Characteristic Variability Timescale Research Articles

The IDV source J 1128+5925, a new candidate for annual modulation?

Context. Short time-scale radio variations of compact extragalactic radio sources, known as IntraDay Variability (IDV), can be explained in at least some sources by a source-extrinsic effect, in which the variations are interpreted as scintillation of radio waves caused by the turbulent interstellar medium of the Milky Way. One of the most convincing observational arguments in favour of propagation-induced variability is the so-called “annual modulation” of the characteristic variability time-scale, which is due to the orbital motion of the Earth. So far there are only two sources known which show such a well-defined seasonal cycle, a few more sources with fewer data can be regarded as possible candidates for this effect. However, source-intrinsic effects, such as structural variations, can also cause the observed changes of the variability time-scale. Data for the new, recently discovered, and highly variable IDV source J 1128+5925 are presented. Aims. We study the frequency and time dependence of the IDV in this compact quasar. We measure the characteristic variability time-scale of the IDV throughout the year, and analyze whether the observed changes in the variability time-scale are consistent with annual modulation. Assuming a radio wave propagation effect as origin, we are able to constrain some physical properties (such as distance, scattering-strength, and possible anisotropy) of the “plasma” screen, which may cause the scintillation. Methods. We monitored the flux density variability of J 1128+5925 with dense time sampling between 2.7 and 10.45 GHz. We observed with the 100 m Effelsberg radio telescope of the Max-Planck-Institut fur Radioastronomie (MPIfR) at 2.70 GHz, 4.85 GHz, and 10.45 GHz, as well as with the 25 m Urumqi radio telescope (China) at 4.85 GHz. From ten observing sessions, each of which lasted several days during the period between 2004–2006, we determine the variability characteristics and time-scales which we investigate in view of possible scintillation and annual modulation. Results. The observed pronounced changes of the variability time-scale of J 1128+5925 are modelled with an anisotropic annual modulation model. The observed frequency dependence of the variation is in good agreement with the prediction from interstellar scintillation. Adopting a simple model for the annual modulation model and also using the frequency dependence of the IDV, we derive a lower limit to the distance the scattering screen and an upper limit for the scintillating source size. The latter is found to be consistent with the measured core size from Very Long Baseline Interferometry (VLBI).

Response of the warm absorber cloud to a variable nuclear flux in active galactic nuclei

Recent modeling of the warm absorber in active galactic nuclei has proved the usefulness of constant total (gas plus radiation) pressure models, which are highly stratified in temperature and density. We explore the consistency of those models when the typical variation of the flux from the central source is taken into account. We have performed a variability study of the warm absorber response, based on timescales and our photoionization code titan. We show that the ionization and recombination timescales are much shorter than the dynamical timescale. Clouds very close to the central black hole will maintain their equilibrium since the characteristic variability timescales of the nuclear source are longer than cloud timescales. For more distant clouds, the density structure has no time to vary in response to the variations in the temperature or ionization structure, and such clouds will show the departure from the constant pressure equilibrium. We explore the impact of this departure on the observed properties of the transmitted spectrum and soft X-ray variability: (i) non uniform velocities, on the order of sound speed, appear due to pressure gradients, up to typical values of 100 km s-1. These velocities lead to the broadening of lines. This broadening is usually observed and very difficult to explain otherwise. (ii) Energy-dependent fractional variability amplitude in soft X-ray range has a broader hump around ~1–2 keV, and (iv) the plot of the equivalent hydrogen column density vs. ionization parameter is steeper than for equilibrium clouds. The results have the character of a preliminary study and should be supplemented in the future with full time-dependent radiation transfer and dynamical computations.

The dependence of quasar variability on black hole mass

In order to investigate the dependence of quasar variability on fundamental physical parameters like black hole mass, we have matched quasars from the Quasar Equatorial Survey Team, Phase 1 (QUEST1) variability survey with broad-lined objects from the Sloan Digital Sky Survey. The matched sample contains ≈100 quasars, and the Sloan spectra are used to estimate black hole masses and bolometric luminosities. Variability amplitudes are measured from the QUEST1 light curves. We find that black hole mass correlates with several measures of the variability amplitude at the 99 per cent significance level or better. The correlation does not appear to be caused by obvious selection effects inherent to flux-limited quasar samples, host galaxy contamination or other well-known correlations between quasar variability and luminosity/redshift. We evaluate variability as a function of rest-frame time lag using structure functions and find further support for the variability–black hole mass correlation. The correlation is strongest for time lags of the order of a few months up to the QUEST1 maximum temporal resolution of ≈2 yr, and may provide important clues for understanding the long-standing problem of the origin of quasar optical variability. We discuss whether our result is a manifestation of a relation between characteristic variability time-scale and black hole mass, where the variability time-scale is typical for accretion disc thermal time-scales, but find little support for this. Our favoured explanation is that more massive black holes have larger variability amplitudes, and we highlight the need for larger samples with more complete temporal sampling to test the robustness of this result.

Optical and IR monitoring of the BL Lac object S5 0716+714 from 2001–2004

Multicolor (BV RIJHK) observations of the BL Lac object S5 0716+714 carried out irom 2001–2004 indicate that the spectral energy distribution of the slowly varying component remained constant over the entire wavelength interval and over the three-year observation period. The distribution can be represented by the power law F v ∼ v −1.12, providing evidence that this component has a synchrotron nature. The color characteristics of very rapid variability detected on several nights are the same as those for the slowly varying component. An analysis of published data on the color variability of the object for previous years indicates that the color characteristics of the slowly varying component are stable, independent of the time intervals considered and the characteristic variability time scales.

Correlated spectral and temporal changes in 3C 390.3: a new link between AGN and Galactic black hole binaries?

We present the results of a systematic search for evidence of temporal changes (i.e., non-stationarity) associated with spectral variations in 3C 390.3, using data from a two-year intensive RXTE monitoring campaign of this broad-line radio galaxy. In order to exploit the potential information contained in a time series more efficiently, we adopt a multi-technique approach, making use of linear and non-linear techniques. All the methods show suggestive evidence of non-stationarity in the temporal properties of 3C 390.3 between 1999 and 2000, where the characteristic time-scale of variability decreases as the energy spectrum of the source softens. However, only the non-linear, “scaling index method” is able to conclusively show that the temporal characteristics of the source do vary, although the physical interpretation of this result is not clear at the moment. Our results indicate that the variability properties of 3C 390.3 may vary with time, in the same way as they do in Galactic black holes in the hard state, strengthening the analogy between the X-ray variability properties of the two types of object. This is the first time that such a behavior is detected in an AGN X-ray light curve. Further work is needed to investigate whether this is a common behavior in AGN, as it is in the Galactic binaries.

Possibilities for studying microlensing of distant quasars using the RADIOASTRON space interferometer

Gravitational lensing is a powerful tool for studies of the distribution of matter (including dark matter) in the Universe. The characteristic angular separation of images and characteristic variability time scale depend on the characteristic masses of the gravitational lenses. The construction of the RADIOASTRON space interferometer in the coming years will provide qualitatively new conditions for investigations of microlensing of distant quasars in the radio, since it will become possible not only to resolve individual microimages of these quasars, but also to observe astrometric microlensing (i.e., the shift of an image of a distant quasar, sometimes called weak microlensing). Astrometric microlensing by compact objects in the Galactic halo is not a very significant effect for the RADIOASTRON interferometer, since the probability of such events is low and the characteristic time scales involved appreciably exceed the proposed operational lifetime of the interferometer.

Specific features of the variability of atmospheric processes and hydrophysical fields near the west and southwest coasts of Crimea on time scales from interdiurnal to long-term

We present the results of analysis of the variability of hydrometeorological and hydrophysical fields near the west and southwest coasts of Crimea on scales varying from 1–100 days to 1–11 yr according to the data of long-term routine (4–8 h) observations performed at the coastal marine hydrometeorological stations. New methods of filtration and spectral analysis are used to reveal the characteristic time scales of variability including the quasiperiodic and periodic components and study the physical processes responsible for the indicated variability. Special attention is given to the analysis of correlation between the periodic variability of the fields in the atmosphere and in the coastal zone on different time scales.

Characteristic Variability Time Scales of Long Gamma-Ray Bursts

Abstract We determined the characteristic variability time scales (${\Delta t}_\mathrm{p}$) of 410 bright and long GRBs, by locating the peaks of their Power Density Spectra, defined and calculated in the time domain. We found that the averaged variability time scale decreases with the peak flux. This is consistent with the time-dilation effect expected for the cosmological origin of GRBs. We also found that the occurrence distribution of the characteristic variability time scale shows bimodality, which might be interpreted as that the long GRB sample is composed of two sub-classes with different variability time scales. However, we found no difference for some other characteristics of these two sub-classes.

Rapid Variability and Annual Cycles in the Characteristic Timescale of the Scintillating Source PKS 1257−326

Rapid radio intraday variability (IDV) has been discovered in the southern quasar PKS 1257-326. Flux density changes of up to 40% in as little as 45 minutes have been observed in this source, making it, along with PKS 0405-385 and J1819+3845, one of the three most rapid IDV sources known. We have monitored the IDV in this source with the Australia Telescope Compact Array (ATCA) at 4.8 and 8.6 GHz over the course of the last year and find a clear annual cycle in the characteristic timescale of variability. This annual cycle demonstrates unequivocally that interstellar scintillation is the cause of the rapid IDV at radio wavelengths observed in this source. We use the observed annual cycle to constrain the velocity of the scattering material and the angular size of the scintillating component of PKS 1257-326. We observe a time delay, which also shows an annual cycle, between the similar variability patterns at the two frequencies. We suggest that this is caused by a small (~10 μas) offset between the centroids of the 4.8 and 8.6 GHz components and may be due to opacity effects in the source. The statistical properties of the observed scintillation thus enable us to resolve source structure on a scale of ~10 μas, resolution orders of magnitude higher than current VLBI techniques allow. General implications of IDV for the physical properties of sources and the turbulent interstellar medium are discussed.

Characteristic Variability Time Scales of Long Gamma-Ray Bursts

We determine the characteristic variability time scales for 410 bright long GRBs by locating the maximums of their Power Density Spectra (PDSs) defined and calculated in the time domain. The averaged characteristic variability time scale decreases with peak fluxe. This is consistent with the time dilation effect expected by cosmological origin of GRBs. The occurrence distribution of the characteristic variability time scale shows bimodality, which might be interpreted as that the long GRB sample is composed of two sub-classes with different intrinsic characteristic variability time scales.

The WEBT BL Lacertae Campaign 2000

We present UBVRI light curves of BL Lacertae from May 2000 to January 2001, obtained by 24 telescopes in 11 countries. More than 15000 observations were performed in that period, which was the extension of the Whole Earth Blazar Telescope (WEBT) campaign originally planned for July-August 2000. 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. Colour indexes have been derived by coupling the highest precision B and R data taken by the same instrument within 20 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 about 7 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.

Optical and radio variability of the BL Lacertae object AO 0235+16: A possible 5-6 year periodicity

The BL Lacertae object AO 0235+16 is well known for its extreme optical and radio variability. New optical and radio data have been collected in the last four years by a wide international collaboration, which conrm the intense activity of this source: on the long term, overall variations of 5 mag in the R band and up to a factor 18 in the radio fluxes were detected, while short-term variability up to 0:5 mag in a few hours and 1: 3m ag in one day was observed in the optical band. The optical data also include the results of the Whole Earth Blazar Telescope (WEBT) rst-light campaign organized in November 1997, involving a dozen optical observatories. The optical spectrum is observed to basically steepen when the source gets fainter. We have investigated the existence of typical variability time scales and of possible correlations between the optical and radio emissions by means of visual inspection and Discrete Correlation Function (DCF) analysis. On the long term, the autocorrelation function of the optical data shows a double-peaked maximum at 4100{4200 days (11:2{11:5 years), while a double-peaked maximum at 3900{4200 days (10:7{11:5 years) is visible in the radio autocorrelation functions. The existence of this similar characteristic time scale of variability in the two bands is by itself an indication of optical-radio correlation. A further analysis by means of Discrete Fourier Transform (DFT) technique and folded light curves reveals that the major radio outbursts repeat quasi-regularly with a periodicity of5:7 years, i.e. half the above time scale. This period is also in agreement with the occurrence of some of the major optical outbursts, but not all of them. Visual inspection and DCF analysis of the optical and radio light curves then reveal that in some cases optical outbursts seem to be simultaneous with radio ones, but in other cases they lead the radio events. Moreover, a deep inspection of the radio light curves suggests that in at least two occasions (the 1992{1993 and 1998 outbursts) flux variations at the higher frequencies may have led those at the lower ones.

Characteristic Ultraviolet/Optical Timescales in Active Galactic Nuclei

The UV/optical light curves of active galactic nuclei (AGNs) may be roughly characterized by aperiodic fractional flux variations of ~10% on timescales of 1 month. The physical mechanism(s) responsible remain ill defined. We present a structure function analysis, i.e., measure the power distribution over a range of timescales τ, of 13 AGNs to constrain the origin of UV/optical emission. On timescales τ ~ 5-60 days, the mean UV and optical power-density spectra (PDS) are equivalent. This may suggest that the underlying energy generating mechanism is identical. The combined UV/optical PDS is P(f) ∝ f-α with α = 2.13. For sources with measured X-ray PDS indices, we find they are indistinguishable from their UV/optical counterparts. This supports scenarios whereby X-rays are generated via Compton upscattering of UV photons, to later radiatively drive optical variations. At the same time, we present evidence for characteristic variability timescales τchar of ~5-100 days in 10 sources. These variability timescales combined with reverberation based masses M suggest a M-τchar relationship; higher mass systems have larger characteristic timescales. The UV τchar may possibly reflect dynamical or accretion disk thermal timescales. We find suggestive evidence for a dichotomy, at τ ~ 30 days and M ~ 107 M☉, between short- and long-timescale optical variations. These optical variations may be attributable to dynamical and accretion disk thermal or starburst activity timescales, respectively.

A Cutoff in the X‐Ray Fluctuation Power Density Spectrum of the Seyfert 1 Galaxy NGC 3516

During 1997 March-July, XTE observed the bright, strongly variable Seyfert 1 galaxy NGC 3516 once every ~12.8 hr for 4.5 months and nearly continuously (with interruptions due to SAA passage but not Earth occultation) for a 4.2 day period in the middle. These were followed by ongoing monitoring once every ~4.3 days. These data are used to construct the first well-determined X-ray fluctuation power density spectrum (PDS) of an active galaxy to span more than 4 decades of usable temporal frequency. The PDS shows no signs of any strict or quasi-periodicity, but does show a progressive flattening of the power-law slope from -1.74 at short time scales to -0.73 at longer time scales. This is the clearest observation to date of the long-predicted cutoff in the PDS. The characteristic variability time scale corresponding to this cutoff temporal frequency is 1 month. Although it is unclear how this time scale may be interpreted in terms of a physical size or process, there are several promising candidate models. The PDS appears similar to those seen for Galactic black hole candidates such as Cyg X-1, suggesting that these two classes of objects with very different luminosities and putative black hole masses (differing by more than a factor of 10^5) may have similar X-ray generation processes and structures.

An analysis of the time scales of variability in centuries-long enso-sensitive records in the last 1000 years

We document the characteristic time scales of variability for seven climate indices whose time-dependent behavior is sensitive to some aspect of the El Nino/Southern Oscillation (ENSO). The ENSO sensitivity arises from the location of these long-term records on the periphery of the Indian and Pacific Oceans. Three of the indices are derived principally from historical sources, three others consist of tree-ring reconstructions (one of summer temperature, and the other two of winter rainfall), and one is an annual record of oxygen isotopic composition for a high-elevation glacier in Peru. Five of the seven indices sample at least portions of the Medieval Warm Period (∼ A.D. 950 to 1250).

On the search for the orbital period in X persei X-ray/Be system

Abstract The polarimetric observations of X Per (Kemp and Barbour, 1983) are reanalysed. Neither the 290/580-day periodicity proposed by these authors, nor the 1-day period (Clarke and McGale, 1988) can be confirmed using these data. Only a 800-day characteristic timescale of variability was present in the period from 1977–1982; the variations were correlated with the changes in optical brightness. It is not clear whether these variations are connected with the orbital rotation or with the mass outflow from the primary.

The University of Michigan radio astronomy data base. I - Structure function analysis and the relation between BL Lacertae objects and quasi-stellar objects

view Abstract Citations (235) References (60) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The University of Michigan Radio Astronomy Data Base. I. Structure Function Analysis and the Relation between BL Lacertae Objects and Quasi-stellar Objects Hughes, P. A. ; Aller, H. D. ; Aller, M. F. Abstract We have performed a structure function analysis on a well-observed group of variable extragalactic sources, monitored in total and polarized flux in the centimeter waveband since 1965. Total flux structure functions of BL Lac objects and QSOs exhibit very similar power-law slopes, suggesting that the same processes are responsible for the variability in both types of source. In view of the recent success of the shocked-jet model for radio source variability, we argue that the current work provides circumstantial evidence for the ubiquity of such shocked flows. This analysis has provided a good estimate of characteristic time scales of variability for the two types of object: there is a very broad distribution of time scales for both BL Lac objects and QSOs with mean time scales of 1.95 and 2.35 yr, respectively. The majority of sources (>~85%) with time scales greater than 10 yr are QSOs. A structure function analysis applied to the Stokes parameters in a frame of reference related to the VLBI jet structure suggests that variations of the polarized flux occur in an orientation related to the underlying jet direction. This is further circumstantial evidence for the domination of the source polarization by a shock phenomenon, as this behavior is what one would expect from the axial compression of a turbulent magnetic field. We interpret our derived time scales for Q and U variations and the relative amplitude of these variations, in terms of emission from shocked jets with a well-defined magnetic field direction, contaminated by a significant but randomly polarized core emission for BL Lacs, and in similar terms, but with a magnetic field less strongly correlated with the jet direction, and a weakly polarized core, for QSOs. Periodogram analyses show little or no evidence of harmonic behavior. Subject Headings BL Lacertae objects: general - galaxies: jets - polarization - quasars: general - radio continuum: galaxies Publication: The Astrophysical Journal Pub Date: September 1992 DOI: 10.1086/171734 Bibcode: 1992ApJ...396..469H Keywords: Bl Lacertae Objects; Data Bases; Quasars; Radio Astronomy; Magnetic Field Configurations; Radio Jets (Astronomy); Radio Spectra; Red Shift; Very Long Base Interferometry; Astrophysics; GALAXIES: BL LACERTAE OBJECTS: GENERAL; GALAXIES: JETS; POLARIZATION; GALAXIES: QUASARS: GENERAL; RADIO CONTINUUM: GALAXIES full text sources ADS | data products NED (51) SIMBAD (50)

The optical variability of PKS 2155 - 304

Results of photometric monitoring of the BL Lacertae object PKS 2155 - 304 between 1979 and 1990 are presented. Optical variations were observed on time scales ranging from several minutes to several years. The variations detected with time scales on the order of minutes are among the fastest optical variations detected for a BL Lacertae object. These rapid variations provided constraints on the mass of the supermassive black hole, assumed to be present at the center of the object, of the range between 5.5 +/-0.4 x 10 exp 8 and 8.1 +/-1.0 x 10 exp 9 solar masses. The results indicate that the characteristic variability time scale for the optical variability is shorter than the characteristic variability time scale established for the UV of approximately 50 d reported by Treves et al. (1989).

Variability analysis in low count rate sources

A method, based on the chi-square statistics, is described for detecting pulselike time variability in low count rate sources observed with photon-counting instruments. This method can be used even in the presence of observational gaps, takes full advantage of the filtering effect due to binning with different bin sizes, and takes into account the arbitrariness introduced by the binning phase. The procedure developed to limit the dependence of the results on the binning phase and ensure statistically correct results is described along with the application of the proposed procedure to a model of a variable source. Monte Carlo simulations are used to show how the method can be used to derive the characteristic variability time scales and that the method is more sensitive than the nonparametric Kolmogorov-Smirnov test in detecting variability to a given confidence level.