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INTEGRAL detection of hard X-ray emission from NGC 1566

ATel #11754; L. Ducci (IAAT and ISDC), T. Siegert, R. Diehl (MPE), C. Sanchez-Fernandez (ESA/ESAC), C. Ferrigno, V. Savchenko, E. Bozzo (ISDC).
on 21 Jun 2018; 17:50 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

Subjects: X-ray, Gamma Ray, AGN, Transient

Referred to by ATel #: 11783, 11786, 11893, 11903, 11913, 11915, 12314, 12826

During the observations of the Reticulum II region carried out from 2018 June 12 to 2018 June 19 (total exposure time of 208 ks), the IBIS/ISGRI instrument on board INTEGRAL detected significant hard X-ray emission from a source with coordinates:
RA: 4:19:49 Dec: -54:56:28, with 3.5 arcmin 90% confinement radius and spatially coincident with the Seyfert galaxy NGC 1566.

The source was detected in the IBIS/ISGRI mosaic images at a significance level of 9.4 (5.5) sigma and a flux of 5.9 ± 0.6 mCrab (6 ± 1 mCrab) in the 20-40 keV (40-80 keV) energy band. The source remains below the sensitivity level in any one of the individual 70 ks datasets if analysed separately. The source was not detected in the JEM-X mosaic images, from which we estimated a 5 sigma upper limit of 3 mCrab in the 3-10 keV band. Assuming a distance of 10 Mpc (Gouliermis et al. 2017 MNRAS 468, 509), the flux measured in June 2018 corresponds to a 20-80 keV X-ray luminosity of about 1042 erg/s.

Hard X-ray radiation from NGC 1566 was also detected by INTEGRAL in a previous observation carried out from 2018 December 26 to 2018 January 27 (total exposure time of 492 ks) at a lower flux: 2.6 ± 0.3 mCrab (2.5 ± 0.7 mCrab) in the 20-40 keV (40-80 keV) energy band. No INTEGRAL observations of that region were performed before.

X-ray emission from NGC 1566 was reported on data from December 2004 to August 2013 in the Swift BAT 105-month catalogue at a lower flux level of ~2×10-11 erg/cm2/s in the 15-200 keV energy band (0.8 mCrab or 15-200 keV luminosity: 2.3×1041 erg/s). Swift/BAT detected a brightening from NGC 1566 with duration of about two months and a peak flux of 4 mCrab only once from 2004 to 2013.

The enhanced hard X-ray emission from NGC 1566 detected by INTEGRAL might be caused by a high luminosity state of the AGN at the center of NGC 1566 or by an extremely bright hard X-ray outburst produced by another source belonging to the galaxy (for example, Liu & Bregmann 2005, ApJSS 157, 59 report the presence of three ULXs in NGC 1566) or spatially coincident with it.

Additional INTEGRAL observations of the region around the source are already planned for the coming weeks. A target of opportunity observation with the Niel Gehrels Swift observatory has been approved.

Further observations of the source in soft X-rays, as well as in other energy domains, are encouraged.