Variability and the X-ray/UV ratio of active galactic nuclei

The Astrophysical Journal, 2014

Variability in the infrared emission from disks around pre-main sequence stars over the course of days to weeks appears to be common, but the physical cause of the changes in disk structure are not constrained. Here we present coordinated monitoring of one young cluster with the Spitzer and Chandra space telescopes aimed at studying the physical source of the variability. In fall 2011 we obtained ten epochs of Chandra ACIS photometry over a period of 30 days with a roughly 3 day cadence contemporaneous with 20 epochs of Spitzer [3.6],[4.5] photometry over 40 days with a roughly 2 day cadence of the IC 348 cluster. This cadence allows us to search for week to month long responses of the infrared emission to changes in the high-energy flux. We find no strong evidence for a direct link between the X-ray and infrared variability on these timescales among 39 cluster members with circumstellar disks. There is no significant correlation between the shape of the infrared and X-ray light curves, or between the size of the X-ray and infrared variability. Among the stars with an X-ray flare none showed evidence of a correlated change in the infrared photometry on timescales of days to weeks following the flare. This lack of connection implies that X-ray heating of the planet forming region of the disk is not significant, although we cannot rule out rapid or instantaneous changes in infrared emission.

The Astrophysical Journal, 2012

NGC 3783 was observed for approximately 210 ks by Suzaku and in this time showed significant spectral and flux variability at both short (20 ks) and long (100 ks) time scales. The full observation is found to consist of approximately six "spectral periods" where the behaviour of the soft (0.3-1.0 keV) and hard (2-10 keV) bands are somewhat distinct. Using a variety of methods we find that the strong warm absorber present in this source does not change on these time scales, confirming that the broadband variability is intrinsic to the central source. The time resolved difference-spectra are well modelled with an absorbed powerlaw below 10 keV, but show an additional hard excess at ≈ 20 keV in the latter stages of the observation. This suggests that, in addition to the variable powerlaw, there is a further variable component that varies with time but not monotonically with flux. We show that a likely interpretation is that this further component is associated with variations in the reflection fraction or possibly ionization state of the accretion disk a few gravitational radii from the black hole.

Astronomy & Astrophysics, 2011

Context. The X-ray variability of the active galactic nuclei (AGN) has been most often investigated with studies of individual, nearby sources, and only a few ensemble analyses have been applied to large samples in wide ranges of luminosity and redshift. Aims. We aim to determine the ensemble variability properties of two serendipitously selected AGN samples extracted from the catalogues of XMM-Newton and Swift, with redshift between ∼ 0.2 and ∼ 4.5, and X-ray luminosities, in the 0.5-4.5 keV band, between ∼ 10 43 erg/s and ∼ 10 46 erg/s. Methods. We used the structure function (SF), which operates in the time domain, and allows for an ensemble analysis even when only a few observations are available for individual sources and the power spectral density (PSD) cannot be derived. The SF is also more appropriate than fractional variability and excess variance, because these parameters are biased by the duration of the monitoring time interval in the rest-frame, and therefore by cosmological time dilation. Results. We find statistically consistent results for the two samples, with the SF described by a power law of the time lag, approximately as SF ∝ τ 0.1 . We do not find evidence of the break in the SF, at variance with the case of lower luminosity AGNs. We confirm a strong anti-correlation of the variability with X-ray luminosity, accompanied by a change of the slope of the SF. We find evidence in support of a weak, intrinsic, average increase of X-ray variability with redshift.

Se examinan la teoría y las observaciones de la emisión de continuo térmico de los AGN. Después de una corrección por enrojecimiento, las distribuciones espectrales de energía (SEDs) en reposo delóptico-UV tienen gran similitud. Las SEDs están dominadas energéticamente por el big blue bump (BBB), peroéste no muestra el espectro ν +1/3 predicho para un disco de acreción delgado con un gradiente radial térmico r −0.75 . En su lugar, la SED delóptico-UV muestra un gradiente térmico de r −0.57 independiente del espesor del disco. Esto indicaría algo de flujo de calor hacia afuera del disco. El disco es grande y la región emisora del continuó optico es tan grande como la región interna de líneas anchas (BLR). Como se observa variabilidadóptica en todos los AGN en el tiempo de cruce de la BLR, las variaciones deben propagarse con velocidad cercana a la de la luz, en lugar de tiempos dinámicos. Esto sostiene que el mecanismo de generación de energía sea electromagnético en lugar de hidrodinámico. Con velocidades casi lumínicas puede haber una anisotropía local significativa en la emisión. Las enormes variaciones rápidas del BBB implican que la generación de energía magnetohidrodinámica es fundamentalmente inestable. Debido al inevitable gradiente radial térmico en el material en acreción, distintas regiones espectrales provienen predominantemente de distintos radios, y variaciones en distintas zonas espectrales corresponden a variabilidad en diferentes radios. Esto explica la independencia observada frecuentemente entre variacionesópticas y en rayos-X, casos de variabilidad a menores energías antecediendo la variabilidad a altas energías, y los cambios rápidos en la demora por reverberación en las líneas de emisión. Se proponen algunas pruebas observacionales de la hipótesis de variabilidad local.

Astronomy and Astrophysics, 2008

Aims. We analyze different data of the variation of the fine structure constant obtained with different methods to check their consistency. Methods. We test consistency using the modified student test and confidence intervals. We split the data sets into smaller intervals. A criterion for this selection is proposed. Results. Results show consistency for reduced intervals for each pair of data sets considered. Conclusions. Results are at variance with the ones obtained considering mean values over the whole interval.

Astronomy and Astrophysics, 2010

Context. The southern "infrared companion" of T Tau is known to show strong photometric variations of several magnitudes on timescales of years, as well as more modest 1 mag variations on timescales as short as one week. The physical mechanism driving these variations is debated, intrinsic luminosity variations due to a variable accretion rate were initially proposed, but later challenged in favor of apparent fluctuations due to time-variable foreground extinction. Aims. We seek to investigate the nature of the observed photometric variability. Based on simple geometric arguments and basic physics laws, a minimum variability timescale can be derived for which variable extinction is a viable mechanism. Because this timescale increases rapidly with wavelength, observations at long wavelengths provide the strongest constraints. Methods. We used VISIR at the VLT to image the T Tau system at two epochs in February 2008, separated by 3.94 days. In addition we compiled an extensive set of near-and mid-infrared photometric data from the literature, supplemented by a number of previously unpublished measurements, and constructed light curves for the various system components. We constructed a 2D radiative transfer model for the disk of T Tau Sa, consisting of a passively irradiated dusty outer part and a central, actively accreting component. Results. Our VISIR data reveal a +26±2% change in the T Tau S/T Tau N flux ratio at 12.8 µm within four days, which can be attributed to a brightening of T Tau Sa. Variable extinction can be excluded as a viable mechanism for the observed flux variation based on the short timescale and the long observing wavelength. We show that also the large long-term photometric variability and its associated color-magnitude behavior can be plausibly explained with variable accretion. However, variable extinction is also a viable mechanism for the long-term variability, and a combination of both mechanisms may be required to explain the collective photometric variability of Sa. Conclusions. We conclude that the observed short-term variability is caused by a variable accretion luminosity in T Tau Sa, which leads to substantial fluctuations in the irradiation of the disk surface and thus induces rapid variations in the disk surface temperature and IR brightness. Both variable accretion and variable foreground extinction can plausibly explain the long-term color and brightness variations. We suggest that the periods of high and variable brightness of Sa that we witnessed in the early and late 1990s were due to enhanced accretion induced by the periastron passage of Sb, which gravitationally perturbed the Sa disk.

Publications of the Astronomical Society of Australia, 2021

We performed a detailed spectral and timing analysis of a Seyfert 1 galaxy Mrk 509 using data from the Neil Gehrels Swift observatory that spanned over $\sim$ 13 years between 2006 and 2019. To study the variability properties from the optical/UV to X-ray emission, we used a total of 275 pointed observations in this work. The average spectrum over the entire duration exhibits a strong soft X-ray excess above the power law continuum. The soft X-ray excess is well described by two thermal components with temperatures of kT $_{\rm BB1}\sim$ 120 eV and kT $_{\rm BB2}\sim$ 460 eV. The warm thermal component is likely due to the presence of an optically thick and warm Comptonizing plasma in the inner accretion disk. The fractional variability amplitude is found to be decreasing with increasing wavelength, i.e., from the soft X-ray to UV/optical emission. However, the hard X-ray (2–8 keV) emission shows very low variability. The strength of the correlation within the UV and the optical ban...

Monthly Notices of the Royal Astronomical Society, 2020

Using a month-long X-ray light curve from RXTE/PCA and 1.5 month-long UV continuum light curves from IUE spectra in 1220–1970 Å, we performed a detailed time-lag study of the Seyfert 1 galaxy NGC 7469. Our cross-correlation analysis confirms previous results showing that the X-rays are delayed relative to the UV continuum at 1315 Å by 3.49 ± 0.22 d, which is possibly caused by either propagating fluctuation or variable Comptonization. However, if variations slower than 5 d are removed from the X-ray light curve, the UV variations then lag behind the X-ray variations by 0.37 ± 0.14 d, consistent with reprocessing of the X-rays by a surrounding accretion disc. A very similar reverberation delay is observed between Swift/XRT X-ray and Swift/UVOT UVW2, U light curves. Continuum light curves extracted from the Swift/GRISM spectra show delays with respect to X-rays consistent with reverberation. Separating the UV continuum variations faster and slower than 5 d, the slow variations at 1825...

The Astronomical Journal, 2014

Based on more than four weeks of continuous high cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high quality, multi-wavelength light curves for young stellar objects (YSOs) whose optical variability is dominated by short duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief -several hour to one day -brightenings at optical and near-infrared (IR) wavelengths with amplitudes generally in the range 5-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a thirty day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u − g vs. g − r color-color diagram with the largest UV excesses. These stars also have large Hα equivalent widths, and either centrally peaked, lumpy Hα emission profiles or profiles with blue-shifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Amongst the stars with the largest UV excesses or largest Hα equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts.

The Astrophysical Journal, 2011

Over three quarters in 2010-2011, Kepler monitored optical emission from four active galactic nuclei (AGN) with ∼30 min sampling, > 90% duty cycle and 0.1% repeatability. These data determined the AGN optical fluctuation power spectral density functions (PSDs) over a wide range in temporal frequency. Fits to these PSDs yielded power law slopes of −2.6 to −3.3, much steeper than typically seen in the X-rays. We find evidence that individual AGN exhibit intrinsically different PSD slopes. The steep PSD fits are a challenge to recent AGN variability models but seem consistent with first order MRI theoretical calculations of accretion disk fluctuations.