Timescales Of Optical Variability Research Articles

Characterizing the γ-Ray Variability of Active Galactic Nuclei with the Stochastic Process Method

Gamma-ray astronomy in the time domain has been by now progressed further as the variabilities of active galactic nuclei (AGNs) on different timescales have been reported a lot. We study the γ-ray variabilities of 23 jetted AGNs by applying a stochastic process method to the ∼12.7 yr long-term light curve (LC) obtained by the Fermi-Large Area Telescope (Fermi-LAT). In this method, the stochastically driven damped simple harmonic oscillator (SHO) and the damped random-walk (DRW) models are used to model the long-term LCs. Our results show that the long-term variabilities of 23 AGNs can be characterized well by both SHO and DRW models. However, the SHO model is restricted in the overdamped mode, and the parameters are poorly constrained. The SHO power spectral densities (PSDs) are the same as those of the typical DRW PSD. In the plot of the rest-frame timescale that corresponds to the broken frequency in the PSD versus black hole mass, the intrinsic, characteristic γ-ray timescales of 23 AGNs occupy almost the same space with the optical variability timescales obtained from the accretion disk emission. This suggests a connection between the jet and the accretion disk. As with the optical variability of the AGN accretion disk, the γ-ray timescale is also consistent with the thermal timescale caused by the thermal instability in the standard accretion disk of AGNs.

Constraining evolution of magnetic field strength in the dissipation region of two BL Lac objects

With the assumption that the optical variability timescale is dominated by the cooling time of the synchrotron process for BL Lac objects, we estimate time dependent magnetic field strength of the emission region for two BL Lac objects. The average magnetic field strengths are consistent with those estimated from core shift measurement and spectral energy distribution modelling. Variation of magnetic field strength in the dissipation region is discovered. Variability of flux and magnetic field strength shows no clear correlation, which indicates the variation of magnetic field is not the dominant reason of the variability origin. The evolution of magnetic field strength can provide another approach to constrain the energy dissipation mechanism in jets.

A characteristic optical variability time scale in astrophysical accretion disks.

Accretion disks around supermassive black holes in active galactic nuclei produce continuum radiation at ultraviolet and optical wavelengths. Physical processes in the accretion flow lead to stochastic variability of this emission on a wide range of time scales. We measured the optical continuum variability observed in 67 active galactic nuclei and the characteristic time scale at which the variability power spectrum flattens. We found a correlation between this time scale and the black hole mass extending over the entire mass range of supermassive black holes. This time scale is consistent with the expected thermal time scale at the ultraviolet-emitting radius in standard accretion disk theory. Accreting white dwarfs lie close to this correlation, suggesting a common process for all accretion disks.

The dual nature of blazar fast variability: Space and ground observations of S5 0716+714

ABSTRACT Blazar S5 0716+714 is well-known for its short-term variability, down to intraday time-scales. We here present the 2-min cadence optical light curve obtained by the TESS space telescope in 2019 December–2020 January and analyse the object fast variability with unprecedented sampling. Supporting observations by the Whole Earth Blazar Telescope Collaboration in B, V, R, and I bands allow us to investigate the spectral variability during the TESS pointing. The spectral analysis is further extended in frequency to the UV and X-ray bands with data from the Neil Gehrels Swift Observatory. We develop a new method to unveil the shortest optical variability time-scales. This is based on progressive de-trending of the TESS light curve by means of cubic spline interpolations through the binned fluxes, with decreasing time bins. The de-trended light curves are then analysed with classical tools for time-series analysis (periodogram, autocorrelation, and structure functions). The results show that below 3 d there are significant characteristic variability time-scales of about 1.7, 0.5, and 0.2 d. Variability on time-scales $\lesssim 0.2$ d is strongly chromatic and must be ascribed to intrinsic energetic processes involving emitting regions, likely jet substructures, with dimension less than about 10−3 pc. In contrast, flux changes on time-scales $\gtrsim 0.5$ d are quasi-achromatic and are probably due to Doppler factor changes of geometric origin.

On the Time Scales of Optical Variability of AGN and the Shape of Their Optical Emission Line Profiles

The mechanism of the optical variability of active galactic nuclei (AGN) is still very puzzling. It is now widely accepted that the optical variability of AGN is stochastic, producing red noise-like light curves. In case they were to be periodic or quasi-periodic, one should expect that the time scales of optical AGN variability should relate to orbiting time scales of regions inside the accretion disks with temperatures mainly emitting the light in this wavelength range. Knowing the reverberation scales and masses of AGN, expected orbiting time scales are in the order of decades. Unfortunately, most of monitored AGN light curves are not long enough to investigate such time scales of periodicity. Here we investigate the AGN optical variability time scales and their possible connections with the broad emission line shapes.

A Method for Locating a High-energy Dissipation Region in a Blazar

Abstract The production site of gamma-rays in a blazar jet is an unresolved problem. We present a method to locate a gamma-ray emission region in the framework of a one-zone emission model. From measurements of the core-shift effect, the relation between the magnetic field strengths (B′) in the radio cores of the jet and the distances (R) of these radio cores from the central supermassive black hole (SMBH) can be inferred. Therefore, once the magnetic field strength in the gamma-ray emission region ( ) is obtained, one can use the relation of B′–R to derive the distance (R diss) of the gamma-ray emission region from the SMBH. Here, we evaluate the lower limit of by using the criteria that the optical variability timescale t var should be longer than or equal to the synchrotron radiation cooling timescale of the electrons that emit optical photons. We test the method with the observations of PSK 1510-089 and BL Lacertae, and derive pc for PSK 1510-089 with t var ∼ a few hours and pc for BL Lacertae with t var ∼ a few minutes. Here, δ D is the Doppler factor and A is the Compton dominance (i.e., the ratio of the Compton to the synchrotron peak luminosities).

Long-term optical variability of high-mass X-ray binaries

We present the results of our monitoring program to study the long-term variability of the Halpha line in high-mass X-ray binaries. We have carried out the most complete optical spectroscopic study of the global properties of high-mass X-ray binaries so far with the analysis of more than 1100 spectra of 20 sources. Our aim is to characterise the optical variability timescales and study the interaction between the neutron star and the accreting material. Our results can be summarised as follows: i) we find that Be/X-ray binaries with narrow orbits are more variable than systems with long orbital periods, ii) we show that a Keplerian distribution of the gas particles provides a good description of the disks in Be/X-ray binaries, as it does in classical Be stars, iii) a decrease in the Halpha equivalent width is generally observed after major X-ray outbursts, iv) we confirm that the Halpha equivalent width correlates with disk radius, v) while systems with supergiant companions display, multi-structured profiles, most of the Be/X-ray binaries show at some epoch double-peak asymmetric profiles, indicating that density inhomogeneities is a common property in the disk of Be/X-ray binaries, vi) the profile variability (V/R ratio) timescales are shorter and the Halpha equivalent width are smaller in Be/X-ray binaries than in isolated Be stars, and vii) we provide new evidence that the disk in Be/X-ray binaries is on average denser than in classical Be stars.

DISCOVERY OF A ∼5 DAY CHARACTERISTIC TIMESCALE IN THEKEPLERPOWER SPECTRUM OF Zw 229–15

We present time series analyses of the full Kepler dataset of Zw 229-15. This Kepler light curve --- with a baseline greater than three years, composed of virtually continuous, evenly sampled 30-minute measurements --- is unprecedented in its quality and precision. We utilize two methods of power spectral analysis to investigate the optical variability and search for evidence of a bend frequency associated with a characteristic optical variability timescale. Each method yields similar results. The first interpolates across data gaps to use the standard Fourier periodogram. The second, using the CARMA-based time-domain modeling technique of Kelly et al. (2014), does not need evenly-sampled data. Both methods find excess power at high frequencies that may be due to Kepler instrumental effects. More importantly both also show strong bends ({\Delta}{\alpha} ~ 2) at timescales of ~5 days, a feature similar to those seen in the X-ray PSDs of AGN but never before in the optical. This observed ~5 day timescale may be associated with one of several physical processes potentially responsible for the variability. A plausible association could be made with light-crossing, dynamical or thermal timescales, depending on the assumed value of the accretion disk size and on unobserved disk parameters such as {\alpha} and H/R. This timescale is not consistent with the viscous timescale, which would be years in a ~10^7 Solar mass AGN such as Zw 229-15. However there must be a second bend on long (>~1 year) timescales, and that feature could be associated with the viscous timescale.

Tests for Standard Accretion Disk Models by Variability in Active Galactic Nuclei

In this paper, standard accretion disk models of AGNs are tested using light curves of 26 objects well observed for reverberation mapping. Time scales of variations are estimated by the most common definition of the variability time scale and the zero-crossing time of the autocorrelation function of the optical light curves for each source. The measured time scales of variations by the two methods are consistent with each other. If the typical value of the viscosity parameter $\alpha \sim 0.1$ is adopted, the measured optical variability time scales are most close to the thermal time scales of the standard disks. If $\alpha$ is allowed to range from $\sim 0.03$ to $\sim 0.2$, the measured time scales are consistent with the thermal time scales of the standard disks. There is a linear relation between the measured variability time scales and black hole masses, and this linear relation is qualitatively consistent with expectation of the standard accretion disk models. The time lags measured by the ZDCF between different bands are on the order of days. The measured time lags of NGC 4151 and NGC 7469 are marginally consistent with the time lags estimated in the case of continuum thermal reprocessing for the standard accretion disk models. However, the measured time lags of NGC 5548 and Fairall 9 are unlikely to be the case of continuum thermal reprocessing. Our results are unlikely to be inconsistent with or are likely to be conditionally in favor of the standard accretion disk models of AGNs.

The Multiple Timescales of Optical Variability of the Blazar 3C 279 during the 2001-2002 Outburst

During 2001-2002 the optically violent variable blazar 3C 279 underwent the most intense outburst seen during the entire 14 yr period that this quasar has been studied at Colgate University's Foggy Bottom Observatory. This study concentrates on ~1600 R-filter images taken during this period of activity. This data set includes 29 nights of microvariability coverage. The outburst began in 2001 March, after 3C 279 had faded to its faintest level, R = 15.5, in 4 yr. The source reached its brightest level, R = 12.5, in the 14 yr of our study in 2001 August, at which time it became unobservable due to its proximity to the Sun. On becoming observable again in 2001 mid-December, 3C 279 fluctuated between R = 13.9 and 14.7, until a dramatic decrease in flux level in 2002 June-July brought the source back down to a level comparable to its preoutburst state. The source exhibited numerous week-long flares of approximately 1 mag during the outburst period. Superposed on these flares were night-to-night variations of up to 0.5 mag and intranight microvariability of up to 0.13 mag in 3 hr. We use visual inspection of the light curve, as well as numerical timescale analysis tools (the autocorrelation function, the structure function, and the power spectrum), to characterize the multiple timescales of variability ranging from 1.5 yr to several hours.

Masses, Dimensionless Kerr Parameters, and Emission Regions in GeV Gamma-Ray-loud Blazars

We have compiled sample of 17 GeV gamma-ray-loud blazars, for which rapid optical variability and gamma-ray fluxes are well observed, from the literature. We derive estimates of the masses, the minimum Kerr parameters a(min), and the size of the emission regions of the supermassive black holes (SMBHs) for the blazars in the sample from their minimum optical variability timescales and gamma-ray fluxes. The results show that (1) the masses derived from the optical variability timescale (M(H)) are significantly correlated with the masses from the gamma-ray luminosity (M(H)(KN)); (2) the values of a(min) of the SMBHs with masses M(H)greater than or equal to10(8.3) M(circle dot)(three out of 17 objects) range from similar to0.5 to similar to1.0, suggesting that these SMBHs are likely to be Kerr black holes. For the SMBHs with M(H) 0, the sizes of the emission regions are almost within the horizon (2r(G)) and marginally bound orbit (4r(G)), while for those with a(min)=0 they are in the range (4.3-66.4)r(G), extending beyond the marginally stable orbit (6r(G)). These results may imply that (1) the rotational state, the radiating regions, and the physical processes in the inner regions for the two kinds of SMBH are significantly different and ( 2) the emission mechanisms of GeV gamma-ray blazars are related to the SMBHs in their centers but are not related to the two different kinds of SMBH.

Photometry of three gamma-ray-loud quasars and implications for supermassive black holes

The results of photometry of three gamma-ray-loud quasars, PKS 1510-089, 3C 279 and OJ248, during 2001 in the B, V and R bands and implications for the supermassive black hole (SMBH) model in these objects are presented. The results show that the three quasars had significant, but irregular, rapid outbursts with a typical time-scale of about 40-60 min and a typical amplitude of about Deltam similar or equal to0.60-1.26 mag during the observations. For 3C 279 and OJ248 the upper limits of the masses of SMBHs are estimated from the observed minimum optical variability time-scale, and the lower limits are also estimated by a method proposed by Dermer & Gehrels. The masses derived from both methods are reasonable. For PKS 1510-089 a new brightness minima was also observed. Analysing the observations from 1999 to 2001, three minima were observed. Although their light curves are poorly sampled, it is intriguing to note that these minimum at intervals of about 43 min seem to indicate the possibility of a periodic phenomenon. So a hypothesis is proposed to interpret the phenomenon, suggesting that there is a binary black hole system at the centre. The parameters of the system are estimated by a very simple method. The periodicity is about 336+/-14 d in the observed frame; the masses of the primary and secondary black holes are 10(8.0) and 10(6.2) M-circle dot, respectively; the distance between two black holes is 10(15.81) cm (378R (g) of the primary black hole), and the lifetime of the system is 10(6.0) yr. These results seem to indicate that PKS 1510-089 is another possible candidate binary black hole system. It should be monitored in future to obtain more data for further analysis to test the hypothesis.

Optical and physical variability on timescales from minutes to the seasonal cycle on the New England shelf: July 1996 to June 1997

High‐resolution time series of physical and bio‐optical data were obtained using moored and bottom‐mounted instruments on the southern New England continental shelf during the Coastal Mixing and Optics experiment (CMO) from July 1996 through June 1997. The most prominent physical and bio‐optical signals observed during the experiment were associated with the seasonal variability. However, several important events interrupted the seasonal cycle. These episodic events appear to have had a great impact on biogenic and nonbiogenic matter. Hurricanes and storms passed over or near the CMO site, resulting in reduced stratification of the water column, particle redistribution, and sediment resuspension. Changing hydrographic conditions that resulted from the influence of several water mass intrusions greatly affected particle concentration on timescales of days to several weeks. The bottom boundary layer had an influence on particle movement in the water column and along the seafloor. The results suggest that there is likely considerable interannual variability in both the physics and bio‐optics because of active and diverse physical forcing. This experiment also sets the context for comparing our coastal ocean results with previous open ocean findings. Important differences arise because of coastal bottom boundary layer effects, large‐scale water mass intrusions, and the relatively greater role of tides on the shelf. Timescales of optical variability (e.g., changes in phytoplankton spectral shapes of absorption) are thus generally shorter for the coastal environment.

Space and time scales of optical variability in and about Mobile Bay as defined by a NOAA AVHRR visible sensor c660 algorithm

Optical values extracted from a NOAA AVHRR database were used to derive the spatial and temporal scales of optical variability of Mobile Bay and its nearby sounds. The study used the attenuation coefficient, c660, (derived from Channels 1 and 2 algorithm) to define the coastal surface turbidity for April 1995 through July 1996. The optical spatial variability of 201 images were investigated to establish 15-day, seasonal, and annual means. The 15-day graphs of the seasonal trends show that minimum turbidity variability occurred in summer and the maximum (by a ratio of about twice in Mobile Bay) in winter. The spring and fall transition periods were not as definite, appearing highly variable and brief. Interannual differences were noticeable between the 1995 and 1996 spring periods. Seasonal c660 composite images show a low-spring turbidity, a comparatively uniform summer season, a fall transition period followed by a highly turbid winter period. Residual images (derived by subtracting the annual mean from each seasonal composite) show that in spring, a much less turbid field was generally found throughout the area than shown by the annual mean. The summer residual showed mixed contributions in different regions of the area, whereas the fall composite most closely resembled the annual mean. The winter months show a much stronger positive residuals in almost all of the area (except in the western Mississippi Sound).

The circumstellar environment of L2 Puppis

Polarimetric observations of the red semi-regular variable L2 Puppis, obtained over a period of several years, confirm significant variations across the CaI 4226 line and several TiO bands. Together with published infra-red data, the observations point to a fundamental symmetry plane to which all variations with wavelength and time are related. The data are also consistent with a combination of photospheric effects, including a non-uniform distribution of calcium across the stellar disk, and scattering from grains in a cloud in which there is a systematic variation in grain size with distance to the star. Grain growth and dissipation, as evidenced by the observations, occur on a time scale of several years, in contrast to the optical variability time scale of months.

The broad-band spectra and variability of compact nonthermal sources

view Abstract Citations (49) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The broad-band spectra and variability of compact nonthermal sources. Jones, T. W. ; Rudnick, L. ; Owen, F. N. ; Puschell, J. J. ; Ennis, D. J. ; Werner, M. W. Abstract It is noted that in approximately a dozen sources, nearly simultaneous infrared and/or visual spectra are available, making it possible to examine the relationship between the infrared-visual and radio regimes. A simple transition to the steeper infrared spectra is seen as likely with a mean break wavelength of approximately 300 microns in the rest frame. These observations indicate a tapered source geometry, with the smallest structures roughly on the order of 10 to the 16th cm as estimated from the spectral break; this is consistent with scales estimated from optical variability time scales. Large amplitude variations on a six-month time interval are found to be fairly infrequent in most of the sources. A relatively model-independent way of categorizing flux variations as due to changes in source scale, or structure or the slope of the electron energy spectrum is suggested. Publication: The Astrophysical Journal Pub Date: January 1981 DOI: 10.1086/158571 Bibcode: 1981ApJ...243...97J Keywords: Extragalactic Radio Sources; Infrared Astronomy; Nonthermal Radiation; Radio Spectra; Spectral Line Width; Energy Spectra; Galactic Nuclei; Infrared Spectra; Millimeter Waves; Variability; Visible Spectrum; Astronomy full text sources ADS | data products NED (21) SIMBAD (19)