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

M. Reshma, C. S. Stalin, Amit Kumar Mandal, S. B. Gudennavar, Senorita Benedict, Prajwel Joseph

Abstract

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 $γ$-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$-$1800 Å) and near-UV (NUV; 2000$-$3000 Å) bands. Of the 24 sources analysed, 18 showed significant UV variability on hour timescales. We found a bluer when brighter (BWB) spectral trend in two sources: the FSRQ CTA 102 and the BL Lac PKS 0447$-$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.

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

Abstract

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 -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 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; 13001800 Å) and near-UV (NUV; 20003000 Å) 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 0447439. 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.
Paper Structure (33 sections, 7 equations, 6 figures, 30 tables)

This paper contains 33 sections, 7 equations, 6 figures, 30 tables.

Table of Contents

  1. Introduction
  2. Observations and data reduction
  3. Flux and Spectral Variability Analysis
  4. Flux Variability
  5. Spectral Variability
  6. Notes on individual sources
  7. 1ES 0120+340
  8. 1ES 0229+200
  9. AO 0235+164
  10. 1H 0323+342
  11. 1ES 0347$-$121
  12. PKS 0352$-$686
  13. PKS 0447$-$439
  14. 1H 0658+595
  15. S5 0836+71
  16. ...and 18 more sections

Figures (6)

  • Figure 1: FUV field images of the blazar sources analyzed in this work. Red circles in each field image shows the target sources. The source name and the respective filter name are shown in all the field images. The field of view of each image is $\sim$$28^\prime$ diameter. In the field image of PKS 0352$-$686, the pattern observed near the target source is due to an unmasked hot pixel showing the drift correction patterns.
  • Figure 2: FUV field images of the blazar sources analyzed in this work. Red circles in each field image shows the target sources. The source name and the respective filter name are shown in all the field images. The field of view of each image is $\sim$$28^\prime$ diameter.
  • Figure 3: FUV (blue stars) and NUV (brown stars) lightcurves of sources. The source name along with the observation ID and filter names are provided in the respective plots.
  • Figure 4: FUV (blue stars) and NUV (brown stars) lightcurves of sources. The source name along with the observation ID and filter names are provided in the respective plots.
  • Figure 5: FUV (blue stars) lightcurves of sources. The source name along with the observation ID and filter names are provided in the respective plots.
  • ...and 1 more figures