39 research outputs found
Mid-infrared variability of changing-look AGN
It is known that some active galactic nuclei (AGNs) transited from type 1 to
type 2 or vice versa. There are two explanations for the so-called changing
look AGNs: one is the dramatic change of the obscuration along the
line-of-sight, the other is the variation of accretion rate. In this paper, we
report the detection of large amplitude variations in the mid-infrared
luminosity during the transitions in 10 changing look AGNs using WISE and newly
released NEOWISE-R data. The mid-infrared light curves of 10 objects echoes the
variability in the optical band with a time lag expected for dust reprocessing.
The large variability amplitude is inconsistent with the scenario of varying
obscuration, rather supports the scheme of dramatic change in the accretion
rate.Comment: Published by ApjL, 7 pages, 3 figures, 2 table
Dissonance in harmony: The UV/optical periodic outbursts of ASASSN-14ko exhibit repeated bumps and rebrightenings
ASASSN-14ko was identified as an abnormal periodic nuclear transient with a
potential decreasing period. Its outbursts in the optical and UV bands have
displayed a consistent and smooth "fast-rise and slow-decay" pattern since its
discovery, which has recently experienced an unexpected alteration in the last
two epochs, as revealed by our proposed high-cadence Swift observations. The
new light curve profiles show a bump during the rising stages and a
rebrightening during the declining stages, making them much broader and
symmetrical than the previous ones. In the last two epochs, there is no
significant difference in the X-ray spectral slope compared to the previous
one, and its overall luminosity is lower than those of the previous epochs. The
energy released in the early bump and rebrightening phases ( erg)
could be due to collision of the stripped stream from partial tidal disruption
events (pTDEs) with an expanded accretion disk. We also discussed other
potential explanations, such as disk instability and star-disk collisions.
Further high-cadence multi-wavelength observations of subsequent cycles are
encouraged to comprehend the unique periodic source with its new intriguing
features.Comment: Accepted for publication in ApJL, 10 pages, 6 figure
Long-Term Decline of the Mid-Infrared Emission of Normal Galaxies: Dust Echo of Tidal Disruption Flare?
We report the discovery of a sample of 19 low redshift (z<0.22)
spectroscopically non-Seyfert galaxies that show slow declining mid-infrared
(MIR) light-curves (LCs), similar to those of tidal disruption event (TDE)
candidates with extreme coronal lines. Two sources also showed a relatively
fast rising MIR LCs. They consist of 61% sample of the WISE MIR variable
non-Seyfert galaxies with SDSS spectra. In a comparison sample of optically
selected Seyfert galaxies, the fraction of sources with such a LC is only 15%.
After rejecting 5 plausible obscured Seyfert galaxies with red MIR colours,
remaining 14 objects are studied in detail in this paper. We fit the declining
part of LC with an exponential law, and the decay time is typically one year.
The observed peak MIR luminosities () after subtracting host
galaxies are in the range of a few 10^42 to 10^44 erg~s^-1 with a median of
5x10^43 erg~s^-1 in the W2 band. The black hole masses distribute in a wide
range with more than half in between 10^7 to 10^8 ~M_sun, but significantly
different from that of optical/UV selected TDEs. Furthermore, MIR luminosities
are correlated with black hole masses, the stellar mass or luminosity of their
host bulges. Most galaxies in the sample are red and luminous with an absolute
magnitude of r between -20 to -23. We estimate the rate of event about 10^-4
gal^-1~yr^-1 among luminous red galaxies. We discuss several possibilities for
the variable infrared sources, and conclude that most likely, they are caused
by short sporadic fueling to the supermassive black holes via either the
instability of accretion flows or tidal disruption of stars.Comment: 32 pages, 12 figures. Accepted to MNRA
Infrared echo and late-stage rebrightening of nuclear transient PS1-10adi: exploring Torus by tidal disruption event in active galactic nuclei
Tidal disruption events (TDEs) in active galactic nuclei (AGNs) have been
overlooked for a long time but tentatively been investigated recently. We
report the discovery of a long-lasting luminous mid-infrared (mid-IR) flare in
PS1-10adi, which is a newly-identified highly energetic transient event
occurred in AGN. The IR luminosity of PS1-10adi, as well as other analogous
events, are at least one order of magnitude higher than all known supernova,
but can be well interpreted as the dust echoes of TDEs, whose ultra-high IR
energy is reprocessed from the dusty torus around the black hole. The torus
dust is sublimating during the early stage of the outburst and probably lead to
the observed rapid emergence of Fe II lines. Moreover, the UV-optical
rebrightening and contemporaneous X-ray onset after rest-frame days
since the optical peak is also an intriguing feature of PS1-10adi, which could
be attributed to the interaction between the high-velocity outflow and torus.
We suggest that the luminous IR echo is a very typical phenomenon of TDEs in
AGNs and may provide us an ideal opportunity to explore the torus properties.Comment: Accepted by ApJ, 1 table, 8 figure
The Carbon and Nitrogen Abundance Ratio in the Broad Line Region of Tidal Disruption Events
The rest-frame UV spectra of three recent tidal disruption events (TDEs), ASASSN-14li, PTF15af, and iPTF16fnl, display strong nitrogen emission lines but weak or undetectable carbon lines. In these three objects, the upper limits of the C III] λ1908/N III] λ1750 ratio are about two orders of magnitude lower than those of quasars, suggesting a high abundance ratio of [N/C]. With detailed photoionization simulations, we demonstrate that C2+ and N2+ are formed in the same zone, so the CIII]/N III] ratio depends only moderately on the physical conditions in the gas and weakly on the shape of the ionizing continuum. There are smaller than 0.5 dex variations in the line ratio over wide ranges of gas densities and ionization parameters at a given metallicity. This allows a robust estimate of the relative abundance ratio of nitrogen to carbon. We derive a relative abundance ratio of [N/C] \u3e 1.5 for ASASSN-14li, and an even higher one for PTF15af and iPTF16fnl. This suggests that the broad line region in those TDE sources is made of nitrogen-enhanced core material that falls back at later times. Based on stellar evolution models, the lower limit of the disrupted star should be larger than 0.6 M⊙. The chemical abundance of the line-emitting gas provides convincing evidence that the flares originate from stellar tidal disruptions. The coincidence of the weakness of the X-ray emission with the strong broad absorption lines in PTF15af and iPTF16fnl, and the strong X-ray emission without such lines in ASASSN-li14, are analogous to quasars with and without broad absorption lines