Dr. Subrata Sarangi

Stellar Intensity Interferometry (SII) as an alternative technique for imaging stars as extended objects with surface morphology was first proposed by Hanbury Brown and Twiss in 1950s. [Hanbury Brown, 1974] also reported the results of the measurements using this technique at the historic Narrabri Stellar Intensity Interfer- ometer. However, the then available electronics limited the observations and imaging of stars to only 32 cases. The technique then went into a slumber. Currently, with fast electronics available, the SII is getting rejuve- nated [SII at 2020]. Interestingly, the Imaging Atmospheric Cherenkov Telescopes (IACT) such as VERITAS, MAGIC and HESS [IACTs4SII, 2021], normally used for Gamma Ray Astronomy but ineffective during moon-lit nights, are being used as SII facilities to systematically measure radii of single, binary or multi-star systems.[Kieda et al., 2022] very recently reported angular size measurements of 44 stars including single, binary and multi-star systems using the VERITAS during moonlit nights. These stars ranged over spectral classes O9-A2 and visual magnitudes 0.06 < mv < 3.74. The accuracy of measurements were better than 5% of statistical resolution. In this background, a systematic and planned Indian effort at SII measurements is in order. As a forerunner, our team has carried out two different Intensity Interferometry (II) simulations of astrometric measurements in two different binary star systems:
• [Rai et al., 2022] have used Bayesian Inference and II to simulate the possibilities of astrometric detection of exoplanets in the habitable zones of the a-Centauri system below the current detection thresholds. This simulation has shown possibilities of detection with currently available electronics at µas level accuracy. In this work, we have also introduced the simulation of a novel method of masking the telescope objectives in order to deal with very closely spaced interference fringes of wide binaries like the a-Centauri.
• In an earlier study, [Rai et al., 2021] used MCMC simulations to generate II fringe patterns for the close- binary Spica system. In this study, it was shown that, II can not only improve upon the astrometry of the system, it can also produce the diameters of the stars in the system. It is worthwhile to note that the High Altitude Gamma Ray (HAGAR) telescope at Hanle, Ladakh is a system of 7 IACTs. Projects need to be drawn up to use this facility during moon-lit nights and join the worldwide efforts to bring SII to centre stage.

References:
- [Hanbury Brown & Twiss, 1954, 1957] Hanbury Brown, R. & Twiss, R. Q. 1954, Philosophical Magazine, 45, 663; Hanbury Brown, R. & Twiss, R. Q. 1957, Proceedings of the Royal Society of London Series A, 242, 300. - [Hanbury Brown, 1974] Hanbury Brown, R. 1974, The intensity interferometer: Its application to astronomy (Taylor and Francis). - [SII at 2020] Acciari, V. A., Bernardos, M. I., Colombo, E., et al. 2020, MNRAS, 491, 1540; Rivet, J. P. et al. 2020, MNRAS, 494, 218; Abeysekara, A. U. et al. 2020, Nature Astronomy. - [IACTs4SII, 2021] Kieda, D. et al., 2021, 37th International Cosmic Ray Conference (ICRC 2021), July 12th – 23rd, 2021, Online – Berlin, Germany; Acciari, V.A. et al., 2019, arXiv:1911.06029v1 [astro-ph.IM]; Kieda, D. & Matthews, N., 2017, 35th International Cosmic Ray Conference - ICRC2017, Bexco, Busan, Korea. - [Kieda et al., 2022] Kieda, D., et al, 2022, The Sharpest Eyes on the Sky Conference, Exeter, UK, April 25-29,2022. - [Rai et al., 2022] Rai, N. K., Sarangi, S., Saha, P., Basak, S., 2022, arXiv: 2022 [to be uploaded shortly] - [Rai et al., 2021] Rai, N.K., Basak, S. and Saha, P., 2021, MNRAS, 507(2), 2813–2824.