分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Mrk 590 is a known changing-look AGN which almost turned off in 2012, and then in 2017 partially re-ignited into a repeat flaring state, unusual for an AGN. Our \emph{Swift} observations since 2013 allow us to characterise the accretion-generated emission and its reprocessing in the central engine of a changing-look AGN. The X-ray and UV variability amplitudes are higher than those typically observed in `steady-state' AGN at similar moderate accretion rates; instead, the variability is similar to that of highly accreting AGN. The unusually strong X-ray to UV correlation suggests that the UV-emitting region is directly illuminated by X-ray outbursts. We find evidence that the X-rays are reprocessed by two UV components, with the dominant one at $\sim$3 days and a faint additional reprocessor at near-zero lag. However, we exclude a significant contribution from diffuse broad line region continuum, known to contribute for bona-fide AGN. A near-zero lag is expected for a standard `lamp-post' disk reprocessing model with a driving continuum source near the black hole. That the overall UV response is dominated by the $\sim$3-day lagged component suggests a complicated reprocessing geometry, with most of the UV continuum not produced in a compact disk, as also found in recent studies of NGC 5548 and NGC 4151. Nonetheless, the observed flares display characteristic timescales of $\sim$100 rest-frame days, consistent with the expected thermal timescale in an accretion disk.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The X-ray source 3XMM~J150052.0+015452 was discovered as a spectacular tidal disruption event candidate during a prolonged ($>11$ yrs) outburst (Lin et al. 2017). It exhibited unique quasi-soft X-ray spectra of characteristic temperature $kT\sim0.3$ keV for several years at the peak, but in a recent Chandra observation (10 yrs into the outburst) a super-soft X-ray spectrum of $kT\sim0.15$ keV was detected. Such dramatic spectral softening could signal the transition from the super-Eddington to thermal state or the temporary presence of a warm absorber. Here we report on our study of four new XMM-Newton follow-up observations of the source. We found that they all showed super-soft spectra, suggesting that the source had remained super-soft for $>5$ yrs. Then its spectral change is best explained as due to the super-Eddington to thermal spectral state transition. The fits to the thermal state spectra suggested a smaller absorption toward the source than that obtained in Lin et al. (2017). This led us to update the modeling of the event as due to the disruption of a 0.75 msun star by a massive black hole of a few$\times10^5$ msun. We also obtained two HST images in the F606W and F814W filters and found that the dwarf star-forming host galaxy can be resolved into a dominant disk and a smaller bulge. No central point source was clearly seen in either filter, ruling out strong optical emission associated with the X-ray activity.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: We present the first results from a 100-day Swift, NICER and ground-based X-ray/UV/optical reverberation mapping campaign of the Narrow-Line Seyfert 1 Mrk 335, when it was in an unprecedented low X-ray flux state. Despite dramatic suppression of the X-ray variability, we still observe UV/optical lags as expected from disk reverberation. Moreover, the UV/optical lags are consistent with archival observations when the X-ray luminosity was >10 times higher. Interestingly, both low- and high-flux states reveal UV/optical lags that are 6-11 times longer than expected from a thin disk. These long lags are often interpreted as due to contamination from the broad line region, however the u band excess lag (containing the Balmer jump from the diffuse continuum) is less prevalent than in other AGN. The Swift campaign showed a low X-ray-to-optical correlation (similar to previous campaigns), but NICER and ground-based monitoring continued for another two weeks, during which the optical rose to the highest level of the campaign, followed ~10 days later by a sharp rise in X-rays. While the low X-ray countrate and relatively large systematic uncertainties in the NICER background make this measurement challenging, if the optical does lead X-rays in this flare, this indicates a departure from the zeroth-order reprocessing picture. If the optical flare is due to an increase in mass accretion rate, this occurs on much shorter than the viscous timescale. Alternatively, the optical could be responding to an intrinsic rise in X-rays that is initially hidden from our line-of-sight.