Large Amplitude X-ray Variability Research Articles

ERASSt J074426.3 + 291606: prompt accretion disc formation in a ‘faint and slow’ tidal disruption event

ABSTRACTWe report on multiwavelength observations of the tidal disruption event (TDE) candidate eRASSt J074426.3 + 291606 (J0744), located in the nucleus of a previously quiescent galaxy at z = 0.0396. J0744 was first detected as a new, ultra-soft X-ray source (photon index ∼4) during the second SRG/eROSITA All-Sky Survey (eRASS2), where it had brightened in the 0.3–2 keV band by a factor of more than ∼160 relative to an archival 3σ upper limit inferred from a serendipitous Chandra pointing in 2011. The transient was also independently found in the optical by the Zwicky Transient Facility (ZTF), with the eRASS2 detection occurring only ∼20 d after the peak optical brightness, suggesting that the accretion disc formed promptly in this TDE. Continued X-ray monitoring over the following ∼400 d by eROSITA, NICER XTI and Swift XRT showed a net decline by a factor of ∼100, albeit with large amplitude X-ray variability where the system fades, and then rebrightens, in the 0.3–2 keV band by a factor ∼50 during an 80-d period. Contemporaneous Swift UVOT observations during this extreme X-ray variability reveal a relatively smooth decline, which persists over ∼400 d post-optical peak. The peak observed optical luminosity (absolute g-band magnitude ∼−16.8 mag) from this transient makes J0744 the faintest optically detected TDE observed to date. However, contrasting the known set of ‘faint and fast’ TDEs, the optical emission from J0744 decays slowly (exponential decay time-scale ∼120 d), making J0744 the first member of a potential new class of ‘faint and slow’ TDEs.

The large amplitude X-ray variability in NGC 7589: possible evidence for accretion mode transition

ABSTRACT We report the discovery of large amplitude X-ray variability in the low-luminosity active galactic nucleus NGC 7589, and present possible observational evidence for accretion mode transition in this source. Long-term X-ray flux variations by a factor of more than 50 are found using X-ray data obtained by Swift/X-Ray Telescope and XMM–Newton over 17 yr. Results of long-term monitoring data in the UV, optical, and infrared bands over ∼20 yr are also presented. The Eddington ratio λEdd increased from 10−3 to ∼0.13, suggesting a transition of the accretion flow from an advection dominated accretion flow to a standard thin accretion disc. Further evidence supporting the thin disc in the high-luminosity state is found by the detection of a significant soft X-ray component in the X-ray spectrum. The temperature of this component ($\sim 19^{+15}_{-7}$ eV, fitted with a blackbody model) is in agreement with the predicted temperature of the inner region for a thin disc around a black hole (BH) with mass of ∼107M⊙. These results may indicate that NGC 7589 had experienced accretion mode transition over a time-scale of a few years, suggesting the idea that similar accretion processes are at work for massive BH and BH X-ray binaries.

A comprehensive long-term study of the radio and X-ray variability of NGC 4051 Paper II

The origin of the low luminosity radio emission in radio-quiet AGN, is unknown. The detection of a positive correlation between the radio and X-ray emission would imply a jet-like origin, similar to that seen in `hard state' X-ray binary systems. In our previous work, we found no believable radio variability in the well known X-ray bright Seyfert 1 galaxy NGC 4051, despite large amplitude X-ray variability. In this study we have carefully re-analysed radio and X-ray observations using the same methods as our previous work, we again find no evidence for core radio variability. In direct contrast to our findings, another study claim significant radio variability and a distinctive anti-correlation between radio and X-ray data for the same source. The other study report only integral flux values and do not consider the effect of the changing array on the synthesised beam. In both our studies of NGC 4051 we have taken great care to account for the effect that the changing beam size has on the measured radio flux and as a result we are confident that our method gives more accurate values for the intrinsic core radio flux. However, the lack of radio variability we find is hard to reconcile because radio images of NGC 4051 do show jet-like structure. We suggest that the radio structures observed are likely the result of a previous period of higher radio activity and that the current level of radio emission from a compact nuclear jet is low.

Highly variable AGN from theXMM-Newtonslew survey

We investigate the properties of a variability-selected complete sample of AGN in order to identify the mechanisms which cause large amplitude X-ray variability on time scales of years. A complete sample of 24 sources was constructed, from AGN which changed their soft X-ray luminosity by more than one order of magnitude over 5--20 years between ROSAT observations and the XMM Slew Survey. Follow-up observations were obtained with the Swift satellite. After removal of two probable spurious sources, we find that the sample has global properties which differ little from a non-varying control sample drawn from the wider XMM-Slew/ROSAT/Veron sample of all secure AGN detections. A wide range of AGN types are represented in the varying sample. The black hole mass distributions for the varying and non-varying sample are not significantly different. This suggests that long timescale variability is not strongly affected by black hole mass. There is marginal evidence that the variable sources have a lower redshift (2$\sigma$) and X-ray luminosity (1.7$\sigma$). Apart from two radio-loud sources, the sample have normal optical-X-ray ratios ($\alpha_{\rm OX}$) when at their peak but are X-ray weak during their lowest flux measurements. Drawing on our results and other studies, we are able to identify a variety of variability mechanisms at play: tidal disruption events, jet activity, changes in absorption, thermal emission from the inner accretion disc, and variable accretion disc reflection. Little evidence for strong absorption is seen in the majority of the sample and single-component absorption can be excluded as the mechanism for most sources.

A systematic search of highly variable X-ray sources with the XMM-Newton serendipitous survey

We describe an ongoing programme to search for X-ray sources with long-term, large amplitude X-ray variability. The method is to compare the X-ray sources from the XMM-Newton Serendipitous Survey with those from the previous ROSAT pointed observations and the ROSAT All-sky Survey. The work is largely motivated by the recent discovery of a new type of X-ray flares characterised by enormous amplitude flux variability and associated with normal galaxies [Komossa, S., Ludwig Biermann Award lecture: X-ray evidence for supermassive black holes at the centers of nearby, non-active galaxies. Rev. Mod. Astron. 15, 27, 2002]. Other objects of interest are black hole accretion systems, such as X-ray binaries and AGN, GRB afterglows, extremes of stellar flares, and other objects undergoing extremely violent physical processes. In this paper, we describe the programme and present some preliminary results.

High-amplitude, long-term X-ray variability in the solar-type star HD 81809: The beginning of an X-ray activity cycle?

We present the initial results from our XMM-Newton program aimed at searching for X-ray activity cycles in solar- type stars. HD 81809 is a G2-type star (somewhat more evolved than the Sun, and with a less massive companion) with a pronounced 8.2 yr chromospheric cycle, as evident from from the Mt. Wilson program data. We present here the results from the initial 2.5 years of XMM-Newton observations, showing that large amplitude (a factor of � 10) modulation is present in the X-ray luminosity, with a clearly defined maximum in mid 2002 and a steady decrease since then. The maximum of the chromospheric cycle took place in 2001; if the observed X-ray variability is the initial part of an X-ray cycle, this could imply a phase shift between chromospheric and coronal activity, although the current descent into chromospheric cycle minimum is well reflected into the star's X-ray luminosity. The observations presented here provide clear evidence for the presence of large amplitude X-ray variability coherent with the activity cycle in the chromosphere in a star other than the Sun.

ROSAT High-Resolution Imager monitoring of extreme X-ray variability in the narrow-line quasar PHL 1092

We report results from an 18-d ROSAT High-Resolution Imager (HRI) monitoring campaign on the ultrasoft narrow-line Seyfert 1 (NLS1) class quasar PHL 1092. This luminous, radio-quiet quasar showed strong X-ray variability in a short ROSAT Position-Sensitive Proportional Counter observation, and ROSAT HRI monitoring of the similar object IRAS 13224-3809 revealed extreme variability on intermediate time-scales. We wanted to determine whether remarkable X-ray variability persistently occurs in PHL 1092, and we also wanted to search for outstanding variability events that constrain emission processes. Given the large luminosity of PHL 1092 (∼ 5 × 1045 erg s-1 in the HRI band), we detect extremely rapid and large-amplitude X-ray variability throughout our monitoring. The maximum observed variability amplitude is a factor of ≈14, and in the most rapid variability event the HRI count rate increases by a factor of ≈ 3.8 in a rest-frame time interval of 1.3 × 1042 erg s-2, making it the most extreme such event we are aware of from a radio-quiet quasar. Standard `radiative efficiency limit' arguments imply a radiative efficiency larger than can be achieved by accretion on to a Kerr black hole rotating at the maximum plausible rate, although we point out that such arguments depend upon the geometry of initial radiation release. Relativistic motions of the X-ray source are probably causing the radiative efficiency limit to break down; such relativistic motions have also been inferred in the similar NLS1-class quasar PKS 0558-504.

A simple new method for analysing gapped time-series: X-ray variability of MCG -6-30-15

We present a new, simple method for analysing gapped time-series which is particularly suited to probe the X-ray power spectra of active galactic nuclei (AGN). The method is applied to a four-day observation of the Seyfert 1 galaxy MCG -6-30-15 with ASCA. MCG -6-30-15 is well-known for rapid, large amplitude X-ray variability. In this source, our method probes the power spectrum in the range 10 −3 Hz to > 5 × 10 −2 Hz, thus extending the high-frequency coverage by a factor of over 50 compared to previous studies. The ASCA data are consistent with a power-law power spectrum with index −1.4, implying a simple extrapolation from the power spectrum measured by EXOSAT at lower frequencies. The data argue against orbital modulation as the origin of the variability since it would imply upper limits on the mass of the putative black hole of less than 5 × 10 5 M⊙, and therefore super-Eddington luminosites. Researchers are encouraged to use the new method on their ASCA, BeppoSAX and archival data. We also propose a new interpretation of the deep minimum in intensity at the end of the long observation of MCG -6-30-15 in terms of an occulation by optically thick matter. If this interpretation is correct, the data allow for the first time, a crude spatial resolution of the X-ray source and possibly of the innermost regions of the putative accretion disk.

A Simple New Method for Analyzing Gapped Time Series: Search for a High-frequency Cutoff in the X-Ray Power Spectrum of the Seyfert Galaxy MCG −6−30−15

We present a new, simple method for analyzing gapped time series that is particularly suited to probe the X-ray power spectra of active galactic nuclei (AGNs). The method is applied to a 4 day observation of the Seyfert 1 galaxy MCG -6-30-15 with ASCA (Advanced Satellite for Cosmology and Astrophysics). MCG -6-30-15 is well known for rapid, large-amplitude X-ray variability and was one of the first AGNs to show evidence of relativistic effects in its Fe-K line profile. In this source, our method probes the power spectrum in the range 10-3-5×10−2 Hz, thus extending the high-frequency coverage by a factor of over 50 compared to previous studies. The ASCA data rule out a cessation of variability up to 1.6×10−2 Hz at greater than 90% confidence. The power-law index of the power spectrum in this range is consistent with ~-1.4. At 2.1×10−2 Hz we can place an upper limit of 7% on the intrinsic fluctuations of the source luminosity at greater than 99% confidence. Above this frequency Poisson noise becomes dominant, but we can place an upper limit of 10% on intrinsic fluctuations up to 5×10−2 Hz at 98.9% confidence. The data argue against orbital modulation as the origin of the variability since it would imply upper limits on the mass of the putative black hole of less than 5×105 M☉, and therefore super-Eddington luminosities.