arXiv subjects

Neelesh Amrutha

Publications and source records attributed to Neelesh Amrutha.

5 recordsLinked to original sources

arXiv2026

ASKAP discovery of a pair of large radio bubbles: on the origin of odd radio circles

We report the serendipitous discovery of a large, low-surface-brightness radio bubble in 944 MHz continuum data from the ASKAP Evolutionary Map of the Universe (EMU) survey. The structure, centred on the elliptical galaxy LEDA 217397 at a redshift of $z=0.040$, spans $\sim$8.4 arcmin, corresponding to a projected diameter of $\sim$399 kpc, and consists of two partly overlapping shells both with radii of $\sim$114 kpc. The integrated flux density of the bubble is $56.8\pm2.9$ mJy at 944 MHz, implying a rest-frame 1.4 GHz luminosity of $\sim$ $1.4\times10^{23}$ W Hz$^{-1}$. Combining the EMU measurement with MWA GLEAM-X data at 88--185 MHz, we derive a steep integrated spectral index of $\alpha=-1.04\pm0.04$, and a two-frequency spectral-index map suggesting a possible exterior flattening. Spectral Energy Distribution (SED) fitting indicates a massive ($\log M_{\ast}/M\odot = 10.97\pm0.09$), quiescent (SFR=$0.025\pm0.083\,M\odot$ yr$^{-1}$) early-type host with no mid-infrared AGN signature and no overdense environment. We compare the bubble with odd radio circles (ORCs) and large radio shells, and discuss three scenarios for its origin: a starburst-driven wind, a merger-driven shock, and AGN jet-inflated bubbles. The starburst wind is disfavoured on energetic grounds ($\gtrsim$$10^{59}$ erg required versus $\sim$$10^{8}$ yr electron lifetimes), and neither a halo-scale merger shock nor a spherical nuclear blast wave can explain the unusually regular, double-shell geometry; a bipolar nuclear outburst -- a relic AGN jet episode, possibly triggered by a supermassive-black-hole merger -- provides the most natural explanation, with later shocks possibly re-energising the plasma. Deeper broad-band radio, polarimetric, spectroscopic and X-ray observations are needed to confirm its nature.

astro-ph.GA↗

arXiv2026

OzSSy1: The Australian Southern Seyfert-1 Spectroscopic Atlas and Catalogue at z < 0.1

We present a spectroscopic atlas of 887 broad-line active galactic nuclei (AGNs) in the Southern sky, spanning redshifts $z < 0.1$, declinations $\delta < 0$ deg and Galactic latitudes $|b| > 10$ deg. The sample aims at being a largely complete census of nearby broad-line AGN. The atlas is constructed from observations with $R\sim3000$ using the integral-field Wide Field Spectrograph at the Australian National University 2.3 m telescope. Spectra are extracted in a 6.7 arcsec aperture and have a median signal-to-noise ratio of 13 and 23 per Angstrom in the blue and red arms, respectively. Each spectrum is accompanied by a spectral decomposition that models the AGN continuum and host galaxy, along with fits to the emission lines H$\gamma$, H$\beta$, H$\alpha$, He II, He I, [O III], [O I], [N II] and [S II], including broad Balmer and Helium components. The data products are publicly available and designed to support studies of population demographics and AGN variability in conjunction with future time-domain surveys.

astro-ph.GA↗

arXiv2026

Optical Variability Structure Function of Low-Luminosity AGN using ATLAS Lightcurves

The origin of the optical flux variability in active galactic nuclei (AGN) is largely unknown. Previous studies have correlated features of the variability structure function (SF) with AGN properties, though they mostly involved high-luminosity AGN to avoid biases from host galaxy flux. In this work, we characterise optical variability in a sample of 246 low-luminosity AGN at $z < 0.1$ from the Six-degree Field Galaxy Survey (6dFGS) through the ensemble variability SF. We use lightcurves from the Asteroid Terrestrial-impact Last Alert System (ATLAS) with a cadence of $\sim$2 days over eight years, and perform host-AGN decomposition on recent spectra to obtain the host fraction. We find that the slope of the SF depends on black hole mass, increasing from $\sim 0.1$ at $\log M_{\mathrm{BH}}/M_\odot \sim 6.5$ to $\sim 0.3$ at $\log M_{\mathrm{BH}}/M_\odot \sim 8$. Contrary to some earlier work, we do not find breaks in the SF, and two-epoch spectra taken 20 years apart suggest that the SF keeps rising into decadal timescales. In addition, we measure an anticorrelation of the amplitude with the luminosity and a positive correlation with the black hole mass. The variability behaviour also suggests that extinction is not the main driver of the variety in Seyfert subtypes.

astro-ph.GA↗

arXiv2024

Discovering Changing-Look AGN in the 6dF Galaxy Survey using ATLAS Light curves

Changing-Look Active Galactic Nuclei (CLAGN) are characterised by extreme variations in line emission over short timescales, mostly affecting broad H$\beta$ lines. While a few hundred CLAGN are known, a complete sample of turn-on CLAGN is still elusive. Here, we present a search for turn-on CLAGN in a complete sample of galaxies, using archival spectra and recent light curves. We obtained light curves from the Asteroid Terrestrial Impact Last Alert System (ATLAS) for 16,232 emission line galaxies, including both star-forming and active galaxies, at $z<0.1$ with spectra from the Six-degree Field Galaxy Survey (6dFGS). We first establish typical variability behaviour for different AGN types, as recorded between 2001 and 2009, and then select outliers from the bulk behaviour as CLAGN candidates. We obtain new spectra for the candidates and identify 12 new turn-on CLAGN (appearing broad H$\beta$ line) and 19 new turn-off CLAGN (disappearing broad H$\beta$ line). We may have missed AGN that changed and reverted their state over the 15$-$20 years since 6dFGS spectra were taken, and thus our CLAGN rates of 1.7$\%$ for turn-on and 9.6$\%$ for turn-off are lower limits. The turn-on rate is naturally much lower as the type 1.9/2 sample is dominated by obscured AGN due to orientation, which are not expected to change. However, the number of turn-on (27) and turn-off (24) CLAGN we find are similar, suggesting that our parent AGN sample is reasonably complete in our search volume at $z<0.1$.

astro-ph.GA↗

arXiv2024

The accretion of a solar mass per day by a 17-billion solar mass black hole

Around a million quasars have been catalogued in the Universe by probing deeper and using new methods for discovery. However, the hardest ones to find seem to be the rarest and brightest specimen. In this work, we study the properties of the most luminous of all quasars found so far. It has been overlooked until recently, which demonstrates that modern all-sky surveys have much to reveal. The black hole in this quasar accretes around one solar mass per day onto an existing mass of $\sim$17 billion solar masses. In this process its accretion disc alone releases a radiative energy of $2\times 10^{41}$ Watts. If the quasar is not strongly gravitationally lensed, then its broad line region (BLR) is expected to have the largest physical and angular diameter occurring in the Universe, and will allow the Very Large Telescope Interferometer to image its rotation and measure its black hole mass directly. This will be an important test for BLR size-luminosity relations, whose extrapolation has underpinned common black-hole mass estimates at high redshift.

astro-ph.CO↗