Research

My Research Areas

Extragalactic astronomy · High-energy astrophysics · AGN & blazars

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Active Galactic Nuclei (AGN)

Studying supermassive black holes and their accretion processes. Focus on the broad line region (BLR), Fe II emission, photoionization modelling with CLOUDY, and the dusty/dustless nature of AGN environments.

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Blazars & Relativistic Jets

Multi-wavelength variability studies of TeV blazars including flux, spectral and polarization variability at optical, X-ray, and gamma-ray energies. Data from NuSTAR, Chandra, XMM-Newton, Fermi-LAT.

X-ray Data →

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Optical Photometry

Intra-night and long-term optical monitoring of blazars using 1.0–1.3m class telescopes at ARIES, Nainital. Observations in BVRI bands to study variability patterns and correlations.

Optical Data →

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X-ray Spectroscopy

X-ray flux and spectral variability studies using NuSTAR, Chandra, XMM-Newton, Swift, and NICER. Analysis of disc-jet interactions, reflection features, and corona properties in AGN and blazars.

X-ray Data →

Active Galactic Nuclei (AGNs)

A small fraction (~10%) of galaxies in our universe has a compact, highly luminous (L~10⁴⁴⁻⁴⁸ ergs/s) central region that emits radiation comparable to or sometimes even higher than the radiation emitted by the rest of the galaxy. These galaxies are called 'active galaxies' and their nuclei are termed as 'Active Galactic Nuclei' (AGNs). Some AGNs host relativistic bipolar outflows or jets that emit non-thermal continuum emission from radio to γ−rays.

1.1 A Brief History of AGN

In 1908, E.A. Fath at Lick Observatory detected strong emission lines in the optical spectra of nebula NGC 1068. Carl Seyfert, in 1943, observed strong broad emission lines in the optical spectra of six spiral galaxies. In the late 1950s, radio surveys published catalogues such as the Third Cambridge Catalogue (3C). Schmidt (1963) identified 3C 273 as an extragalactic source with redshift z = 0.158 — a new class named quasars (QSOs).

The idea of accretion onto a supermassive black hole was proposed in the 1960s by Salpeter (1964) and Lynden-Bell (1969). Rees (1984) confirmed AGN are powered by accreting SMBHs at galactic centers.

1.2 Basic Components of AGN

Supermassive Black Hole (SMBH): M_BH ~ 10^6-9 M_☉ at the center, pulling surrounding matter by gravity.
Accretion Disk (AD): Spiraling disk of infalling matter that radiates thermally across UV/optical bands.
X-ray Corona: Hot plasma sandwiching the disk — inverse-Comptonizes disk photons to produce X-rays.
Broad Line Region (BLR): Dense (~10¹⁰ cm⁻³) gas clouds at 0.1–1 pc, producing Doppler-broadened emission lines.
Obscuring Torus: Dusty torus at 1–10 pc; obscures central regions and re-emits in infrared.
Narrow Line Region (NLR): Lower density (~10³ cm⁻³) clouds at ~100 pc producing narrow emission lines.
Relativistic Jets: Magnetically collimated bipolar outflows extending to kpc–Mpc scales, seen in jetted AGN.

1.3 AGN Taxonomy & Blazars

About 15% of AGN are jetted-AGN (Padovani et al. 2017). Blazars are jetted-AGN aligned at small (≤ 15–20°) viewing angles (Urry & Padovani 1995), and their spectra are dominated by non-thermal radiation. They are subdivided into FSRQs (flat-spectrum radio quasars) and BL Lac objects.

Artist impression of AGN — Artist's impression of an active galactic nucleus. Image: NASA/Fermi

References

Ghisellini, G., Maraschi, L., & Tavecchio, F. 2009, MNRAS, 396, L105
Padovani, P., Alexander, D. M., Assef, R. J., et al. 2017, A&ARv, 25, 2
Rees, M. J. 1984, ARA&A, 22, 471
Urry, C. M., & Padovani, P. 1995, PASP, 107, 803

Optical photometric observations of TeV blazars using the 1.3 m Devasthal Fast Optical Telescope (DFOT) and 1.04 m Sampuranand Telescope, both at ARIES, Nainital, India.

1ES 0229+200

RA: 02h 32m 53.2s | Dec: +20° 16' 21"

Date	JD	Telescope	Data pts (B,V,R,I)
24-10-2016	2457686.30358	DFOT (1.3m)	1,0,40,0
25-10-2016	2457687.31715	DFOT (1.3m)	1,0,32,0
26-10-2016	2457688.30863	DFOT (1.3m)	1,0,30,0
24-11-2016	2457717.28042	DFOT (1.3m)	0,0,41,0
25-11-2016	2457718.22020	DFOT (1.3m)	1,0,50,0
10-10-2018	2458402.32068	DFOT (1.3m)	1,0,42,0
28-12-2018	2458481.05990	DFOT (1.3m)	0,0,34,0
29-12-2018	2458482.05153	DFOT (1.3m)	1,0,47,0

1ES 0414+009

RA: 04h 16m 53.0s | Dec: +01° 05' 20"

Date	JD	Telescope	Data pts (B,V,R,I)
30-12-2016	2457753.09527	DFOT (1.3m)	0,1,35,1
18-01-2017	2457772.11041	DFOT (1.3m)	0,2,02,1
19-01-2017	2457773.09260	DFOT (1.3m)	0,1,32,1
31-10-2018	2458423.62996	ST (1.04m)	0,1,38,0
15-12-2018	2458468.23930	DFOT (1.3m)	0,1,56,1
28-12-2018	2458481.30444	DFOT (1.3m)	0,1,30,1

1ES 2344+514

RA: 23h 47m 04.8s | Dec: +51° 42' 17.9"

Date	JD	Telescope	Data pts (B,V,R,I)
24-10-2016	2457686.27076	DFOT (1.3m)	0,1,01,1
25-10-2016	2457687.08169	DFOT (1.3m)	0,1,59,1
26-10-2016	2457688.07811	DFOT (1.3m)	0,1,70,1
08-11-2016	2457701.13915	ST (1.04m)	0,1,41,1
10-10-2018	2458402.07662	DFOT (1.3m)	0,0,95,1
15-10-2018	2458407.12018	DFOT (1.3m)	0,1,70,1
28-12-2018	2458481.04459	DFOT (1.3m)	0,1,01,1