| Name: | Giridharan L |
| Affiliation: | Christ University |
| Conference ID : | ASI2024_172 |
| Title : | Millisecond phenomena observed in 4U 1820−30 by AstroSat |
| Authors : | L. Giridharan, Neal Titus Thomas, S. B. Gudennavar, S. G. Bubbly |
| Authors Affiliation: | L. Giridharan, Neal Titus Thomas, S. B. Gudennavar, S. G. Bubbly (CHRIST (Deemed to be University), Bengaluru-560029, India) |
| Mode of Presentation: | Oral |
| Abstract Category : | High Energy Phenomena, Fundamental Physics and Astronomy |
| Abstract : | Ultracompact X-ray binaries (UCXB) are systems consisting of a compact object and a hydrogen deficient helium burning star or a white dwarf as its companion, having a very short orbital period (<1 hr). One such UCXB is 4U 1820−30 located in the globular cluster NGC 6624. It hosts a neutron star as its compact object accompanied by a helium rich companion of mass 0.058 − 0.078 M⊙. The source was observed three times between 2016−2017, using Soft X-ray Telescope and Large Array X-ray Proportional Counter instruments onboard AstroSat. Here, we present in-depth spectral and temporal analysis of the same. The source traced patterns of both banana and island states of atoll sources in its hardness-intensity diagram. Spectral modeling in the 0.6−23.0 keV energy range revealed the source to be in a hard state ( Γ <1.5 ) during all three observations. Temporal analysis revealed the presence of twin kHz quasi-periodic oscillations (QPOs) having frequency ranging between ~630−1100 Hz. Moreover, flux-resolved spectro-temporal analysis showed the emission radii of upper kHz QPOs to be ~18 km and the frequency of the lower kHz QPO to be positively correlated with source flux indicating their origin to be in the inner part of the accretion disk. Furthermore, through the relativistic precession model the mass of the compact object was constrained to be 1.8−2.0 M⊙. These results will be presented during the meeting.
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