J. Majumdar · ResearchPreprint, manuscript no. 26xx.134xx

Multi-wavelength studies of astrophysical transients, from interacting supernovae to relativistic jets

J. Majumdar1

1 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland

Last revised 24 October 2026

ABSTRACT

Context. The sky changes on every timescale we care to look at, and the most energetic changes happen far outside our Galaxy.

Aims. To understand what powers extragalactic transients and how long they keep doing it.

Methods. Optical photometry and spectroscopy from a global network of telescopes, combined with ultraviolet, X-ray and γ-ray data from space observatories.

Results. Current work focuses on long-lived interacting supernovae and flares in active galactic nuclei. Earlier work covers blazar variability and the response of the ionosphere to solar flares.

Key words. supernovae: individual: SN 2019qem – galaxies: nuclei – tidal disruption events (TDE) – changing-look AGN (CL-AGN) – Bowen fluorescence flares (BFF) – BL Lacertae objects: individual: BL Lacertae – Sun: flares – telescopes

1.Introduction

I am interested in multi-wavelength studies of astrophysical transients and high-energy astrophysics. At present I work mostly on core-collapse supernovae (Sect. 2). In parallel I am expanding the features of BHTOM (Sect. 4), which supplies much of the photometry behind this work.

2.Supernovae that refuse to fade

When a massive star explodes inside a dense shell of material it shed before death, the ejecta crash into that shell and convert kinetic energy into light for months to years. These are the Type IIn supernovae, named for the narrow hydrogen lines that the slow circumstellar gas imprints on their spectra. SN 2019qem is a luminous example that I have followed through three years of interaction, presented at the Gaia Science Alerts and ACME workshop and at the ePESSTO+ meeting in 2026.

3.Flares in active galactic nuclei

Supermassive black holes that are already accreting can still surprise us. AT 2020xys is a Bowen fluorescence flare, a class of nuclear transient in which a sudden rise in ultraviolet flux drives the Bowen mechanism and lights up He ii and N iii lines. I presented its discovery, classification and multi-wavelength observations at HERA 2026 in Warsaw. Related work studies the jets of peculiar narrow-line Seyfert 1 galaxies through their multi-wavelength variability (Ojha et al. 2026).

4.Instrumentation and follow-up

BHTOM coordinates observations from a global network of professional and amateur telescopes and calibrates their photometry in a common system. The same infrastructure has produced photometry for the binary-lens microlensing event Gaia20fnr (Wicker et al. 2026a), for Gaia Science Alerts microlensing events whose lens masses will be testable with Gaia Data Release 4 (Wicker et al. 2026b) and for the interstellar comet 3I/ATLAS (Gillan et al. 2026), and its follow-up of supernovae, tidal disruption events and Galactic novae is reported through Transient Name Server AstroNotes and Astronomer’s Telegrams.

4.1.BHTOM3

BHTOM3 is the next generation of the platform, rebuilt on the TOM Toolkit. I develop its new data services and its photometry and spectroscopy features, which I presented at the 17th Gaia Science Alerts and ACME Time-Domain Workshop in Fréjus in 2026.

5.Blazars and relativistic jets

Most galaxies host a supermassive black hole at their centre. When it is active, the central region is called an active galactic nucleus, and some of these launch bipolar relativistic jets. If the jet points within about 15° of our line of sight, we call the source a blazar, and relativistic beaming makes its variability fast and extreme across the whole spectrum.

My first papers followed the 2024 flare of BL Lacertae, down to minute-scale variability in γ-rays (Majumdar et al. 2024, 2025a), the variability of Ton 599 (Maurya et al. 2025) and the drivers of flaring in 4C +27.50 (Majumdar et al. 2025c). I have also presented a multi-wavelength study of the transitioning blazar OP 313.

5.1.An excursion to the Sun

Before turning to distant galaxies I studied our nearest flaring source, using several instruments to trace how intense solar flares in the declining phase of solar cycle 24 disturbed the Earth’s ionosphere (Majumdar et al. 2025b).

6.Collaborations

References

Full entries with links are on the publications page.