Extreme variability in an active galactic nucleus: Gaia16aax
G. Cannizzaro, M. Fraser, P. G. Jonker, J. E. Pringle, S. Mattila, P. C. Hewett, T. Wevers, E. Kankare, Z. Kostrzewa-Rutkowska, Ł. Wyrzykowski, F. Onori, J. Har… 展开作者
G. Cannizzaro, M. Fraser, P. G. Jonker, J. E. Pringle, S. Mattila, P. C. Hewett, T. Wevers, E. Kankare, Z. Kostrzewa-Rutkowska, Ł. Wyrzykowski, F. Onori, J. Harmanen, K. E. S. Ford, B. McKernan, C. J. Nixon
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21 pages, 17 figure - accepted for publication in MNRAS main journal
We present the results of a multi-wavelength follow up campaign for the luminous nuclear transient Gaia16aax, which was first identified in January 2016. The transient is spatially consistent with the nucleus of an active galaxy at z=0.25, hosting a black hole of mass $\rm \sim6\times10^8M_\odot$. The nucleus brightened by more than 1 magnitude in the Gaia G-band over a timescale of less than one year, before fading back to its pre-outburst state over the following three years. The optical spectra of the source show broad Balmer lines similar to the ones present in a pre-outburst spectrum. During the outburst, the $\rm Hα$ and $\rm Hβ$ emission lines develop a secondary peak. We also report on the discovery of two transients with similar light curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by a microlensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disk. We consider variability in the accretion flow in the inner part of the disk, or a tidal disruption event of a star $\geq 1 M_{\odot}$ by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria.
We report short-cadence monitoring of a radio-quiet Active Galactic Nuclei (AGN), NGC7469, at 95 GHz (3 mm) over a period of 70 days with the CARMA telescope. The AGN varies significantly ($\pm3σ$ from the mean) by a factor of two within 4-5 days. The intrinsic 95 GHz variability amplitude in excess of the measurement noise (10%) and relative to the mean flux is comparable to that in the X-rays, and much higher than at 8.4 GHz. The mm-band variability and its similarity to the X-ray variability adds to the evidence that the mm and X-ray emission have the same physical origin, and are associate…
Migration Traps as Variability Attractors. I. Optical/UV Signatures of Embedded Stellar-Mass Black Holes in Active Galactic Nucleus Disks
Xiamen University（厦门大学）；SHAO-XMU Joint Center for Astrophysics, Xiamen University（厦大-上海天文台联合天体物理中心）；Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；University of Science and Technology of China（中国科学技术大学）；Guangxi University（广西大学）
The active galaxy NGC1275 lies at the centre of the Perseus cluster of galaxies, which is the X-ray brightest cluster in the Sky. The nucleus shows large variability over the past few decades. We compile a lightcurve of its X-ray emission covering about 40 years and show that the bright phase around 1980 explains why the inner X-ray bubbles were not seen in the images taken with the Einstein Observatory. The flux had dropped considerably by 1992 when images with the ROSAT HRI led to their discovery. The nucleus is showing a slow X-ray rise since the first Chandra images in 2000. If it brighten…
A key characteristic of many active galactic nuclei (AGN) is their variability, but its origin is poorly understood, especially in the radio domain. Williams et al. (2017) reported a ~50 per cent increase in peak flux density of the AGN in the Seyfert galaxy NGC 4151 at 1.5 GHz with the e-MERLIN array. We present new high resolution e-MERLIN observations at 5 GHz and compare these to archival MERLIN observations to investigate the reported variability. Our new observations allow us to probe the nuclear region at a factor three times higher-resolution than the previous e-MERLIN study. We separa…
We present results from a new optical/UV/X-ray monitoring campaign of the luminous Seyfert galaxy Fairall 9 using the Swift satellite. Using the UV-Optical Telescope (UVOT) on Swift, we find correlated optical/UV variability on all time scales ranging from the sampling time (4-days) to the length of the campaign (2.5 months). In one noteworthy event, the UW2-band flux dips by 20% in 4-days, and then recovers equally quickly; this event is not seen in either the optical or the X-ray bands. We argue that this event provides further evidence that a significant fraction of the UV-emission must be …
Rapid, large amplitude variability at optical to X-ray wavelengths is now seen in an increasing number of Seyfert galaxies and luminous quasars. The variations imply a global change in accretion power, but are too rapid to be communicated by inflow through a standard thin accretion disc. Such discs are long known to have difficulty explaining the observed optical/UV emission from active galactic nuclei. Here we show that alternative models developed to explain these observations have larger scale heights and shorter inflow times. Accretion discs supported by magnetic pressure in particular are…
When a black hole accretes close to the Eddington limit, the astrophysical jet is often accompanied by radiatively driven, wide-aperture and mildly relativistic winds. Powerful winds can produce significant non-thermal radio emission via shocks. Among the nearby critical accretion quasars, PDS 456 has a very massive black hole (about one billion solar masses), shows a significant star-forming activity (about seventy solar masses per year) and hosts exceptionally energetic X-ray winds (power up to twenty per cent of the Eddington luminosity). To probe the radio activity in this extreme accretio…
We present a detailed study of the highly obscured active galaxy NGC 4507, performed using four Nuclear Spectroscopic Telescope Array (NuSTAR) observations carried out between May and August in 2015 (~ 130 ks in total). Using various phenomenological and physically motivated torus models, we explore the properties of the X-ray source and those of the obscuring material. The primary X-ray emission is found to be non-variable, indicating a stable accretion during the period of the observations. We find the equatorial column density of the obscuring materials to be ~ 2 x 10^24 cm^-2 while the lin…
We analyze 1598 serendipitous Chandra X-ray observations of 462 radio-quiet quasars to constrain the frequency of extreme amplitude X-ray variability that is intrinsic to the quasar corona and innermost accretion flow. The quasars in this investigation are all spectroscopically confirmed, optically bright ($m_i \leq$ 20.2), and contain no identifiable broad absorption lines in their optical/ultraviolet spectra. This sample includes quasars spanning $z \approx$ 0.1 - 4 and probes X-ray variability on timescales of up to $\approx$ 12 rest-frame years. Variability amplitudes are computed between …
We present results of a long-term optical, UV and X-ray study of variability of the nearby changing-look (CL) Seyfert NGC 1566 which was observed with the Swift Observatory from 2007 to 2020. We summarize our previously published spectroscopic and photometric results and present new observations. We reported on the alteration in the spectral type of NGC 1566 in 2018 (REF1). Moreover, we focused on the exceptional postmaximum behavior after 2018 July, when all bands dropped with some fluctuations (REF2). We observed four significant re-brightenings in the post-maximum period. We have found diff…
We report the discovery of extreme X-ray variability in a type 1 quasar: SDSS J$075101.42+291419.1$. It has a black hole mass of $1.6\times 10^7~\rm M_\odot$ measured from reverberation mapping (RM), and the black hole is accreting with a super-Eddington accretion rate. Its XMM-Newton observation in 2015 May reveals a flux drop by a factor of $\sim 22$ with respect to the Swift observation in 2013 May when it showed a typical level of X-ray emission relative to its UV/optical emission. The lack of correlated UV variability results in a steep X-ray-to-optical power-law slope ($α_{\rm OX}$) of -…
NGC 1566 is a changing look AGN known to exhibit recurrent X-ray outbursts with each lasting for several years. The most recent X-ray outburst is observed on 2018, with a substantial increase of 2--10 keV flux by a factor of ~24 than the historical minimum. We re-analyze the XMM-Newton and NuSTAR observations covering the pre-outburst, outburst and post-outburst epochs, and confirm the discovery of the broad feature in the ~5--7 keV band during the period of outburst that could be interpreted as a relativistic Fe K_alpha emission line. Our analysis suggests that its flux has increased in tande…
Young stellar objects are known to exhibit strong radio variability on timescales of weeks to months, and a few reports have documented extreme radio flares, with at least an order of magnitude change in flux density on timescales of hours to days. However, there have been few constraints on the occurrence rate of such radio flares or on the correlation with pre-main sequence X-ray flares, although such correlations are known for the Sun and nearby active stars. Here we report simultaneous deep VLA radio and Chandra X-ray observations of the Orion Nebula Cluster, targeting hundreds of sources …
We examine archival XMM-Newton data on the extremely variable narrow-line Seyfert 1 (NLS1) active galactic nucleus (AGN) 1H 0707-495. We construct fractional excess variance (Fvar) spectra for each epoch, including the recent 2019 observation taken simultaneously with eROSITA. We explore both intrinsic and environmental absorption origins for the variability in different epochs, and examine the effect of the photoionised emission lines from outflowing gas. In particular, we show that the unusual soft variability first detected by eROSITA in 2019 is due to a combination of an obscuration event …
We report the discovery of an extreme X-ray flux rise (by a factor of > 20) of the weak-line quasar SDSS J153913.47+395423.4 (hereafter SDSS J1539+3954) at z = 1.935. SDSS J1539+3954 is the most-luminous object among radio-quiet type 1 AGNs where such dramatic X-ray variability has been observed. Before the X-ray flux rise, SDSS J1539+3954 appeared X-ray weak compared with the expectation from its UV flux; after the rise, the ratio of its X-ray flux and UV flux is consistent with the majority of the AGN population. We also present a contemporaneous HET spectrum of SDSS J1539+3954, which demons…
We present a multi-wavelength study of the active nucleus and the off-nuclear X-ray sources in the nearby spiral galaxy, NGC 1365 using three simultaneous UV/X-ray observations by AstroSat over a two months period and archival IR observations performed with Spitzer and Herschel. Utilising the data from the Soft X-ray Telescope (SXT) on-board AstroSat, we find spectral variability mainly caused by the variation in the X-ray column density, (N$_H$ $\sim$ 10$^{22}$ - 10$^{23}$ cm$^{-2}$). With the accurate spatial resolution of the UVIT onboard AstroSat, we separate the intrinsic AGN flux from th…
The ultra-soft narrow-line Seyfert 1 galaxy 1H 0707-495 is a well-known and highly variable active galactic nucleus (AGN), with a complex, steep X-ray spectrum, and has been studied extensively with XMM-Newton. 1H 0707-495 was observed with the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) aboard the Spectrum-Roentgen-Gamma (SRG) mission on October 11, 2019, for about 60,000 seconds as one of the first calibration and pointed verification phase (CalPV) observations. The eROSITA light curves show significant variability in the form of a flux decrease by a factor of 58 with …
We present results of optical observations of the extremely violently variable blazar S4 0954+65 on intra-night time scales. The object showed flux changes of up to 100% within a few hours. Time delays between optical bands, color changes and "rms-flux" relations are investigated and the results are discussed in terms of existing models of blazar variability.
The physical properties of the accretion flow and of the X-ray emitting plasma, in supermassive black holes accreting at extreme Eddington rates, are still very unclear. Here we present the analysis of simultaneous XMM-Newton and NuSTAR observations of the hyper-Eddington Seyfert 1 galaxy IRAS 04416+1215, carried out in 2020. The main goal of these observations is to investigate the properties of the X-ray corona, as well as the structure of the accretion flow and of the circumnuclear environment, in this regime of extreme accretion. IRAS 04416+1215 has one of the highest Eddington ratio ($λ_{…
Rapid variability in the radio flux density of the BL Lac object PKSB1144-379 has been observed at four frequencies, ranging from 1.5 to 15 GHz, with the VLA and the University of Tasmania's Ceduna antenna. Intrinsic and line of sight effects were examined as possible causes of this variability, with interstellar scintillation best explaining the frequency dependence of the variability timescales and modulation indices. This scintillation is consistent with a compact source 20-40 microarcseconds, or 0.15-0.3 pc in size. The inferred brightness temperature for PKSB1144-379 (assuming that the ob…
The VERITAS array of imaging atmospheric Cherenkov telescopes has collected nearly 6000 hours of observations of active galactic nuclei (AGNs). It has detected 39 very-high-energy (VHE, >100 GeV) AGNs at redshifts up to z = 0.9, of which 24 are classified as high-frequency-peaked BL Lacertae objects (HBLs). VERITAS has obtained an extensive dataset of HBL, including extreme HBL (xHBL), observations, with lightcurves spanning up to 11 years, allowing the characterization of their long-term and short-term variability. A study of various xHBLs/HBLs in the VERITAS dataset is presented, and the cor…
Systematic Monitoring of Extreme X-ray Variability from Weak-line Quasars
The Pennsylvania State University（宾夕法尼亚州立大学）；Nanjing University（南京大学）；Max-Planck-Institut für extraterrestrische Physik (MPE)（马克斯·普朗克地外物理研究所）；University of Nevada, Reno（内华达大学雷诺分校）；University of Nevada, Las Vegas（内华达大学拉斯维加斯分校）；University of North Texas（北德克萨斯大学）；INAF – Osservatorio di Astrofisica e Scienza dello Spazio（意大利国家天体物理研究所空间科学天文台）
We analyse the properties of the X-ray emission from Seyfert 2 NGC 7172 which are based on the 32 ks observational data from NuSTAR in 2014. This source showed a constant Compton-thin obscuration $N_{H}\approx 8\cdot 10^{22} cm^{-2}$, which is similar to that observed by XMM-Newton, Suzaku, ASCA and BeppoSAX over past 30 years. We revealed the presence of a moderate reflection component with <R>~0.44 and a narrow FeKα line with $67^{+13}_{-14}$ eV. The applied numerical models of torus confirmed the Compton-thin type of the Seyfert nucleus and allowed us to obtain an estimate of the torus open…
RX J1301.9+2747 is an ultrasoft active galactic nucleus (AGN) with unusual X-ray variability that is characterized by a long quiescent state and a short-lived flare state. The X-ray flares are found to recur quasi-periodically on a timescale of 13-20 ks. Here, we report the analysis of the light curve in the quiescent state from two XMM observations spanning 18.5 years, along with the discovery of a possible quasi-periodic X-ray oscillation (QPO) with a period of ~1500s. The QPO is detected at the same frequency in the two independent observations, with a combined significance of >99.89%. The …
We study the long-term variability in the optical monitoring database of Ark~120, a nearby radio-quiet active galactic nucleus (AGN) at a distance of 143 Mpc (z=0.03271). We compiled the historical archival photometric and spectroscopic data since 1974 and conducted a new two-year monitoring campaign in 2015-2017, resulting in a total temporal baseline over four decades. The long-term variations in the optical continuum exhibit a wave-like pattern and the Hbeta integrated flux series varies with a similar behavior. The broad Hbeta profiles have asymmetric double peaks, which change strongly wi…
The majority of the active galactic nuclei (AGN) detected at very-high-energies above 100 GeV belong to the class of blazars with a small angle between the jet-axis and the line-of-sight. Only about 10 percent of the gamma-ray AGN are objects with a larger viewing angle resulting in a smaller Doppler boosting of the emission. Originally, it was believed that gamma-ray emission can only be observed from blazars and those are variable in its brightness. Instead, the last years have shown that non-blazar active galaxies also show a fascinating variability behaviour which provide important new ins…
We report on observations of the optical variability of the radio-loud, narrow-line Seyfert 1 galaxy J0948+0022 on time scales ranging from minutes to years. Implications regarding recent suggestions that the object may constitute a prototype for an emerging class of blazar-like objects similar to FSRQs are discussed. The optical microvariability observed for J0948+0022 is found to be similar to that found for a typical LBL blazar. Based on observations of J0948+0022 in a flaring state and a significantly lower state, one can demonstrate that these rapid variations are most likely originating …
We investigate the physical mechanisms of high-energy gamma-ray emission from the TeV-emitting misaligned active galactic nucleus IC 310. Eight years of data from the Fermi Large Area Telescope (Fermi-LAT) between 100 MeV and 500 GeV are reduced and analysed to study the temporal and spectral characteristics of IC 310. Point spread function-partitioned instrument response functions are used to improve the resolvability of IC 310 from nearby NGC 1275. Systematic effects due to this choice of instrument response functions and the proximity of NGC 1275 are investigated. We find strong spectral va…
We present new X-ray spectral data for the Seyfert 1 nucleus in NGC 4151 observed with Chandra for 200 ks. A significant ACIS pileup is present, resulting in a non-linear count rate variation during the observation. With pileup corrected spectral fitting, we are able to recover the spectral parameters and find consistency with those derived from unpiled events in the ACIS readout streak and outer region from the bright nucleus. The absorption corrected 2-10 keV flux of the nucleus varied between 6E-11 and 1E-10 erg s^{-1} cm^{-2}. Similar to earlier Chandra studies of NGC 4151 at a historical …
Variable X-ray and gamma-ray emission is characteristic of the most extreme physical processes in the Universe, and studying the sources of these energetic photons has been a major driver in astronomy for the past 50 years. Here we present multiwavelength observations of a unique gamma-ray selected transient, discovered by Swift, which was accompanied by bright emission across the electromagnetic spectrum, and whose properties are unlike any previously observed source. We pinpoint the event to the center of a small, star-forming galaxy at redshift z=0.3534. Its high-energy emission has lasted …
We present the results of optical photometric observations of three extreme TeV blazars, 1ES 0229$+$200, 1ES 0414$+$009, and 1ES 2344$+$514, taken with two telescopes (1.3 m Devasthal Fast Optical Telescope, and 1.04 m Sampuranand Telescope) in India and two (1.4 m Milanković telescope and 60 cm Nedeljković telescope) in Serbia during 2013--2019. We investigated their flux and spectral variability on diverse timescales. We examined a total of 36 intraday $R-$band light curves of these blazars for flux variations using the power-enhanced {\it F}-test and the nested ANOVA test. No significant in…
NGC 5033 is an intriguing Seyfert galaxy because its sub-classification may change with time, and because optical and sub-mm observations find that the massive black hole does not sit at the dynamical center of the galaxy, pointing to a past merger. We obtained a new optical spectrum of NGC 5033 using the 200'' Hale telescope at Palomar that clearly reveals a broad H$β$ line (FWHM$=5400\pm 300~{\rm km}~{\rm s}^{-1}$). This signals a clear view of the optical broad line region (BLR) and requires Seyfert-1.5 designation. Some spectra obtained in the past suggest a Seyfert-1.9 classification, pot…
The X-ray spectra of the narrow line Seyfert 1 galaxy, 1H 0707-495, obtained with XMM-Newton, from time periods of varying X-ray luminosity are analysed in the context of understanding the changes to the X-ray emitting corona that lead to the extreme variability seen in the X-ray emission from active galactic nuclei (AGN). The emissivity profile of the accretion disc, illuminated by the X-ray emitting corona, along with previous measurements of reverberation time lags are used to infer the spatial extent of the X-ray source. By fitting a twice-broken power law emissivity profile to the relativ…
Relativistically blurred reflection from the accretion disc provides a powerful probe of the extreme environments close to supermassive black holes; the inner regions of the accretion flow and the corona that produces the intense X-ray continuum. Techniques by which the geometry and extent of the corona can be measured through the observed X-ray spectrum are reviewed along with the evolution in the structure of the corona that is seen to accompany variations in the X-ray luminosity both on long and short timescales. Detailed analyses of the narrow line Seyfert 1 galaxies Markarian 335 and 1H07…
AT 2018dyk: tidal disruption event or active galactic nucleus? Follow-up observations of an extreme coronal line emitter with the Dark Energy Spectroscopic Instrument
通过暗能量光谱仪后续观测和宿主星系分析，确认 AT 2018dyk 是发生在富气体环境中的潮汐瓦解事件，并发现此类 TDE 的中红外特性与局部环境物质含量及密度相关。
The author presents a model for variability of the flux and polarization of blazars in which turbulent plasma flowing at a relativistic speed down a jet crosses a standing conical shock. The shock compresses the plasma and accelerates electrons to energies up to gamma(max) > 1E4 times their rest-mass energy, with the value of gamma(max) determined by the direction of the magnetic field relative to the shock front. The turbulence is approximated in a computer code as many cells, each with a uniform magnetic field whose direction is selected randomly. The density of high-energy electrons in the …
EXO 1745-248 is a transient neutron-star low-mass X-ray binary that resides in the globular cluster Terzan 5. We studied the transient during its quiescent state using 18 Chandra observations of the cluster acquired between 2003 and 2016. We found an extremely variable source, with a luminosity variation in the 0.5-10 keV energy range of $\sim3$ orders of magnitude (between $3\times10^{31}$ erg s$^{-1}$ and $2\times10^{34}$ erg s$^{-1}$) on timescales from years down to only a few days. Using an absorbed power-law model to fit its quiescent spectra, we obtained a typical photon index of $\sim1…
We explore the origin of the strong soft X-ray excess in Narrow Line Seyfert 1 galaxies using spectral-timing information from a 120ks {\it XMM-Newton} observation of PG 1244+026. Spectral fitting alone cannot distinguish between a true additional soft X-ray continuum component and strongly relativistically smeared reflection, but both models also require a separate soft blackbody component. This is most likely intrinsic emission from the disc extending into the lowest energy X-ray bandpass. The {\it RMS} spectra on short timescales (200-5000s) contain both (non-disk) soft excess and power law…
Extreme AGN Variability in WISE: Powerful Flares and Candidate Tidal Disruption Events in AGN
University of Missouri – Kansas City（密苏里大学堪萨斯城分校）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）；Columbia University（哥伦比亚大学）；Flatiron Institute（平顿研究所）；Massachusetts Institute of Technology（麻省理工学院）；California Institute of Technology（加州理工学院）；American Museum of Natural History（美国自然历史博物馆）；CUNY Borough of Manhattan Community College（纽约市立学院曼哈顿社区学院）；CUNY Graduate Center（纽约市立大学研究生中心）
Context. The radio galaxy IC 310 has recently been identified as a gamma-ray emitter based on observations at GeV energies with Fermi-LAT and at very high energies (VHE, E>100GeV) with the MAGIC telescopes. Originally classified as a head-tail radio galaxy, the nature of this object is subject of controversy since its nucleus shows blazar-like behavior. Aims. In order to understand the nature of IC 310 and the origin of the VHE emission we studied the spectral and flux variability of IC 310 from the X-ray band to the VHE gamma-ray regime. Methods. The light curve of IC 310 above 300GeV has bee…
