• Kuntal Misra

Articles written in Journal of Astrophysics and Astronomy

• Explosive and Radio-Selected Transients: Transient Astronomy with Square Kilometre Array and its Precursors

With the high sensitivity and wide-field coverage of the Square Kilometre Array (SKA), large samples of explosive transients are expected to be discovered. Radio wavelengths, especially in commensal survey mode, are particularly well-suited for uncovering the complex transient phenomena. This is because observations at radio wavelengths may suffer less obscuration than in other bands (e.g. optical/IR or X-rays) due to dust absorption. At the same time, multiwaveband information often provides critical source classification rapidly than possible with only radio band data. Therefore, multiwaveband observational efforts with wide fields of view will be the key to progress of transients astronomy from the middle 2020s offering unprecedented deep images and high spatial and spectral resolutions. Radio observations of Gamma Ray Bursts (GRBs) with SKA will uncover not only much fainter bursts and verifying claims of sensitivity-limited population versus intrinsically dim GRBs, they will also unravel the enigmatic population of orphan afterglows. The supernova rate problem caused by dust extinction in optical bands is expected to be lifted in the SKA era. In addition, the debate of single degenerate scenario versus double degenerate scenario will be put to rest for the progenitors of thermonuclear supernovae, since highly sensitive measurements will lead to very accurate mass loss estimation in these supernovae. One also expects to detect gravitationally lensed supernovae in far away Universe in the SKA bands. Radio counterparts of the gravitational waves are likely to become a reality once SKA comes online. In addition, SKA is likely to discover various new kinds of transients.

• Upcoming 4m ILMT facility and data reduction pipeline testing

The 4m international liquid mirror telescope (ILMT) installation activities have recently been completed at the Devasthal observatory (Uttarakhand, India). The ILMT will perform continuous observation of a narrow strip of the sky ($\sim$27$'$) passing over the zenith in the SDSS $g'$, $r'$ and $i'$ bands. Incombination with a highly efficient 4k$\times$4k CCD camera and an optical corrector, the images will be secured at the prime focus of the telescope using the time delayed integration technique. The ILMT will reach $\sim$22.5mag ($g'$-band) in a single scan and this limiting magnitude can be further improved by co-adding the nightly images. The uniqueness of the one-day cadence and deeper imaging with the ILMT will make it possible to discover and study various galactic and extra-galactic sources, specially variable ones. Here, we present the latest updates of the ILMT facility and discuss the preparation for the first light, which is expected during early 2022. We also briefly explain different steps involved in the ILMT data reduction pipeline.

• Revealing nature of GRB 210205A, ZTF21aaeyldq (AT2021any) and follow-up observations with the 4K$\times$4K CCD imager $+$ 3.6m DOT

Optical follow-up observations of optical afterglows of gamma-ray bursts are crucial to probe the geometry of outflows, emission mechanisms, energetics and burst environments. We performed the follow-up observations of GRB 210205A and ZTF21aaeyldq (AT2021any) using the 3.6m Devasthal opticaltelescope (DOT) around one day after the burst to deeper limits due to the longitudinal advantage of the place. This paper presents our analysis of the two objects using data from other collaborative facilities, i.e., 2.2m Calar Alto Astronomical Observatory (CAHA) and other archival data. Our analysis suggests that GRB 210205A is a potential dark burst once compared with the X-ray afterglow data. Also, comparing results with other known and well-studied dark GRBs samples indicate that the reason for the optical darkness of GRB210205A could either be intrinsic faintness or a high redshift event. Based on our analysis, we also found that ZTF21aaeyldq is the third known orphan afterglow with a measured redshift except for ZTF20aajnksq (AT2020blt) and ZTF19abvizsw (AT2019pim). The multiwavelength afterglow modeling of ZTF21aaeyldq using the afterglowpy package demands a forward shock model for an ISM-like ambient medium with a rather wider jet opening angle. We determine circumburst density of $n_0 =0.87$ cm$^{-3}$, kinetic energy $E_k=3.80 \times 10^{52}$ erg and the afterglow modeling also indicates that ZTF21aaeyldq is observed on-axis ($\theta_{\rm obs}$ < $\theta_{\rm core}$) and a gamma-ray counterpart was missed by GRBs satellites. Our results emphasize that the 3.6m DOT has a unique capability for deep follow-up observations of similar and other new transients for deeper observations as a part of time-domain astronomy in the future.

• GRB 210217A: a short or a long GRB?

Gamma-ray bursts are traditionally classified as short and long bursts based on their $T_{90}$ value (the time interval during which an instrument observes 5% to 95% of gamma-ray/hard X-ray fluence). However, $T_{90}$ is dependent on the detector sensitivity and the energy range in which the instrument operates. As a result, different instruments provide different values of $T_{90}$ for a burst. GRB 210217A is detected with different duration by Swift and Fermi. It is classified as a long/soft GRB by Swift-BAT with a $T_{90}$ value of 3.76 s. On the other hand, the sub-threshold detection by Fermi-GBM classified GRB 210217A as a short/hard burst with a duration of 1.024 s. We present the multi-wavelength analysis of GRB 210217A (lying in the overlapping regime of long and short GRBs) to identify its actual class using multi-wavelength data. We utilized the $T_{90}$-hardness ratio, $T_{90}-E_p$ and $T_{90}-t_{\rm mvts}$ distributions of the GRBs to find the probability of GRB 210217A being a short GRB. Further, we estimated the photometric redshift of the burst by fitting the joint XRT/UVOT SED and placed the burst in the Amati plane. We found that GRB 210217A is an ambiguous burst showing properties of both short and long class of GRBs.

• A global look into the world of interacting supernovae

Interacting supernovae (SNe) IIn and Ibn show narrow emission lines and have always been a mysterious and unsolved genre in SNe physics. We present a comprehensive analysis of the temporal and spectroscopic behavior of a group of interacting SNe IIn and Ibn. We choose SNe 2012ab, 2020cui, 2020rc and2019uo as representative members of these SN sub-types to probe the nature of explosion. Our study reveals that SNe IIn are heterogeneous, bright depicting multi-staged temporal evolution while SNe Ibn are moreover homogeneous, comparatively fainter than SNe IIn and short lived, but limited in a sample to firmly constrain thehomogeneity. The spectroscopic features display a great diversity in Ha and He profiles for both SNe IIn and Ibn. The representative SN Ibn also show flash ionization signatures of CIII and NIII. Modeling of H$\alpha$ reveals that SNe IIn have, in general, an asymmetric CSM which interacts with SN ejecta resulting in diversity in H$\alpha$ profiles.

• Modeling the late-time merger ejecta emission in short gamma ray bursts

The short gamma ray bursts (GRBs) are the aftermath of the merger of binary compact objects (neutron star–neutron star or neutron star–black hole systems). With the simultaneous detection of gravitational wave (GW) signal from GW 170817 and GRB 170817A, the much-hypothesized connection between GWs and short GRBs has been proved beyond doubt. The resultant product of the merger could be amillisecond magnetar or a black hole depending on the binary masses and their equation of state. In the case of a magnetar central engine, fraction of the rotational energy deposited to the emerging ejecta produces latetime synchrotron radio emission from the interaction with the ambient medium. In this paper, we present ananalysis of a sample of short GRBs located at a redshift of $z\leq 0.16$, which were observed at the late-time to search for the emission from merger ejecta. Our sample consists of seven short GRBs, which have radio upper limits available from very large array and Australian telescope compact array observations. We generate the model light curves using the standard magnetar model incorporating the relativistic correction. Using the model light curves and upper limits we constrain the number density of the ambient medium to be 10$^{-1}$–10$^{-3}$ cm$^{-3}$ for rotational energy of the magnetar $E_{\rm rot}\sim 5\times 10^{51}$ erg. Variation in ejecta mass does not play a significant role in constraining the number density.

• Photometric studies on the host galaxies of gamma-ray bursts using 3.6m Devasthal optical telescope

In this paper, we present multi-band photometric observations and analysis of the host galaxies for a sample of five interesting gamma-ray bursts (GRBs) observed using the 3.6mDevasthal optical telescope (DOT) and the back-end instruments. The host galaxy observations of GRBs provide unique opportunities to estimatethe stellar mass, ages, star-formation rates and other vital properties of the burst environments and hence, progenitors. We performed a detailed spectral energy distribution (SED) modeling of the five host galaxies using an advanced tool called $\mathtt{Prospector}$, a stellar population synthesis model. Furthermore, we comparedthe results with a larger sample of well-studied host galaxies of GRBs, supernovae and normal star-forming galaxies. Our SED modeling suggests that GRB 130603B, GRB 140102A, GRB 190829A and GRB 200826A have massive host galaxies with high star-formation rates (SFRs). On the other hand, a supernovae-connectedGRB 030329 has a rare low-mass galaxy with a low star-formation rate.We also find that GRB 190829A has the highest (in our sample) amount of visual dust extinction and gas in its local environment of the host, suggesting that the observed very high-energy emission from this burst might have a unique local environment. Broadly,the five GRBs in our sample satisfy the typical correlations between host galaxies parameters and these physical parameters are more common to normal star-forming galaxies at the high-redshift Universe. Our results also demonstrate the capabilities of 3.6m DOT and the back-end instruments for the deeper photometric studies ofthe host galaxies of energetic transients, such as GRBs, supernovae and other transients in the long run.

• # Journal of Astrophysics and Astronomy

Volume 44, 2023
All articles
Continuous Article Publishing mode

• # Continuous Article Publication

Posted on January 27, 2016

Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019