Spectro-temporal variability studies of GX 340+0 using AstroSat

Neutron Star Low Mass X-ray Binaries (NS-LMXBs) are stellar systems comprised of a weakly magnetized neutron star as the compact object and a donor star of mass ~ 1 - 2 (M_\odot). Based on their correlated spectral and temporal behaviour and the pattern they trace on their X-ray Hardness-Intensity Diagram (HID), NS-LMXBs are classified into Z and Atoll type sources. The exact nature of the physical mechanisms that drive the Z and Atoll type of patterns are not known. Although there are several theories with regard to this, none of them have been firmly established without contention. This has been a long-standing problem in the study of NS-LMXBs. One such NS-LMXB is GX 340+0, which was discovered as a galactic X-ray source in 1967. Based on its spectro-temporal properties, it is classified as a Cygnus-like Z-type NS-LMXB. Its peculiar properties include the presence of Quasi Periodic Oscillations (QPOs) in the Flaring Branch, which is usually a signature of Scorpius-like Z sources; and quasi stable photon indices, similar to some previously established atoll sources. In addition to this, the Z pattern traced by GX 340+0 in its HID shows distinct secular shifts. Motivated by this, we carried out in-depth spectro-temporal variability studies of GX 340+0 using $\sim$ 235 ks of data from the LAXPC and SXT instruments on-board the AstroSat mission. The HID shows that during the observations, the source was undergoing a transition from the Horizontal branch to the Normal branch. We observe QPOs whose frequencies agree well with the frequency of Horizontal Branch Oscillations (HBO) and Normal Branch Oscillations (NBO). These are thought to be caused due to the perturbations in the accretion disk and/or possibly the corona and act as probes into changes in the accretion geometry of the source. Further, dynamic temporal analysis shows that the HBO is dominant in lower energy whereas the NBO is dominant in the higher energy. This indicates that these QPOs originate from different sites of the system. We have also correlated these temporal variations with the spectral parameters of the source through flux and time resolved spectroscopy. These results will be presented during the meeting.