Variations In Different Bands Research Articles

Multi-band optical variability of BL Lac object OQ 530

Historical optical data are combined on the BL Lac object OQ 530. Verifying the existence of correlations among the flux variations in different bands serves as an important tool to investigate emission processes. To examine the possible existence of a lag between variations in different optical bands from this source, a statistical analysis is performed through the discrete correlation function (DCF) method and the z-transformed discrete correlation function (ZDCF) method. Monte Carlo simulations called ``flux redistribution/random subset selection'' (FR/RSS) are performed to obtain statistically meaningful values for the cross-correlation time lags and their related uncertainties. The analysis confirms that the variations in different optical light curves are strongly correlated, with zero-lag within the errors. Long term variability of color indexes is also analyzed. The result suggests a strong correlation between color index and brightness. A clear bluer-when-brighter chromatism is found, in the sense that the spectrum steepens as the brightness decreases.

The Correlated Multi-color Optical Variations of BL Lac Object S5 0716+714

AbstractS5 0716+714 is a well-studied BL Lac object in the sky. Verifying the existence of correlations among the flux variations in different bands serves as an important tool to investigate the emission processes. To examine the possible existence of a lag between variations in different optical bands on this source, we employ a discrete correlation function analysis on the light curves. In order to obtain statistically meaningful values for the cross-correlation time lags and their related uncertainties, we perform Monte Carlo simulations called ‘flux redistribution/random subset selection’. Our analysis confirms that the variations in different optical light curves are strongly correlated. The time lags show a hint of the variations in high frequency band leading those in low frequency band of the order of a few minutes.

Dissipation of jet bulk kinetic energy in powerful blazars

We investigate the dissipation of the bulk kinetic energy of a relativistic jet at different distances from the central power--house and analyse in detail how the dissipated energy is radiated away. We assume that the location of the dissipation region is a function of the bulk Lorentz factor Gamma of the jet, being closer to the center for smaller Gamma. This assumption is naturally fulfilled in the internal shock scenario. The dissipated energy is partially used to accelerate electrons and to amplify the magnetic field. This process creates a source inside the jet (blob). Such blobs may efficiently produce synchrotron and inverse Compton emission. We find that even if the blobs or shells responsible for the blazar activity carry the same energy (in bulk kinetic form), the fact that they move at different Gamma can produce dramatic variations in different bands, even if the bolometric luminosity is instead very similar. This is due to the relative importance of the synchrotron, self-Compton and external Compton radiation processes, which greatly changes by changing Gamma and the compactness of the source, even if the total radiated energy is constant. We then find that the jet can produce most of its radiative output at small distances from the putative black-hole and its accretion disk, if this implies a low level of emitted MeV-GeV flux. Our findings, which we apply for illustrative purposes to the blazar 3C 454.3, will be easily testable by the coming gamma-ray satellite, such as AGILE and GLAST.

Observational Constraints on Cool Disk Material in Quiescent Black Hole Binaries

We consider current observational constraints on the presence of cool, optically thick disk material in quiescent black hole binaries, specifically focusing on a case study of the prototypical system A0620� 00. Such material might be expected to be present theoretically but is usually claimed to make a negligible contribution at optical and IR wavelengths. The primary argument is based on measurements of the veiling of stellar photospheric absorption lines,inwhichitisassumedthatthediskspectrumisfeatureless.Weusesimulatedspectratoexplorethesensitivity of veiling measurements to uncertainties in companion temperature, gravity, and metallicity. We find that the derived veiling is extremely sensitive to a mismatch between the temperature and metallicity of the companion and template but that the effect of a plausible gravity mismatch is much smaller. In general, the resulting uncertainty in the amount of veiling is likely to be much larger than the usually quoted statistical uncertainty. We also simulate spectra in which the disk has an emergent spectrum similar to the star and find that in this case, optical veiling constraints are moderately robust. This is because the rotational broadening of the disk is so large that the two line profiles effectively decouple, and the measurement of the depth of stellar lines is largely unbiased by the disk component. We note, however, that this is only true at intermediate resolutions or higher and that significant bias might still affect low-resolution IR observations. Assuming that the optical veiling is reliable, we then examine the constraints on the temperature and covering factor of any optically thick disk component. These are stringent if the disk is warm (TeA k3500 K), but very temperature sensitive, and cooler disks are largely unconstrained by optical measurements. Current IR veiling estimates do not help much, representing rather high upper limits. Probably the best constraint comes from the relative amplitudes of ellipsoidal variations in different bands, as these are sensitive to differences in veiling that are expected for disks cooler than the companion star. A significant disk contribution in the IR, up to � 25%, is not ruled out in this or any other way considered, however. Subject headingg accretion, accretion disks — binaries: close — black hole physics

Rapid continuum variability in NGC 4151

Intensive monitoring of NGC 4151 showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in ∼1.5 keV X-rays, with weaker variations at ∼100 keV and no significant variability in the 0.1–1 keV bands. In the ultraviolet/optical regime, the fractional variability amplitude decreased from 9% to 1% as the wavelength increased from 1275 Å to 6900 Å. The phase differences between variations in different bands were consistent with zero lag, with upper limits of ≲0.15 day between 1275 Å and the other ultraviolet bands, ≲0.3 day between 1275 Å and 1.5 keV, and ≲1 day between 1275 Å and 5125 Å, an order of magnitude improvement over limits determined in previous multi-waveband AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well-fitted with a very red power-law. The tight limits on the lags indicate that size of the putative reprocessing region is smaller than ∼0.15 lt-day in size. After correcting for absorption, the X-ray luminosity variations appear adequate to drive the ultraviolet/optical variations, but energy budget and other arguments indicate that only a fraction of the total low-energy flux could be reprocessed high-energy emission. The data are also consistent with the variable emission coming from the hotter inner regions of an accretion disk and non-variable emission from the cooler outer regions.

Multiwavelength Observations of Short-Timescale Variability in NGC 4151. IV. Analysis of Multiwavelength Continuum Variability

For pt.III see ibid., vol.470, no.1, p.349-63 (1996). Combines data from the three preceding papers in order to analyze the multi wave-band variability and spectral energy distribution of the Seyfert 1 galaxy NGC 4151 during the 1993 December monitoring campaign. The source, which was near its peak historical brightness, showed strong, correlated variability at X-ray, ultraviolet, and optical wavelengths. The strongest variations were seen in medium-energy (~1.5 keV) X-rays, with a normalized variability amplitude (NVA) of 24%. Weaker (NVA=6%) variations (uncorrelated with those at lower energies) were seen at soft gamma-ray energies of ~100 keV. No significant variability was seen in softer (0.1-1 keV) X-ray bands. In the ultraviolet/optical regime, the NVA decreased from 9% to 1% as the wavelength increased from 1275 to 6900 Aring. These data do not probe extreme ultraviolet (1200 Aring to 0.1 keV) or hard X-ray (250 keV) variability. The phase differences between variations in different bands were consistent with zero lag, with upper limits of lsim0.15 day between 1275 Aring and the other ultraviolet bands, lsim0.3 day between 1275 Aring and 1.5 keV, and lsim1 day between 1275 and 5125 Aring. These tight limits represent more than an order of magnitude improvement over those determined in previous multi-wave-band AGN monitoring campaigns. The ultraviolet fluctuation power spectra showed no evidence for periodicity, but were instead well fitted with a very steep, red power law (ales-2.5)