DETECTION OF MILLI-SECOND TIMING PHENOMENA BY LAXPC

X-ray binaries are a special class of binary stars that are very bright in X-rays. The X-rays are produced when matter is accreted from the donor star (usually a relatively normal star) onto the accretor, which is very compact – a Neutron Star or Black Hole. 4U 1728-34 is a Neutron Star low mass X-ray binary (NS-LMXB) which is known to exhibit regular thermo-nuclear bursts (Type-I) of accreted matter onto the Neutron Star surface. The burst oscillations (BO) observed during the initial phase of the Type-1 burst is one of the important diagnostic tool to measure the spin period of the NS-LMXB. In addition to BO, which originates at the surface of NS, quasi-periodic oscillations (QPO) of X-ray radiation from the accreting gas is also a common phenomenon observed in X-ray binaries. QPOs in milliseconds timescale are very important tool to understand the dynamics of accretion flow at the close vicinity of the compact objects. The source 4U 1728-34 was observed by LAXPC onboard AstroSat on 8th March 2016 for ~ 3 ksec duration. Dynamical power density spectrum in the 3-20 keV band during the observation, reveals the presence of a high frequency QPO (HFQPO) whose frequency drifted from ~ 815 Hz at the beginning of the observation to about 850 Hz (fig.1). The QPO is also detected, for the first time in the 10-20 keV band by LAXPC (fig.2).

In the early phase of the burst, a coherent burst oscillation was observed at around 363 Hz. The frequency varies from ~361.5 to ~363.5 Hz which has been reported by Rossi X-ray Timing Explorer (RXTE) during earlier burst observations. Thus LAXPC demonstrated the capability of detecting millisecond timing phenomenon even from short observations.

Reference:Jai Verdhan Chauhan et.al., The Astrophysical Journal, 841:41 (5pp), 2017 May 20