Swift follow-up observations determine enhanced nuclear activity from NGC 1566

ATel #11783; C. Ferrigno (ISDC, CH), T. Siegert (MPE, Germany), C Sanchez-Fernandez, E. Kuulkers (ESAC/ESA, Spain), L. Ducci (IAAT, Germany/ ISDC, CH), V. Savchenko, E. Bozzo (ISDC, CH)
on 26 Jun 2018; 12:31 UT
Credential Certification: Carlo Ferrigno (Carlo.Ferrigno@unige.ch)

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We observed the Seyfert galaxy NGC 1566 with the Neil Gehrels Swift observatory on 2018-06-24 from 17:12 to 17:27 and from 18:48 to 19:01 UT for a total exposure time of 1.3 ks. This target of opportunity observation was triggered by an enhanced flux level with respect to the secular flux average, as already reported using INTEGRAL data in ATeL #11754.

The Swift/XRT observation was carried out in photon counting mode to determine the source position which resulted to be RA=65.00165, Dec=-54.93793 (04h 20m 00.4s, -54° 56′ 16.6′′) with an uncertainty radius of 2.2′′ (90% confidence level), using the enhanced UVOT field astrometry. The X-ray spectrum is well described by an absorbed power law with photon index Γ=1.79±0.14, N_H < 3.1 ×10²⁰cm^-2, and 0.3-10 keV flux of 9.3±0.7×10^-11 erg cm^-2 s^-1 (L_X=1.11±0.08 ×10⁴² erg/s). No significant variability has been detected on a 50-1000 s time scale. The joint fit of the Swift/XRT and INTEGRAL/IBIS data yields Γ=1.80^+0.10_-0.6 and N_H < 2 ×10²⁰cm^-2.

The source location is coincident with the known active nucleus of NGC 1566 and excludes the ultra luminous X-ray sources detected by Rosat (Liu et al., ApJS, 157, 1, 2005). The spectral shape is typical of Seyfert galaxies and marginally harder and less absorbed than in archival Suzaku and Swift/BAT observations, when the source was about 15 times weaker (Kawamuro et al., 2013, ApJ 770,2). We conclude that the luminosity enhancement is most probably due to increased nuclear activity.

We thank the Neil Gehrels Swift observatory staff for accepting and performing this target of opportunity observation. We acknowledge the service by the UK Swift Science Data Centre at the University of Leicester for data reduction (Evans et al. 2009, MNRAS, 397, 1177).