arXiv subjects

Kriti K. Gupta

Publications and source records attributed to Kriti K. Gupta.

5 recordsLinked to original sources

arXiv2026

The Millimeter/X-ray Relation in Rapidly Accreting Supermassive Black Holes at $z < 0.16$

A tight correlation between nuclear millimeter and X-ray emission has recently been found in nearby ($z < 0.01$) and low-Eddington ratio ($\rm λ_{Edd} < 0.1$) radio-quiet Active Galactic Nuclei (AGN), suggesting a common origin in the hot X-ray corona. We test this relation in nine more distant RQ AGN ($z \sim 0.06-0.16$) with higher bolometric luminosities ($\log(L_{\rm bol}/\mathrm{erg\,s^{-1}})=45.3-46.3$), Eddington ratios ($\rm λ_{Edd} = 0.19-0.85$), and X-ray bolometric corrections ($κ_{2-10}=29-194$), selected from the Burst Alert Telescope (BAT) survey. We obtained quasi-simultaneous observations with Swift at 2-10 keV and the Atacama Large Millimeter/submillimeter Array (ALMA) at 100 GHz and with high angular resolution ($<0.14$"). We find that these high-luminosity AGN lie above the millimeter/X-ray correlation defined by lower-luminosity sources. A joint fit to both samples yields a second-degree polynomial with an intrinsic scatter of 0.32 dex. Furthermore, the millimeter emission correlates linearly with both the UV disk luminosity and $L_{\rm bol}$, with intrinsic scatters of 0.45 and 0.35 dex, respectively. We propose that the deviation from the linear millimeter/X-ray relation arises from a two-component coronal electron population: thermal electrons that produce X-rays, but become less efficient at higher luminosities, and nonthermal electrons that produce millimeter emission and remain tied to $L_{\rm bol}$. Additional millimeter emission from outflow-driven shocks may also contribute, though spectral energy distribution modeling and spectral index studies favor a coronal origin.

astro-ph.HE↗

arXiv2026

Resolved UV-Optical HST Imaging and Spectral Energy Distribution Modeling of Nearby BAT Active Galactic Nuclei

We use high-resolution UV-to-optical imaging from the Hubble Space Telescope (HST) to construct spatially resolved spectral energy distributions (SEDs) for seven nearby ($z<0.07$) hard (14--195$\,$keV) X-ray-selected broad-line active galactic nuclei (AGN) with $L_{\rm bol}=10^{43.26}-10^{45.34}\,\rm{erg\,s^{-1}}$. The high spatial resolution of HST, which physically resolves structures on the scale of $\sim$50$\,$pc at $z=0.05$, enables the separation of AGN and host-galaxy emission through morphological decomposition with GALFIT, yielding improved measurements of AGN properties compared to those obtained with lower-resolution Swift UV/Optical Telescope (UVOT) data. AGN UV magnitudes derived from HST imaging (e.g., F225W) can differ by more than a magnitude from those from Swift/UVOT UVM2 due to extended nuclear emission. Additionally, the inclusion of high-resolution data at longer wavelengths (e.g., F814W) can significantly affect the resulting SED fit. Comparing fits of accretion disk and extinction models using HST and Swift/UVOT data, we find significant differences in the resulting parameters, with average differences of 2.0$\,$eV in the maximum disk temperature and 2.2$\,$mag in the AGN host-galaxy extinction. These differences ultimately lead to significant changes in bolometric luminosities and X-ray bolometric corrections, with the HST-based fits yielding average increases of $\sim$0.57$\,$dex and $\sim$0.66$\,$dex respectively. This demonstrates host-galaxy contamination in unresolved UV--optical data can strongly bias SED-based estimates of disk temperatures, extinction, bolometric luminosities, and X-ray bolometric corrections in AGN. Large-area, high-resolution imaging surveys from Euclid and the Nancy Grace Roman Space Telescope will extend these techniques to much larger AGN samples, enabling uniform, high-precision SED measurements in the near-IR.

astro-ph.GA↗

arXiv2025

BASS. XLIV. Morphological preferences of local hard X-ray selected AGN

We present morphological classifications for the hosts of 1189 hard X-ray selected (14-195 keV) active galactic nuclei (AGNs) from the Swift-BAT 105-month catalog as part of the BAT AGN Spectroscopic Survey (BASS). BASS provides a powerful all-sky census of nearby AGN, minimizing obscuration biases and providing a robust dataset for studying AGN-host galaxy connections. Classifications are based on volunteer-based visual inspection on the Zooniverse platform, adapted from Galaxy Zoo DECaLS (GZD). Dual-contrast grz color composite images, generated from public surveys (e.g., NOAO Legacy Survey, Pan-STARRS, SDSS) and dedicated observations enabled key morphological features to be identified. Our analysis reveals that, with respect to a control sample of inactive galaxies matched in redshift and i-band magnitude, BASS AGN hosts show a deficiency of smooth ellipticals (~70%) and disks with prominent arms (~80%), while displaying an excess of mergers or disturbed systems (~400%), and disk galaxies without a spiral structure (~300%). These trends suggest a preference for AGN activity in gas-rich, dynamically disturbed environments or transitional disk systems. We also find a higher bar fraction among AGN hosts than the control sample (~50% vs. ~30%). We further explore the relations between AGN properties (e.g., X-ray luminosity, black hole mass, and Eddington ratio) and host morphology, and find that high-luminosity and high-accretion AGN preferentially reside in smooth or point-like hosts. In parallel, lower-luminosity AGN are more common in disk galaxies. These results underscore the importance of morphological studies in understanding the fueling and feedback mechanisms that drive AGN activity and their role in galaxy evolution. Our dataset provides a valuable benchmark for future multiwavelength surveys (e.g. LSST, Roman, and Euclid) and automated morphological classification efforts.

astro-ph.GA↗

arXiv2024

BASS. XLIII: Optical, UV, and X-ray emission properties of unobscured Swift/BAT active galactic nuclei

We present one of the largest multiwavelength studies of simultaneous optical-to-X-ray spectral energy distributions (SEDs) of unobscured active galactic nuclei (AGN) in the local Universe. Using a representative sample of hard-X-ray-selected AGN from the 70-month Swift/BAT catalog, with optical/UV photometric data from Swift/UVOT and X-ray spectral data from Swift/XRT, we constructed broadband SEDs of 236 nearby AGN (0.001<z<0.3). We employed GALFIT to estimate host galaxy contamination in the optical/UV and determine the intrinsic AGN fluxes. We used an absorbed power law with a reflection component to model the X-ray spectra and a dust-reddened multi-temperature blackbody to fit the optical/UV SED. We calculated total bolometric luminosities ($L_{bol}$), optical-to-X-ray spectral indices ($α_{ox}$), and multiple bolometric corrections (BCs) in the optical, UV, and X-rays. We used black hole masses obtained by reverberation mapping and the virial method to estimate Eddington ratios ($λ_{Edd}$) for all our AGN. We confirm the tight correlation between UV and X-ray luminosity for our sample. We observe a significant decrease in $α_{ox}$ with $L_{bol}$ and $λ_{Edd}$, suggesting that brighter sources emit more UV photons per X-rays. We report a second-order regression relation between the 2-10 keV BC and $α_{ox}$, which is useful to compute $L_{bol}$ in the absence of multiband SEDs. We also investigate the dependence of optical/UV BCs on the physical properties of AGN and obtain a significant increase in the UV BCs with $L_{bol}$ and $λ_{Edd}$, unlike those in the optical, which are constant across five orders of $L_{bol}$ and $λ_{Edd}$. We obtain significant dispersions (~0.1-1 dex) in all BCs, and hence recommend using appropriate relations with observed quantities while including the reported scatter, instead of their median values.

astro-ph.GA↗

arXiv2023

A Tight Correlation Between Millimeter and X-ray Emission in Accreting Massive Black Holes from <100 Milliarcsecond-resolution ALMA Observations

Recent studies have proposed that the nuclear millimeter continuum emission observed in nearby active galactic nuclei (AGN) could be created by the same population of electrons that gives rise to the X-ray emission that is ubiquitously observed in accreting black holes. We present the results of a dedicated high spatial resolution ($\sim$60-100 milliarcsecond) ALMA campaign on a volume-limited ($<50$ Mpc) sample of 26 hard X-ray ($>10$ keV) selected radio-quiet AGN. We find an extremely high detection rate (25/26 or $94^{+3}_{-6}\%$), which shows that nuclear emission at mm-wavelengths is nearly ubiquitous in accreting SMBHs. Our high-resolution observations show a tight correlation between the nuclear (1-23 pc) 100GHz and the intrinsic X-ray emission (1$σ$ scatter of $0.22$ dex). The ratio between the 100GHz continuum and the X-ray emission does not show any correlation with column density, black hole mass, Eddington ratio or star formation rate, which suggests that the 100GHz emission can be used as a proxy of SMBH accretion over a very broad range of these parameters. The strong correlation between 100GHz and X-ray emission in radio-quiet AGN could be used to estimate the column density based on the ratio between the observed 2-10keV ($F^{\rm obs}_{2-10\rm\,keV}$) and 100GHz ($F_{100\rm\,GHz}$) fluxes. Specifically, a ratio $\log (F^{\rm obs}_{2-10\rm\,keV}/F_{100\rm\,GHz})\leq 3.5$ strongly suggests that a source is heavily obscured [$\log (N_{\rm H}/\rm cm^{-2})\gtrsim 23.8$]. Our work shows the potential of ALMA continuum observations to detect heavily obscured AGN (up to an optical depth of one at 100GHz, i.e. $N_{\rm H}\simeq 10^{27}\rm\,cm^{-2}$), and to identify binary SMBHs with separations $<100$ pc, which cannot be probed by current X-ray facilities.

astro-ph.HE↗