Rapid Accretion Episode in Seyfert Galaxy 1ES 1927+654

ATel #12169; Erin Kara (UMD, NASA/GSFC), Michael Loewenstein (UMD/GSFC), Ronald A. Remillard (MIT), Keith Gendreau (NASA/GSFC), Zaven Arzoumanian (NASA/GSFC), Iair Arcavi (Tel Aviv University), Benny Trakhtenbrot (Tel Aviv University), Claudio Ricci (Universidad Diego Portales/KIAA), Andy Fabian (University of Cambridge), Chelsea MacLeod (Harvard, CfA), Diego Altamirano (University of Southampton), Edward M. Cackett (Wayne State University), Dheeraj R. Pasham (MIT), James F. Steiner (MIT), Kent Wood (Praxis, Inc.), Daichi Hiramatsu (Las Cumbres Obs.) for the NICER Team
on 5 Nov 2018; 12:39 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Erin Kara (ekara@astro.umd.edu)

Tweet

We report on recent NICER X-ray follow-up observations of the nuclear transient AT2018zf (ASASSN-18el; ATel #11391), initially classified as a Type II supernova (TNSCR #499) but which later displayed emission lines typical of broad-line AGN (Trakhtenbrot et al., in prep). The transient is spatially coincident with Seyfert galaxy 1ES 1927+654 (z=0.017), and may be evidence of an extremely rapid accretion episode.

Archival observations of 1ES 1927+654 in 2011 showed an already remarkable source, which defies AGN unification models. It was optically classified as a Type-2 AGN, but showed very little X-ray obscuration (Gallo et al., 2013, MNRAS, 433, 421). The archival X-ray spectrum was described with a \Gamma~2.4 power-law, and soft excess below 1 keV. The average 0.3-10 keV flux was ~1e-11 erg/cm^2/s. The flux varied rapidly, with a fractional excess variance of 25% on timescales ranging from 10 ks down to 500 seconds. This spectrum and variability is typical of an unobscured, high Eddington-ratio Type 1 AGN.

NICER began observing 1ES 1927+654 on 2018-05-22 (2.5 months after the ASAS-SN flare on 2018-03-03), and continues monitoring with 1-3 day cadence. The 0.3-10 keV flux in May was similar to archival observations (~1e-11 erg/cm^2/s), but the spectral shape had changed dramatically. Compared to archival spectra, the new spectra are extremely soft, described roughly as a 0.1 keV blackbody with very little hard X-ray emission above 2 keV. This is confirmed with an XMM-Newton DDT observation on 2018-06-05 (PI: C. Ricci). The 2-10 keV flux had decreased by ~2 orders of magnitude or more compared to the 2011 archival observations. We find little change in intrinsic absorption (N_H~4.0e20 cm^-2). The source varies rapidly on timescales from 10 ks to 500 seconds, with fractional excess variance of 35%.

Then from May-July 2018, the average X-ray flux decreased by >2 orders of magnitude, with inter-day flux variability from 1e-12 to 5e-11 erg/cm^2/s in the 0.5--2 keV band (see NICER light curve in link below). Over this time, there was little evolution in spectral shape. In August, the X-ray flux began to increase again, and the source is now brighter in X-rays than in May. The source continues to exhibit inter-day flux variability by as much as an order of magnitude. The weekly mean continues to increase through 2018-10-26, accompanied by a spectral hardening inferred by an increase in the NICER soft color hardness ratio (1-2 keV / 0.4-1.0 keV).

Those wishing to conduct simultaneous observations with NICER at other wavelengths can submit coordination requests through the TOO request process described at https://heasarc.gsfc.nasa.gov/docs/nicer/index.html

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.

NICER 0.5-2 keV lightcurve