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Register a new userCharacterizing Long-term Optical Variability Properties of $gamma$-ray Bright Blazars
تشخيص خصائص الاختلاف البصري الطويل الأجل للبلازار الشديدة في الإشعاع غاما
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Gopal Bhatta
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تم جمع الملاحظات البصرية لعينة من 12 بلازار باهتة بالإشعاع جاما من أربع مخزونات بيانات بصرية، AAVSO، SMARTS، Catalina و Steward Observatory، لإنشاء منحنيات ضوء مشبعة بشكل كثيف تشمل أكثر من عقد. كجزء من دراسات البلازار عبر الأشعة المتغيرة، تم تنفيذ عدة طرق للتحليل، على سبيل المثال، توزيع الطاقة وعلاقة RMS-flux، على الملاحظات مع هدف مقارنة النتائج مع النتائج المشابهة في النطاق الجاما المعروض في Bhatta & Dhital 2020. وتبين أنه، مثل النطاق الجاما، يظهر البلازار تباينا كبيرا في النطاق البصري الذي يمكن تحديده بتوزيع طاقة معمول بالسجل وعلاقة قوة-قانونية ل RMS على الطاقة. قد يكون هذا علامة على علاقة RMS-flux خطية ذات أصل قابل للاقتراب، لكن التبعات في البيانات لا تسمح لرفض إمكانيات أخرى. عند مقارنة خصائص التباين في الطبقتين، وجد أن البلازار في الجاما تبدو أكثر تباينا مع علاقة RMS-flux خطية أكثر شدة وتوزيع الطاقة الذي يتجاوز إلى الطاقات العالية بشكل أكثر. وأظهرت الدراسة التقاطعية أن باستثناء المصدر 3C 273، فإن الإشعاع البصري والجاما في المصادر تتشابه بشكل كبير، مشيرة إلى وجود موحد للذرات المسؤولة عن الإشعاع البصري والجاما. بالإضافة إلى ذلك، أظهرت المصادر S5 0716+714، Mrk 421، Mrk 501، PKS 1424-418 و PKS 2155-304 دليلا محتملا على وجود تهاويج نظرية متكررة في الإشعاع البصري مع وقت الخصوصية الذي يتساوي بالنسبة لذلك في النطاق الجاما الملحوظ في عملنا السابق.
Optical observations of a sample of 12 $gamma$-ray bright blazars from four optical data archives, AAVSO, SMARTS, Catalina, and Steward Observatory, are compiled to create densely sampled light curves spanning more than a decade. As a part of the blazar multi-wavelength studies, several methods of analyses, e. g., flux distribution and RMS-flux relation, are performed on the observations with an aim to compare the results with the similar ones in the gama-ray band presented in Bhatta & Dhital 2020. It is found that, similar to $gamma$-ray band, blazars display significant variability in the optical band that can be characterized with log-normal flux distribution and a power-law dependence of RMS on flux. It could be an indication of possible inherent linear RMS-flux relation, yet the scatter in the data does not allow to rule out other possibilities. When comparing variability properties in the two bands, the blazars in the gama-rays are found to exhibit stronger variability with steeper possible linear RMS-flux relation and the flux distribution that is more skewed towards higher fluxes. The cross-correlation study shows that except for the source 3C 273, the overall optical and the $gamma$-ray emission in the sources are highly correlated, suggesting a co-spatial existence of the particles responsible for both the optical and $gamma$-ray emission. Moreover, the sources S5 0716+714, Mrk 421, Mrk 501, PKS 1424-418 and PKS 2155-304 revealed possible evidence for quasi-periodic oscillations in the optical emission with the characteristic timescales, which are comparable to those in the $gamma$-ray band detected in our previous work.
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We present long term optical and near infrared flux variability analysis of 37 blazars detected in the $gamma$-ray band by the {it Fermi Gamma-Ray Space Telescope}. Among them, 30 are flat spectrum radio quasars (FSRQs) and 7 are BL Lac objects (BL Lacs). The photometric data in the optical (BVR) and infrared (JK) bands were from the Small and Moderate Aperture Research Telescope System acquired between 2008$-$2018. From cross-correlation analysis of the light curves at different wavelengths, we did not find significant time delays between variations at different wavelengths, except for three sources, namely PKS 1144$-$379, PKS B1424$-$418 and 3C 273. For the blazars with both B and J-band data, we found that in a majority of FSRQs and BL Lacs, the amplitude of variability ($sigma_m$) in the J-band is larger than B-band consistent with the dominance of the non-thermal jet over the thermal accretion disc component. Considering FSRQs and BL Lacs as a sample, there are indications of $sigma_m$ to increase gradually towards longer wavelengths in both, however, found to be statistically significant only between B and J-bands in FSRQs. In the B$-$J v/s J colour magnitude diagram, we noticed complicated spectral variability patterns. Most of the objects showed a redder when brighter (RWB) behaviour. Few objects showed a bluer when brighter (BWB) trend, while in some objects both BWB and RWB behaviours were noticed. These results on flux and colour characteristics indicate that the jet emission of FSRQs and BL Lacs is indistinguishable.
Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in the high (HE, 100 MeV < E < 300 GeV) and very high energy (VHE, E > 200 GeV) gamma-ray domain. Over the course of ~9 yr of H.E.S.S observations the VHE light curve in the quiescent state is consistent with a log-normal behavior. The VHE variability in this state is well described by flicker noise (power-spectral-density index {ss}_VHE = 1.10 +0.10 -0.13) on time scales larger than one day. An analysis of 5.5 yr of HE Fermi LAT data gives consistent results ({ss}_HE = 1.20 +0.21 -0.23, on time scales larger than 10 days) compatible with the VHE findings. The HE and VHE power spectral densities show a scale invariance across the probed time ranges. A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established. These long-term-variability properties are discussed and compared to the red noise behavior ({ss} ~ 2) seen on shorter time scales during VHE-flaring states. The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes, while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection.
We use optical data from the Palomar Transient Factory (PTF) and the Catalina Real-Time Transient Survey (CRTS) to study the variability of gamma-ray detected and non-detected objects in a large population of active galactic nuclei (AGN) selected from the Candidate Gamma-Ray Blazar Survey and Fermi Gamma-Ray Space Telescope catalogs. Our samples include 714 sources with PTF data and 1244 sources with CRTS data. We calculate the intrinsic modulation index to quantify the optical variability amplitude in these samples. We find the gamma-ray detected objects to be more variable than the non-detected ones. The flat spectrum radio quasars (FSRQs) are more variable than the BL Lac objects in our sample, but the significance of the difference depends on the sample used. When dividing the objects based on their synchrotron peak frequency, we find the low synchrotron peaked (LSP) objects to be significantly more variable than the high synchrotron peaked (HSP) ones, explaining the difference between the FSRQs and BL Lacs. This could be due to the LSPs being observed near their electron energy peak, while in the HSPs the emission is caused by lower energy electrons, which cool more slowly. We also find a significant correlation between the optical and gamma-ray fluxes that is stronger in the HSP BL Lacs than in the FSRQs. The FSRQs in our sample are also more Compton dominated than the HSP BL Lacs. These findings are consistent with models where the gamma-ray emission of HSP objects is produced by the synchrotron self-Compton mechanism, while the LSP objects need an additional external Compton component that increases the scatter in the flux-flux correlation.
We present 10 years of R-band monitoring data of 31 northern blazars which were either detected at very high energy (VHE) gamma rays or listed as potential VHE gamma-ray emitters. The data comprise 11820 photometric data points in the R-band obtained in 2002-2012. We analyze the light curves by determining their power spectral density (PSD) slopes assuming a power-law dependence with a single slope $beta$ and a Gaussian probability density function (PDF). We use the multiple fragments variance function (MFVF) combined with a forward-casting approach and likelihood analysis to determine the slopes and perform extensive simulations to estimate the uncertainties of the derived slopes. We also look for periodic variations via Fourier analysis and quantify the false alarm probability through a large number of simulations. Comparing the obtained PSD slopes to values in the literature, we find the slopes in the radio band to be steeper than those in the optical and gamma rays. Our periodicity search yielded one target, Mrk 421, with a significant (p<5%) period. Finding one significant period among 31 targets is consistent with the expected false alarm rate, but the period found in Mrk~421 is very strong and deserves further consideration}.
We present an in-depth and systematic variability study of a sample of 20 powerful blazars, including 12 BL Lacs and 8 flat spectrum radio quasars, applying various analysis tools such as flux distribution, symmetry analysis, and time series analysis on the decade-long Fermi/LAT observations. The results show that blazars with steeper gama-ray spectral indexes are found to be more variable; and the gama-ray flux distribution closely resembles lognormal probability distribution function. The statistical variability properties of the sources as studied by power spectral density analysis are consistent with emph{flicker noise} ($P( u)propto1/ u$) -- an indication of long-memory processes at work. Statistical analysis of the distribution of flux rise and decay rates in the light curves of the sources, aimed at distinguishing between particle acceleration and energy dissipation timescales, counter-intuitively suggests that both kinds of rates follow a similar distribution and the derived mean variability timescales are in the order of a few weeks. The corresponding emission region size is used to constrain location of gama-ray production sites in the sources to be a few parsecs. Additionally, using Lomb-Scargle periodogram and weighted wavelet z-transform methods and extensive Monte Carlo simulations, we detected year timescale quasi-periodic oscillations in the sources S5 0716+714, Mrk 421, ON +325, PKS 1424-418 and PKS 2155-304. The detection significance was computed taking proper account of the red-noise and other artifacts inherent in the observations. We explain the results in the light of current blazar models with relativistic shocks propagating down the jet viewed close to the line of sight.
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