Observation of hard X-ray emission from GRO J1744-28 in the 20–200 keV band during declining phase

Abstract

We report spectral measurements of the bursting X-ray pulsar GRO J1744-28, made during a balloon flight experiment using high sensitivity scintillation counter telescope operating in the 20–200 keV energy band. The observations were made on March 30th, 1997 near to fading away of the source during its second outburst in December 1996 to April 1997. The X-ray flux in the 25–100 keV energy range was a factor of 10 lower than the averaged flux during the burst. The continuum spectrum was found to be extremely hard and is best fitted with a power law of the form dN/dE=KE^-α with spectral index α = 2.05 ± 0.1 and is very different compared to the averaged burst spectrum. The form of the spectrum compares with the transient X-ray pulsating source 4U0535+26 during its quiescent phase. At a distance of 8 kpc, the observed source luminosity in hard X-rays is ∼ 10³⁷ erg s⁻¹. The power law nature of the X-ray emission, close to fading of the source during the declining phase suggests that unpulsed emission arises in the accretion disk which gradually disappears.