Palomar Gattini-IR: Dawn of wide-field time domain astronomy in the near infrared
The infrared transient sky remains largely unexplored due to the high cost of wide-field detectors as well as the brightness of the sky. I serve as the data system lead for the Palomar Gattini-IR survey, a wide-field (25 square degrees field of view), all sky time domain survey scanning the entire northern sky every two nights to a depth of 16 AB mag in J band (De et al. 2020a). Operating in the near-infrared J band, Palomar Gattini-IR is sensitive to a range of astrophysical phenomena invisible to optical time domain surveys due to their red colors, either because of extinction by dust in the foreground or cool temperatures.
I am currently using Palomar Gattini-IR to find large amplitude transients from compact objects (e.g. classical novae, X-ray binaries, magnetars) in dust obscured regions of the Galaxy (De et al. 2020c) . I contribute to a range of other scientific projects with the survey such as outbursts in obscured Young Stellar Objects (M. Hankins, Caltech postdoc), variability in dust forming R Coronae Borealis stars (V. Karambelkar, Caltech grad), RR Lyrae variability (J. Soon, ANU grad) and reddened microlensing events in the Galactic plane (P. Mroz, Caltech postdoc).
Collage of J band light curves of large amplitude transients from Palomar Gattini-IR (De et al. 2020a)
Supernova demographics in the local universe
I am working with the Zwicky
Transient Facility (ZTF), and leading a large
spectroscopic campaign to build a complete (spectroscopically
classified) sample of transients in the local universe (in
galaxies less than 200 Mpc away) by cross-matching alerts from
ZTF to a catalog of nearby galaxies (the CLU catalog in Cook
et al. 2017). Since ZTF science operations started in
early 2018, this experiment is regularly finding infant
supernovae discovered within hours of explosion, rare fast
evolving transients, as well as building up a large
volume-limited sample of supernovae to measure their rates and
luminosity functions in the local universe.
Using this experiment (De et al. 2020b), I am focusing on faint thermonuclear supernovae in the nearby universe. Although the prototypical thermonuclear Type Ia supernovae have long been used as cosmological probes, their explosion mechanisms remain poorly understood. A subset of these explosions likely arise from explosions triggered by He shell burning on the surfaces of white dwarfs, although the evidence has so far remained unclear. In De et al. 2019, we showed conclusive evidence of this mechanism with observations of a unique thermonuclear transient (SN 2018byg) found in this experiment.
(Left) Discovery location and host galaxy of SN2018byg. (Right) Comparison of observed spectra of SN 2018byg (black) with He shell double detonation models (green and red) from Polin et al. 2019.
With a systematic search for faint thermonuclear supernovae in
the largest volume-limited sample of supernovae constructed till
date, I showed that the previously mysterious and heterogeneous
class of 'Ca-rich transients' show a continuum of properties that
are consistent with helium shell detonations on low mass white
et al. 2020b) involving a range of shell and core masses.
Together, these observations open up a completely new window
into the explosive fates of helium accreting white dwarfs, and
demonstrate that the long sought helium shell explosions
proposed for Type Ia supernovae may be hiding in events that are
nearly as common (~ 20% of SN Ia rate) but have been difficult
to find in previous surveys.
Supernova progenitors with high cadence observations
Before the start of ZTF, I worked on transients in nearby galaxies with the intermediate Palomar Transient Factory (iPTF) during my first two years at Caltech. The iPTF was a wide field transient survey operating from the 48 inch Schmidt telescope at Palomar observatory in California. The wide field and high cadence of the survey allowed us to find young supernovae that were discovered within a few hours of explosion. Such early observations provide vital clues about the immediate surroundings and nature of the progenitor star at the time explosion.
On behalf of the iPTF team, I led works on two intriguing faint and young transients discovered by iPTF. By analyzing the properties of the fast evolving Type Ic supernova iPTF 14gqr, we showed that it was consistent with being the first confirmed candidate for an ultra-stripped supernova -- a class that was predicted to lead to compact neutron star binary systems in the universe (De et al. 2018a).
Press releases on the discovery: Caltech,
Oskar Klein Center,
Carnegie. The discovery was featured in several news
Gizmodo. You can also read my article at
Stellar evolution leading from two massive stars to a binary neutron star system. The penultimate step in the chain involves an ultra-stripped supernova like iPTF 14gqr. (De et al. 2018a)
On the other hand, the faint and double-peaked Type Ib supernova iPTF 16hgs was shown to be a unique member of the class of Ca-rich gap transients -- faint and fast evolving explosions that exhibit strong Ca emission lines in their late-time spectra (De et al. 2018b). The early excess emission detected in the data constraints the mixing of radioactive material in the outer ejecta, or could even be indicative of a giant progenitor at the time of explosion.
The double peaked light curve of iPTF 16hgs (De et al. 2018b)
High time resolution pulsar studies with the uGMRT
As an undergraduate, I worked on developing high performance signal processing pipelines for the pulsar backend at the Giant Metrewave Radio Telescope (GMRT), specifically to enable science possible with coherent dedispersion and high time resolution observations of radio pulsars at low radio frequencies. Coherent dedispersion is a powerful technique that allows one to remove the effects of the interstellar medium (ISM) that are imprinted on to radio pulsar signals when they travel through the ISM. Using the massively parallel architecture of Graphics Processing Units (GPUs), I developed GPU-based real-time coherent dedispersion systems for both the legacy GMRT Software Backend and the upgraded GMRT Wideband backend (De & Gupta 2016; Reddy et al. 2017).
These pipelines are now being regularly used as standard observing modes at the GMRT. As a first demonstration of science possible with system, we carried out a survey of single pulse emission from a large sample of bright pulsars. These observations resulted in the first detections of microstructure emission in rapidly spinning millisecond pulsars (De, Gupta & Sharma 2016). I continue to help the Indian Pulsar Timing Array (InPTA; Joshi et al. 2018) collaboration which uses the uGMRT (enabled with the coherent dedispersion system) and the Ooty radio telescope for high precision timing of pulsars.
Galactic X-ray binaries
I enjoyed a summer at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany as a DAAD WISE fellow working on using X-ray eclipses as a probe of the orbital evolution of galactic low mass X-ray binaries (LMXBs). Using more than two decades of data from the XMM Newton and ASCA observatories, we showed that the orbit of the outbursting galactic center LMXB AX J1745.6-2901 was decaying at a rate much higher than expected by models, leading to some important questions regarding the evolution and merger times of LMXBs (Ponti et al. 2017). I also ventured into exploring several side projects during this time, working on high ionization absorption and radio emission in LMXBs (Ponti et al. 2016; Ponti et al. 2018).
Previously, as a freshman undergrad, I was at the Aryabhatta Research Institute of Observational Science (ARIES) in Nainital, India working on the photometric properties of the high mass X-ray binary (HMXB) SS 433 with Indranil Chattopadhyay and Jeewan Pandey. SS 433 is a unique galactic system consisting of a high mass star accreting on to a low mass compact object, and is famous for its precessing radio jets (video). Using a 1 m telescope at the observatory, I observed and reduced photometric data taken over a duration of 1 month. Analysis of the multi-color photometric behavior allowed us to estimate the orbital properties of the system and the companion star.