J/A+A/672/A167 eRASSt J234402.9-352640 optical photometry (Homan+, 2023)
Discovery of the luminous X-ray ignition eRASSt J234402.9-352640.
I. Tidal disruption event or a rapid increase in accretion in an
active galactic nucleus?
Homan D., Krumpe M., Markowitz A., Saha T., Gokus A., Partington E.,
Lamer G., Malyali A., Liu Z., Rau A., Grotova I., Cackett E.M.,
Buckley D.A.H., Ciroi S., Di Mille F., Gendreau K., Gromadzki M.,
Krishnan S., Schramm M., Steiner J.F.
<Astron. Astrophys. 672, A167 (2023)>
=2023A&A...672A.167H 2023A&A...672A.167H (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei; X-ray sources ; Photometry ; Optical
Keywords: X-rays: individuals: eRASSt J234402.9-352640 -
accretion, accretion disks - galaxies: active
Abstract:
In November 2020, a new, bright object, eRASSt J234402.9-352640, was
discovered in the second all-sky survey of SRG/eROSITA. The object
brightened by a factor of at least 150 in 0.2-2.0keV flux compared to
an upper limit found six months previous, reaching an observed peak of
1.76+0.03-0.24x10-11erg/cm2/s. The X-ray ignition is
associated with a galaxy at z=0.10, making the peak luminosity
log10(L0.2-2keV/[erg/s])=44.7±0.1. Around the time of the rise in
X-ray flux, the nucleus of the galaxy brightened by approximately
3mag. in optical photometry, after correcting for the host
contribution. We present X-ray follow-up data from Swift, XMM-Newton,
and NICER, which reveal a very soft spectrum as well as strong
0.2-2.0keV flux variability on multiple timescales. Optical spectra
taken in the weeks after the ignition event show a blue continuum with
broad, asymmetric Balmer emission lines, and high-ionisation
([OIII]4959,5007) and low-ionisation ([NII]6585, [SII]6716,6731)
narrow emission lines. Following the peak in the optical light curve,
the X-ray, UV, and optical photometry all show a rapid decline. The
X-ray light curve shows a decrease in luminosity of ∼0.45 over 33 days
and the UV shows a drop of ∼0.35 over the same period. eRASSt
J234402.9-352640 also shows a brightening in the mid-infrared, likely
powered by a dust echo of the luminous ignition. We find no evidence
in Fermi-LAT gamma-ray data for jet-like emission. The event displays
characteristics of a tidal disruption event (TDE) as well as of an
active galactic nucleus (AGN), complicating the classification of this
transient. Based on the softness of the X-ray spectrum, the presence
of high-ionisation optical emission lines, and the likely infrared
echo, we find that a TDE within a turned-off AGN best matches our
observations.
Description:
Overview of the optical photometry gathered for the object
J234402.9-352640. The data consist of results from public surveys, as
well as from new follow-up. The public data are drawn from the POSS2,
Gaia, and ATLAS surveys. New follow-up data were gathered using the
PROMPT-6 telescope, as part of Skynet. None of the data are corrected
for the contribution of the host galaxy (i.e. the ATLAS data are not
from differential imaging). The data were corrected for Galactic
extinction as detailed in the paper. Gaia data were provided as part
of the Gaia Alerts web server. These Gaia data do not come with an
estimate of the uncertainty. POSS2-B photographic magnitudes were
extracted using source decomposition, as several objects were blended
with the target object in the image. The data presented here come from
various sources and were taken using different filters. The data are
limited to period up to February 2020, with later data to be included
in a follow-up paper. This the full version of the data presented in
Table 4 of the paper.
Objects:
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RA (2000) DE Designation(s)
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23 44 02.9 -35 26 40 eRASSt J234402.9-352640
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table4.dat 37 421 Optical photometric data for eRASSt J234402.9-352640,
combining data from several instruments
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 8 F8.2 d MJD Observing date
10- 16 A7 --- Survey Name of survey/instrument from which the entry
originates
18- 24 A7 --- Filter Name of the filter used for the observation (1)
26- 31 F6.2 mag mag AB magnitude
34- 37 F4.2 mag e_mag ? 1sigma error on AB magnitude
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Note (1): Filters are B, R, G, c, o, POSS2-B and POSS2-R.
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Acknowledgements:
David Homan, dhoman(at)aip.de
(End) Patricia Vannier [CDS] 14-Feb-2023