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

Laser-driven very high energy electron/photon beam radiation therapy in conjunction with a robotic system

We present a new external-beam radiation therapy system using very-high-energy (VHE) electron/photon beams generated by a centimeter-scale laser plasma accelerator built in a robotic system. Most types of external-beam radiation therapy are delivered using a machine called a medical linear accelerator driven by radio frequency (RF) power amplifiers, producing electron beams with an energy range of 6-20 MeV, in conjunction with modern radiation therapy technologies for effective shaping of three-dimensional dose distributions and spatially accurate dose delivery with imaging verification. However, the limited penetration depth and low quality of the transverse penumbra at such electron beams delivered from the present RF linear accelerators prevent the implementation of advanced modalities in current cancer treatments. These drawbacks can be overcome if the electron energy is increased to above 50 MeV. To overcome the disadvantages of the present RF-based medical accelerators, harnessing recent advancement of laser-driven plasma accelerators capable of producing 1-GeV electron beams in a 1-cm gas cell, we propose a new embodiment of the external-beam radiation therapy robotic system delivering very high-energy electron/photon beams with an energy of 50-250 MeV; it is more compact, less expensive, and has a simpler operation and higher performance in comparison with the current radiation therapy system

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 ($\nu L_\nu$) 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 $\sim1500$ 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