| Issue |
A&A
Volume 691, November 2024
|
|
|---|---|---|
| Article Number | L5 | |
| Number of page(s) | 6 | |
| Section | Letters to the Editor | |
| DOI | https://doi.org/10.1051/0004-6361/202451609 | |
| Published online | 29 October 2024 | |
Letter to the Editor
Hidden by a star: The redshift and the offset broad line of the flat-spectrum radio quasar PKS 0903–57⋆
1
Université Paris Cité, CNRS, CEA, Astroparticule et Cosmologie, F-75013 Paris, France
2
Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, Université Paris Cité, CNRS, F-92190 Meudon, France
3
Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, France
4
INAF – Istituto di Radioastronomia, Via Gobetti 101, I-40129 Bologna, Italy
5
Department of Physics, Chemistry & Material Science, University of Namibia, Private Bag 13301, Windhoek, Namibia
6
Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
7
Centre for Space Research, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa
8
Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, United Kingdom
⋆⋆ Corresponding author; goldoni@apc.in2p3.fr
Received:
22
July 2024
Accepted:
5
October 2024
Context. PKS 0903−57 is a little-studied γ-ray blazar that has recently attracted considerable interest due to the strong flaring episodes observed since 2020 in high energy (HE; 100 MeV ≤ E ≤ 100 GeV) and very high-energy (VHE; 100 GeV ≤ E ≤ 10 TeV) γ-rays. Its nature and properties are still not well determined. In particular, it is unclear whether PKS 0903−57 is a BL Lac or a flat-spectrum radio quasar (FSRQ), while its redshift estimation relies on a possibly misassociated low signal-to-noise ratio spectrum.
Aims. Our aims were to reliably measure the redshift of the blazar, and to determine its spectral type and luminosity in the optical range.
Methods. We performed spectroscopy of the optical counterpart of the blazar using the South African Large Telescope (SALT) and the Very Large Telescope (VLT), and monitored it photometrically with the Rapid Eye Mount (REM) telescope.
Results. We firmly measured the redshift of the blazar as z = 0.2621 ± 0.0006 thanks to the detection of five narrow optical lines. The detection of a symmetric broad Hα line with full width at half maximum (FWHM) of 4020 ± 30 km/s together with a jet-dominated continuum leads us to classify it as a FSRQ. Finally, we detected with high significance a redshift offset (∼1500 km/s) between the broad line and the host. This is the first time that such an offset has been unequivocally detected in a VHE blazar, possibly pointing to a very peculiar accretion configuration, a merging system, or a recoiling black hole.
Key words: galaxies: active / galaxies: distances and redshifts / BL Lacertae objects: general
Based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, Chile, under programs P10822CJ.002 and P112.25SN.001. The raw FITS data files are available in the ESO archive. Based on observations made with the Southern African Large Telescope (SALT) under program 2020-1-SCI-027 (PI: E. Kasai).
© The Authors 2024
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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