The Seyfert 1 galaxy II ZW 229.015 has been observed with the Kepler spacecraft since quarter 4 of Kepler science operations. The results of the quarters 4-7 (1 year) Kepler observations are ...presented in this paper. We find the source to be highly variable on multiple timescales, with discrete variations occurring on timescales as short as tens of hours with amplitudes as small as 0.5%. Such small amplitude, rapid variability has never before been detected in active galactic nuclei. The presence of a strong galaxy component dilutes the variability determined from the photometric aperture used in the standard Kepler PDC analysis. Using the tools provided by the Kepler Guest Observer Office and simultaneous V-band photometry found in the literature, we determine an optimal customized aperture for photometry of this source with Kepler. The results of a PSRESP analysis reveal tentative evidence of a characteristic variability timescale in the power spectrum. Using this timescale, we estimate the mass of the central supermassive black hole and this estimate is consistent with the virial mass estimate from reverberation mapping studies.
ABSTRACT
We present the results from a dense multwavelength optical/UV, near-infrared (IR), and X-ray follow-up campaign of the nuclear transient AT 2017gge, covering a total of 1698 d from the ...transient’s discovery. The bolometric light curve, the blackbody temperature and radius, the broad H and He i λ5876 emission lines and their evolution with time, are all consistent with a tidal disruption event (TDE) nature. A soft X-ray flare is detected with a delay of ∼200 d with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He ii λ4686 and by a number of long-lasting high ionization coronal emission lines. This indicate a clear connection between a TDE flare and the appearance of extreme coronal line emission (ECLEs). An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He i λ10830 and, for the first time in a TDE, a transient high-ionization coronal NIR line (the Fe xiii λ10798) is also detected. The data are well explained by a scenario in which a TDE occurs in a gas-and-dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disc formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.
We report the results of a study of multicolor optical microvariability in the Blazar S5 0716+714. S5 0716+714 was observed in the B and I bands continuously for five consecutive nights in 2003 ...March. This is the first study to apply color analysis, structure function, and cross-correlation analysis on an optical data set with the temporal coverage and photometric quality that characterizes this data. The source displayed variability on timescales from days to tens of minutes in both bands. Discrete events in the light curves lead to the determination of a range in the size of the emission regions of R <= 1.07 X1015 cm to R <= 9.8 X1015 cm, adopting Delta *d = 20 from Nesci et al. Hysteresis loops were found in plots of the B-band flux (F B ) and the ratio of the B to the I-band flux (F B/I ); these loops were found to be clockwise for dips in the light curves and counterclockwise for bursts. Their directionality and the symmetric nature of the flares are consistent with the flux variations being dominated by the light crossing time of the emission region and not its intrinsic electron cooling or acceleration timescales. The variability between B and I bands is highly correlated with no significant lags between the B and I-band flux variations detected. Significant lags were detected between the flux in the B band (F B ) and the B/I flux ratio (F B/I ). A structure function analysis shows similar slopes in both bands, lying in the range of --1.0 to --2.0, indicating that the observed variability is the result of a fractional noise process, consistent with the variations arising from a turbulent process. A differential I-band calibration for comparison star 4 from the sequence of Villata et al. is also provided.
The radio source 3C 264, hosted by the giant elliptical galaxy NGC 3862, was observed with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) between 2017 February and 2019 May. ...These deep observations resulted in the discovery of very high energy (VHE; E > 100 GeV) γ-ray emission from this active galaxy. An analysis of ∼57 hr of quality-selected live time yields a detection at the position of the source, corresponding to a statistical significance of 7.8 standard deviations above background. The observed VHE flux is variable on monthly timescales, with an elevated flux seen in 2018 observations. The VHE emission during this elevated state is well characterized by a power-law spectrum with a photon index Γ = 2.20 0.27 and flux F(>315 GeV) = ( cm−2 s−1, or approximately 0.7% of the Crab Nebula flux above the same threshold. 3C 264 (z = 0.0217) is the most distant radio galaxy detected at VHE, and the elevated state is thought to be similar to that of the famously outbursting jet in M87. Consequently, extensive contemporaneous multiwavelength data were acquired in 2018 at the time of the VHE high state. An analysis of these data, including Very Long Baseline Array, Very Large Array, Hubble Space Telescope, Chandra, and Swift observations in addition to the VERITAS data, is presented, along with a discussion of the resulting spectral energy distribution.
ABSTRACT We present ground-based optical photometric monitoring data for NGC 5548, part of an extended multiwavelength reverberation mapping campaign. The light curves have nearly daily cadence from ...2014 January to July in nine filters (BVRI and ugriz). Combined with ultraviolet data from the Hubble Space Telescope and Swift, we confirm significant time delays between the continuum bands as a function of wavelength, extending the wavelength coverage from 1158 Å to the z band (~9160 Å). We find that the lags at wavelengths longer than the V band are equal to or greater than the lags of high-ionization-state emission lines (such as He ii λ 1640 and λ 4686 ), suggesting that the continuum-emitting source is of a physical size comparable to the inner broad-line region (BLR). The trend of lag with wavelength is broadly consistent with the prediction for continuum reprocessing by an accretion disk with τ ∝ λ 4 / 3 . However, the lags also imply a disk radius that is 3 times larger than the prediction from standard thin-disk theory, assuming that the bolometric luminosity is 10% of the Eddington luminosity ( L = 0.1 L Edd ). Using optical spectra from the Large Binocular Telescope, we estimate the bias of the interband continuum lags due to BLR emission observed in the filters. We find that the bias for filters with high levels of BLR contamination (~20%) can be important for the shortest continuum lags and likely has a significant impact on the u and U bands owing to Balmer continuum emission.
We present the results of an optical spectroscopic monitoring program targeting NGC 5548 as part of a larger multiwavelength reverberation mapping campaign. The campaign spanned 6 months and achieved ...an almost daily cadence with observations from five ground-based telescopes. The Hβ and He ii λ4686 broad emission-line light curves lag that of the 5100 optical continuum by and , respectively. The Hβ lag relative to the 1158 ultraviolet continuum light curve measured by the Hubble Space Telescope is ∼50% longer than that measured against the optical continuum, and the lag difference is consistent with the observed lag between the optical and ultraviolet continua. This suggests that the characteristic radius of the broad-line region is ∼50% larger than the value inferred from optical data alone. We also measured velocity-resolved emission-line lags for Hβ and found a complex velocity-lag structure with shorter lags in the line wings, indicative of a broad-line region dominated by Keplerian motion. The responses of both the Hβ and He ii emission lines to the driving continuum changed significantly halfway through the campaign, a phenomenon also observed for C iv, Ly , He ii(+O iii), and Si iv(+O iv) during the same monitoring period. Finally, given the optical luminosity of NGC 5548 during our campaign, the measured Hβ lag is a factor of five shorter than the expected value implied by the RBLR-LAGN relation based on the past behavior of NGC 5548.
ABSTRACT We conduct a multiwavelength continuum variability study of the Seyfert 1 galaxy NGC 5548 to investigate the temperature structure of its accretion disk. The 19 overlapping continuum light ...curves ( to ) combine simultaneous Hubble Space Telescope, Swift, and ground-based observations over a 180 day period from 2014 January to July. Light-curve variability is interpreted as the reverberation response of the accretion disk to irradiation by a central time-varying point source. Our model yields the disk inclination , temperature K at 1 light day from the black hole, and a temperature-radius slope ( ) of . We also infer the driving light curve and find that it correlates poorly with both the hard and soft X-ray light curves, suggesting that the X-rays alone may not drive the ultraviolet and optical variability over the observing period. We also decompose the light curves into bright, faint, and mean accretion-disk spectra. These spectra lie below that expected for a standard blackbody accretion disk accreting at .
The WEBT BL Lacertae Campaign 2000 Villata, M.; Raiteri, C. M.; Kurtanidze, O. M. ...
Astronomy & astrophysics,
08/2002, Letnik:
390, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present UBVRI light curves of BL Lacertae from May 2000 to January 2001, obtained by 24 telescopes in 11 countries. More than 15 000 observations were performed in that period, which was the ...extension of the Whole Earth Blazar Telescope (WEBT) campaign originally planned for July–August 2000. The exceptional sampling reached allows one to follow the flux behaviour in fine detail. Two different phases can be distinguished in the light curves: a first, relatively low-brightness phase is followed by an outburst phase, after a more than $1\rm\,mag$ brightening in a few weeks. Both the time duration (about $100\rm\,d$) and the variation amplitude (roughly $0.9\rm\,mag$) are similar in the two phases. Rapid flux oscillations are present all the time, involving variations up to a few tenths of mag on hour time scales, and witnessing an intense intraday activity of this source. In particular, a half-mag brightness decrease in about $7\rm\,h$ was detected on August 8–9, 2000, immediately followed by a ~$0.4 \rm \, mag$ brightening in $1.7\rm\,h$. Colour indexes have been derived by coupling the highest precision B and R data taken by the same instrument within $20\rm\,min$ and after subtracting the host galaxy contribution from the fluxes. The 620 indexes obtained show that the optical spectrum is weakly sensitive to the long-term trend, while it strictly follows the short-term flux behaviour, becoming bluer when the brightness increases. Thus, spectral changes are not related to the host galaxy contribution, but they are an intrinsic feature of fast flares. We suggest that the achromatic mechanism causing the long-term flux base-level modulation can be envisaged in a variation of the relativistic Doppler beaming factor, and that this variation is likely due to a change of the viewing angle. Discrete correlation function (DCF) analysis reveals the existence of a characteristic time scale of variability of ~$7\rm\,h$ in the light curve of the core WEBT campaign, while no measurable time delay between variations in the B and R bands is found.
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