Multi-wavelength Intra-day Variability and Quasi-periodic Oscillation in Blazars

We reviewed multi-wavelength blazars variability and detection of quasi-periodic oscillations on intra-day timescales. The variability timescale from few minutes to up to less than a days is commonly known as intra-day variability. These fast variations are extremely useful to constrain the size of emitting region, black hole mass estimation, etc. It is noticed that in general blazars show intra-day variability in the complete electromagnetic spectrum. But some class of blazars either do not show or show very little intra-day variability in a specific band of electromagnetic spectrum. Blazars show rarely quasi-periodic oscillations in time series data in optical and X-ray bands. Other properties and emission mechanism of blazars are also briefly discussed.


Introduction
It is commonly accepted that super massive black holes (SMBHs, with masses between 10 6 − 10 10 M ) are present in the nuclei of all galaxies with stellar bulges. At any given time a few percent of these SMBHs are fed with a sufficient amount of gas that they will possess significant accretion discs. The emission of these discs is comparable to the total emission of stars in the entire host galaxy because of a very high efficiency for the conversion of matter into radiation as it spirals into a BH. This is the fundamental mechanism underlying "active galactic nucleus", or AGN.
Roughly 85−90 % of AGNs have very little radio emission (F 5GHz /F B ≤ 10, here F 5GHz = flux at radio 5 GHz and F B = flux at optical B band 4400Å) and are therefore called radio-quiet AGNs (RQAGNs). The remaining ∼ 10-15 % of AGNs are radio-loud AGNs (RLAGNs). It has been proposed that different types of AGNs can be explained by the idea that different line of sight (LOS) angles can play an important role in understanding their different properties (Antonucci 1993;Urry & Padovani 1995).
Blazars belongs to the RLAGN class and its LOS is pointed close to the observer. Blazars show rapid variability at almost all wavelengths of the electromagnetic (EM) spectrum with the emission being strongly polarized (optical linear polarization ≥ 3%). Due to their strong and large amplitude variability nature in the complete EM spectrum, they are considered as transient astronomical objects. BL Lacertae objects (BLLs) and flat spectrum radio quasars (FSRQs) are collectively known as blazars. BLLs show featureless optical continua (no prominent emission or absorption lines) while FSRQs show prominent emission lines in their optical spectra. The radiation from blazars is dominated by non-thermal emission at all wavelengths, consisting of two broad spectral bumps The study of variability is one of the most powerful tools for understanding the nature and processes occurring in blazars. Variability of blazars can be broadly divided into three classes. Significant variations in flux may occur over few tens of minutes to the course of less than a day, often called micro-variability, intra-night variability or intra-day variability (IDV) (Wagner & Witzel 1995). Short term variability (STV) can range time scales from days to few months and long term variability (LTV) can have time scales of several months to several years (Gupta et al. 2004).
Variability observations on IDV timescales in AGN is the most puzzling. Variability of blazars on IDV time scale can provide important clues to the physics of the innermost nuclear regions in these objects. Blazar properties are consistent with relativistic beaming caused by bulk relativistic motion of the jet plasma at small angles to the line of sight, which gives rise to a strong amplification and rapid variability in the observer's frame. With simultaneous multi-wavelength observations of blazars in the entire EM spectrum is an important tool to test several The possible models for IDV: shock-in-jet models, accretion-disk based models, and models based on plasma instabilities in shear layers, etc.

Intra-day Variability in different EM Bands
There have been several dedicated monitoring campaigns in which the IDV of blazars has been studied over the entire EM spectrum (e.g. Miller et al. 1989;Carini et al. 1990Carini et al. , 1991Quirrenbach, et al. 1991;Heidt & Wagner 1996Sagar et al. 1999 There are several methods which can be used to find the genuine IDV and variability parameters in time series data. These methods are described by different groups and relevant source references are cited in their papers. Romero et al. (1999)  . This method takes care of large brightness difference in blazar and comparison / standard stars or large brightness difference in comparison / standard stars. It is used by Gaur et al. (2015). χ 2 −Test is explained by (Gaur et  IDV in blazars can be intrinsic to the source or due to extrinsic origin. Interstellar scintillation and gravitational microlensing are the main extrinsic cause of IDV. Interstellar scintillation is only relevant in low-frequency radio observations. Gravitational microlensing is only applicable in a few blazars which are lensed system e.g. the blazar AO 0235+135 at z = 0.94 have revealed foreground-absorbing systems at z = 0.524 and z = 0.851 (Cohen et al. 1987;Nilsson et al. 1996). In the blazars where IDV is detected in low-frequency radio observations or gravitationally lensed sources can also have some intrinsic origin, and to find that we need to do simultaneous multi-wavelength of the blazar, and co-related variability in different EM bands will be helpful to find the nature of variability.
IDV in the high state (pre/post to outburst state) of blazars can be explained by jet based models e.g. helical instabilities in the jet (Marscher & Gear 1985;Qian et al. 1991) or turbulence behind the shock in the jet (Marscher, Gear, Travis 1992). Jet based models can explain IDV over the entire range of EM wavelengths. Other theoretical models seek to explain IDV in blazars (mainly in their low-state) involving accretion-disk based models. These models include pulsations of the gravitational modes of the gaseous disk (Kato & Fukue 1980;Nowak & Wagoner 1992) or orbital signatures from "hot-spots" in the gas surrounding the black hole, either from the disk itself or the corona above it (Zhang & Bao 1991;Mangalam & Wiita 1993). Accretion-disk based models can explain the variations in optical, UV and X-ray bands, but are difficult to connect to the observed rapid variability in γ−rays. Plasma instabilities in the jet (e.g., Kelvin-Helmholtz type instabilities due to the interaction of a fast inner spine of the jet with a slower, outer layer) could play an important role in the production of IDV at a variety of wavelengths.

IDV in Optical and infrared (IR) bands
The first evidence of optical micro-variability is reported in BL Lacertae by Miller et al. (1989). The result of this paper motivated several groups around the globe to start dedicated project to search for optical micro-variability in blazars. Optical IDV in blazars is pioneered by the USA group in which they studied optical micro-variability in five blazars (e.g. Miller et al. 1989;Carini et al. 1990Carini et al. , 1991, and reported that the probability of finding genuine micro-variability is about 80% for the blazar continuously monitored for < 8 hours. An extensive search for optical IDV in a sample of 34 BLLs from 1 Jy catalog was done by (Heidt & Wagner 1996). IDV was detected in 28 out of 34 BLLs (82%), and 75% of the variable BLLs changed significantly over a time span < 6 hours. But this data lacks continuity in the LCs. Gupta  Wagner et al. (1990) did simultaneous optical and radio monitoring of blazars.  reported for the first time the correlated optical and radio IDV in the blazar S5 0716+714. To test the inverse-Compton (IC) catastrophe scenario in the blazar S5 0716+714 extensive observational campaign in radio and mm wavelengths were coordinated (Ostorero, et al. 2006;Agudo et al. 2006;Fuhrmann et al. 2008), and the lower limits to brightness temperature was derived from the inter-day variations exceed the 10 12 K IC-limit by up to 2-4 orders of magnitude. Gabányi et al. (2007) reported radio IDV of the blazar J 1128+5925 in three frequencies i.e. 2.7 and 10.45 GHz observations using 100m Effelsberg radio telescope in Germany, and 4.8 GHz observations using 25m radio telescope in Urumqi, China. The observed frequency dependent IDV in the source was in good agreement with prediction from interstellar scintillation. VLBA observation of the blazar J1128+592 is reported by Gabányi et al. (2009), and with VLBA observations they detected an east-west oriented core-jet structure with no significant motion in its jet. Radio IDV in blazars are studied where variability characteristics have changed abruptly by interstellar scintillation (Marchili et al. 2011(Marchili et al. , 2013. Radio IDV at 4.8 GHz using 25m Urumqi, China telescope for the blazars S5 0716+714 and 1156+295 are reported by Liu et al. 2013). Simultaneous IDV in X-ray, optical four bands and three frequencies in radio are reported by Gupta et al. (2012). IDV detected in all three radio frequencies and also noticed that low and high frequencies correlation does not peak at zero lag which show that low frequency radio observation is combined effect of intrinsic and extrinsic mechanism. Optical and radio IDV observations were carried out by Liu et al. (2017). IDV observation along with VLBI analysis is carried out for the blazar S4 0917+624 (Liu et al. 2015).

Quasi Periodic Oscillations in Intra-day Time Series Multi-wavelength Data
Detection of quasi-periodic oscillations (QPOs) in time series data are very rare in AGN. In last one decade, several detection of QPOs in AGN on diverse timescales ranging as short as few minutes and as long as few years using γ−ray, X-ray, optical and radio time series data are made (e.g. QPOs in blazars on IDV timescales can be explained by several standard models of AGN. One of the simplest models in which the central BHs of AGN would attribute the QPOs can be explained by presence of a single dominating hot-spot on the accretion disk (e.g., Mangalam & Wiita 1993;Chakrabarti & Wiita 1993). Using QPO or nearly periodic signal, the period can be used to estimate the BH mass for non-rotating (Schwarzschild) BH, and maximally rotating (Kerr) BH. The detailed explanation is given in Gupta et al. (2009). Other alternative possible mechanisms for QPOs in blazars on IDV timescales can also have a disk origin or can arise from relativistic jets. The former class includes small epicyclic deviations in both radial and vertical directions from exact planar motions within a thin accretion disk (e.g., Abramowicz 2005), and trapped pulsational modes within a disk (e.g., Perez et al. 1997;Espaillat et al. 2008). Using detailed explanation of Perez et al. (1997), one can also get the BH mass of the blazar. There are various jet models which also can explain the QPO detection in blazars on IDV timescales e.g. a shock propagating down a jet in which jet structure is quasi-helical and change in electron density or magnetic field can produce QPO, a short lived QPO can be due to turbulence behind the shock in the relativistic jet (e.g. Camenzind (Montagni et al. 2006). They used wavelet analysis along with randomization test and found strong evidence for nearly periodic variations on 5 light curves with probability > 99%. The period for these five light curves are found in the range of 25 minutes to 73 minutes which lead to BH mass ranging 2.47-7.35 × 10 6 M and 1.57-4.67 × 10 7 M for non-rotating BH and maximally rotating BH, respectively. Another evidence of QPO detection in optical band on the same blazar S5 0716+714 is reported by Rani et al. (2010). They found QPO period of ∼ 15 minutes using various techniques (e.g. SF, LSP, PSD, data folding). This period yield the BH mass 1.5 × 10 6 M and 9.6 × 10 6 M for non-rotating BH and maximally rotating BH, respectively.

In X-rays
Using wavelet technique, (Espaillat et al. 2008) analyzed 19 observations of 10 AGN observed with EPIC/pn detector on board to XMM-Newton, and detected QPO period of 3.3 ks in one light curve of the blazar 3C 273. The QPO period is used to get the black hole (BH) mass of the blazar. They estimated the BH mass of the blazar is 7.3 × 10 6 M and 8.1 × 10 7 M for non-rotating BH and maximally rotating BH, respectively. In another observation of EPIC/pn of XMM-Newton for the blazar PKS 2155-304, (Lachowicz et al. 2009) detected QPO period of 4.6 h in which period was present for ∼ 3.8 cycles. This QPO detection was verified by various techniques (e.g. SF, PSD, MHAoV, data folding and wavelet). The BH mass of the blazar is estimated to be 3.29 × 10 7 M and 2.09 × 10 8 M for non-rotating BH and maximally rotating BH, respectively.

Conclusions
With the extensive studies of IDV in radio to optical bands in last about three decades and in high energies (X-ray and γ−rays) in last one decade, we reach on the following conclusion: • Blazars show large amplitude IDV in radio bands which is basically the mixture of extrinsic and intrinsic nature. • LBLs and IBLs show large amplitude IDV in optical/IR bands with high duty cycle. • HBLs either don't show optical/IR IDV or if show the amplitude is low and the duty cycle is very less compare to LBLs/IBLs. • In general blazars don't show color variation on IDV timescales. But occasionally it is seen. • Optical inter-band cross-correlation show that in general there is no time lag in different optical bands. On some occasions the time lag of a few minutes are reported in different optical bands. • Optical/IR SEDs are well fitted with single power law. • Sometimes optical/IR SEDs show big blue bump which is a signature of emission from accretion disk.
• In X-rays, HBLs show large amplitude IDV, and the duty cycle is high.
• In X-rays, LBLs/IBLs either don't show IDV or if show the amplitude is less. The duty cycle of X-ray IDV for LBLs/IBLs are much less compare to duty cycles for HBLs. • In general hard and soft X-ray LCs of blazars show strong cross-correlation with zero lag which imply the emission in hard and soft bands are co-spatial. • IDV timescales in different EM bands are used to get black hole mass of the blazars, size of emitting region, and the Doppler factor δ. • In general sensitivity of very high energy γ−ray facilities are poor but occasionally blazars are observed on time resolutions of a few minutes to few hours. The best time resolution γ−ray light curves give high Doppler factor ∼ 100 for the blazar PKS 2155-304. • QPOs in blazars on IDV timescales are rare. • Occasionally QPOs on IDV timescales are detected in a few blazars in X-ray and optical bands.
Author Contributions: The author was asked by the journal to write the review article. The author planned and wrote the article.

Conflicts of Interest:
The author declares no conflict of interest.