high energy spectral cut-off at 12.9 ± 0.7 keV. A broad iron line at ∼ 6.3 keV is detected in the energy spectrum. We briefly discuss the implications of these results.