arXiv subjects

Prajwel Joseph

Publications and source records attributed to Prajwel Joseph.

12 recordsLinked to original sources

arXiv2026

Ultraviolet variability in Radio-Loud Active Galactic Nuclei observed by UVIT onboard AstroSat

Radio-loud active galactic nuclei (AGN) are among the most luminous objects in the Universe, emitting radiation from low-energy radio waves to high energy $\gamma$-rays. They are well known to exhibit flux variations at nearly all accessible wavelengths. However, their variability properties in the ultraviolet (UV) band remain relatively less explored compared to other wavebands. Here, we present the results of a systematic investigation of the UV flux and spectral variability characteristics of 24 radio-loud AGN spanning the redshift range 0.018 $\le$ $z$ $\le$ 2.218. The sample comprises 17 BL Lac objects, 6 flat spectrum radio quasars (FSRQs) and one radio-loud narrow line Seyfert 1 galaxy. We used observations obtained with the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat during its first ten years of operation, covering both the far-UV (FUV; 1300$-$1800 \AA) and near-UV (NUV; 2000$-$3000 \AA) bands. Of the 24 sources analysed, 18 showed significant UV variability on hour timescales. We found a bluer when brighter (BWB) spectral trend in two sources: the FSRQ CTA 102 and the BL Lac PKS 0447$-$439. The observed UV variability in our sample of radio-loud AGN, together with the BWB trend detected in these two sources, supports a scenario in which the hour timescale UV variations are driven by intrinsic processes within their relativistic jets.

astro-ph.GA↗

arXiv2026

Detection of time delay between UV and X-ray variability in Mrk 1044 using AstroSat observations

Active galactic nuclei are known to exhibit flux variations across the entire electromagnetic spectrum. Among these, correlations between UV/optical and X-ray flux variations serve as a key diagnostics for understanding the physical connection between the accretion disk and the corona. In this work, we present the results of analysis of ultraviolet (UV) and X-ray flux variations in the narrow line Seyfert 1 galaxy Mrk 1044. Simultaneous observations in the far-UV band (FUV: 1300$-$1800 \AA) and the X-ray band (0.5$-$7 keV) obtained during 31 August $-$ 8 September 2018 with the Ultraviolet Imaging Telescope and the Soft X-ray Telescope onboard \textit{AstroSat} were used for this study. Significant flux variability was detected in both FUV and X-ray bands. The fractional root mean square variability amplitude ($F_{\rm var}$) was found to be 0.036 $\pm$ 0.001 in the FUV band and 0.384 $\pm$ 0.004 in the X-ray band. To explore potential time lag between the two bands, cross-correlation analysis was performed using both the interpolated cross-correlation function (ICCF) and just another vehicle for estimating lags in nuclei (JAVELIN) methods. Results from both approaches are consistent within 2$\sigma$ uncertainty, indicating that X-ray variations lead the FUV variations, with measured lags of 2.25$\pm$0.05 days (ICCF) and $2.35_{-0.01}^{+0.02}$ days (JAVELIN). This is the first detection of a time delay between UV and X-ray variations in Mrk 1044. The observed UV lag supports the disk reprocessing scenario, wherein X-ray emission from the corona irradiates the accretion disk, driving the observed UV variability.

astro-ph.GA↗

arXiv2025

Nine years of UVIT: assessing sensitivity variation

The Ultra-Violet Imaging Telescope (UVIT) is one of the five payloads onboard the first Indian multiwavelength astronomical observatory, AstroSat, launched by the Indian Space Research Organisation on 28 September 2015. UVIT, designed for simultaneous imaging in the far-ultraviolet (FUV; 1300-1800 {\AA}) and near-ultraviolet (NUV; 2000-3000 {\AA}) channels, has completed nine years in orbit in 2024 despite the failure of the NUV channel in 2018. As the FUV optics is subject to possible reduction in sensitivity due to microscopic amounts of contaminants, we used the FUV data acquired by UVIT over the past nine years on the open cluster NGC 188 and the white dwarf HZ 4 to study sensitivity variations in the UVIT FUV channel. Our findings indicate no significant reduction in the sensitivity of the FUV channel over the last nine years, with no significant episodic variations due to unknown causes.

astro-ph.IM↗

arXiv2025

Redshift ~2.7 is not special

Galikyan et al. (2025) reported a statistically significant change in galaxy spectral properties at redshift $z$ $\simeq$ 2.7 based on a Kolmogorov Stochasticity Parameter analysis of JWST spectroscopic data of galaxies. In this comment, we demonstrate that their result is critically driven by a single outlier in the dataset. This outlier arises from the use of a questionable redshift estimate for one spectrum. When the outlier is removed or the redshift is corrected, the claimed transition at $z$ $\simeq$ 2.7 disappears entirely. By independently reproducing their analysis, we demonstrate that the claimed feature is not a robust statistical signal, but an artefact of this anomalous data point.

astro-ph.GA↗

arXiv2025

UVIT Data Release version 7: Regenerated high-level UVIT data products

The Ultra-Violet Imaging Telescope (UVIT) on board AstroSat is an active telescope capable of high-resolution far-ultraviolet imaging (<1.5'') and low resolution (${\lambda}/{\delta}{\lambda}$ $\approx$ 100) slitless spectroscopy with a field of view as large as ~0.5 degrees. Now almost a decade old, UVIT continues to be operational and generates valuable data for the scientific community. UVIT is also capable of near-ultraviolet imaging (<1.5''); however, the near-ultraviolet channel stopped working in August 2018 after providing data for nearly three years. This article gives an overview of the latest version (7.0.1) of the UVIT pipeline and UVIT Data Release version 7. The high-level products generated using pipeline versions having a major version number of seven will be called "UVIT Data Release version 7". The latest pipeline version overcomes the two limitations of the previous version (6.3), namely (a) the inability to combine all episode-wise images and (b) the failure of the astrometry module in a large fraction of the observations. The procedures adopted to overcome these two limitations, as well as a comparison of the performance of this new version over the previous one, are presented in this paper. The UVIT Data Release version 7 products are available at the Indian Space Science Data Center of the Indian Space Research Organisation for archival and dissemination from 01 June 2024. The new pipeline version is open source and made available on GitHub.

astro-ph.IM↗

arXiv2024

Ultraviolet flux and spectral variability study of blazars observed with UVIT/AstroSat

Blazars, the peculiar class of active galactic nuclei (AGN), are known to show flux variations across the accessible electromagnetic spectrum. Though they have been studied extensively for their flux variability characteristics across wavelengths, information on their ultraviolet (UV) flux variations on time scales of hours is very limited. Here, we present the first UV flux variability study on intraday time scales of a sample of 10 blazars comprising 2 flat spectrum radio quasars (FSRQs) and 8 BL Lacertae objects (BL Lacs). These objects, spanning a redshift (z) range of 0.034 <= z <= 1.003, were observed in the far-UV (FUV: 1300 - 1800 \text{\AA}) and near-UV (NUV: 2000 - 3000 \text{\AA}) wavebands using the ultraviolet imaging telescope on board AstroSat. UV flux variations on time scales of hours were detected in 9 sources out of the observed 10 blazars. The spectral variability analysis showed a bluer-when-brighter trend with no difference in the UV spectral variability behavior between the studied sample of FSRQs and BL Lacs. The observed UV flux and spectral variability in our sample of both FSRQs and BL Lacs revealed that the observed UV emission in them is dominated by jet synchrotron process.

astro-ph.HE↗

arXiv2023

UVIT view of NGC 5291: Ongoing star formation in tidal dwarf galaxies at ~ 0.35 kpc resolution

NGC 5291, an early-type galaxy surrounded by a giant HI ring, is believed to be formed from collision with another galaxy. Several star forming complexes and tidal dwarf galaxies are distributed along the collisional ring which are sites of star formation in environments where extreme dynamical effects are involved. Dynamical effects can affect the star formation properties and the spatial distribution of star forming complexes along the tidal features. To study and quantify the star formation activity in the main body and in the ring structure of the NGC 5291 system, we use high spatial resolution FUV and NUV imaging observations from the Ultraviolet Imaging Telescope onboard AstroSat. A total of 57 star-forming knots are identified to be part of this interacting system out of which 12 are new detections (star forming complexes that lie inside the HI contour) compared to the previous measurements from lower resolution UV imaging. We estimate the attenuation in UV for each of the resolved star-forming knots using the UV spectral slope $\beta$, derived from the FUV-NUV colour. Using the extinction corrected UV fluxes, we derive the star formation rate of the resolved star forming complexes. The extinction corrected total star formation rate of this system is estimated as 1.75 $\pm$ 0.04 $M_{\odot}/yr$. The comparison with dwarf galaxy populations (BCD, Sm and dIm galaxies) in the nearby Universe shows that many of the knots in the NGC 5291 system have SFR values comparable to the SFR of BCD galaxies.

astro-ph.GA↗

arXiv2023

UVIT observations of the Small Magellanic Cloud: Point source catalogue

Three fields in the outskirts of the Small Magellanic Cloud were observed by the Ultra-Violet Imaging Telescope (UVIT) on board AstroSat, during 31 December 2017 and 01 January 2018. The observations were carried out on a total of seven filters, three in the far ultra-violet (FUV; 1300$-$1800 \r{A}) band and four in the near ultra-violet (NUV; 2000$-$3000 \r{A}) band. We carried out photometry of these observations that have a spatial resolution better than 1.5$^{\prime\prime}$. We present here the first results of this work, which is a matched catalogue of 11,241 sources detected in three FUV and four NUV wavelengths. We make the catalogue available online, which would be of use to the astronomical community to address a wide variety of astrophysical problems. We provide an expression to estimate the total count rate in the full point spread function of UVIT that also incorporate the effect of saturation.

astro-ph.GA↗

arXiv2022

Active galactic nucleus feedback in NGC 3982

The energetic feedback from supermassive black holes can influence star formation at the centres of galaxies. Observational evidence for active galactic nucleus (AGN) impact on star formation can be searched for in galaxies by combining ultraviolet imaging and optical integral field unit data. The ultraviolet flux directly traces recent star formation, and the integral field unit data can reveal dust attenuation, gas ionisation mechanisms, and gas kinematics from the central regions of the galaxy disk. A pilot study on NGC 3982 shows star formation suppression in the central regions of the galaxy, likely due to negative AGN feedback, and enhanced star formation in the outer regions. The case of NGC 3982 could be observational evidence of AGN feedback operating in a Seyfert galaxy.

astro-ph.GA↗

arXiv2022

Smallest scale clumpy star formation in Stephan's Quintet revealed from UV and IR imaging

The spatial distribution and physical sizes of star forming clumps at the smallest scales provide valuable information on hierarchical star formation (SF). In this context, we report the sites of ongoing SF at ~120 pc along the interacting galaxies in Stephan's Quintet (SQ) compact group using AstroSat-UVIT and JWST data. Since ultraviolet radiation is a direct tracer of recent SF, we identified star forming clumps in this compact group from the FUV imaging which we used to guide us to detect star forming regions on JWST IR images. The FUV imaging reveals star forming regions within which we detect smaller clumps from the higher spatial resolution images of JWST, likely produced by PAH molecules and dust ionised by FUV emission from young massive stars. This analysis reveals the importance of FUV imaging data in identifying star forming regions in the highest spatial resolution IR imaging available.

astro-ph.GA↗

arXiv2022

UVIT view of Centaurus A; a detailed study on positive AGN feedback

$\require{mediawiki-texvc}$ Supermassive black holes at the centre of active galactic nuclei (AGN) produce relativistic jets that can affect the star formation characteristics of the AGN hosts. Observations in the ultraviolet (UV) band can provide an excellent view of the effect of AGN jets on star formation. Here, we present a census of star formation properties in the Northern Star-forming Region (NSR) that spans about 20 kpc of the large radio source Centaurus A hosted by the giant elliptical galaxy NGC 5128. In this region, we identified 352 UV sources associated with Cen A using new observations at an angular resolution of $<$1.5 arcseconds observed with the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat. These observations were carried out in one far-ultraviolet (FUV; $\lambda_{\text{mean}}$ = 1481 $\AA$) and three near-ultraviolet (NUV; with $\lambda_{\text{mean}}$ of 2196 $\AA$, 2447 $\AA$, and 2792 $\AA$, respectively) bands. The star-forming sources identified in UV tend to lie in the direction of the jet of Cen A, thereby suggesting jet triggering of star formation. Separating the NSR into Outer and Inner regions, we found the stars in the Inner region to have a relatively younger age than the Outer region, suggesting that the two regions may have different star formation histories. We also provide the UVIT source catalogue in the NSR.

astro-ph.GA↗

arXiv2022

The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package

The Astropy Project supports and fosters the development of open-source and openly-developed Python packages that provide commonly needed functionality to the astronomical community. A key element of the Astropy Project is the core package $\texttt{astropy}$, which serves as the foundation for more specialized projects and packages. In this article, we summarize key features in the core package as of the recent major release, version 5.0, and provide major updates for the Project. We then discuss supporting a broader ecosystem of interoperable packages, including connections with several astronomical observatories and missions. We also revisit the future outlook of the Astropy Project and the current status of Learn Astropy. We conclude by raising and discussing the current and future challenges facing the Project.

astro-ph.IM↗