Research

10 pages, 411 KiB

Open AccessFeature PaperArticle

Fast γ-ray Variability: A Common Feature and Powerful Probe for Jetted AGNs

by Neng-hui Liao

Galaxies 2018, 6(3), 68; https://doi.org/10.3390/galaxies6030068 - 03 Jul 2018

Cited by 4 | Viewed by 2519

Among the highly dynamical non-thermal universe, flashes of

γ

-ray photons from jets in active galactic nuclei (AGNs) are always attractive. Not only are they extraordinary observational phenomena, but they also become powerful probes of the jets. Benefiting from the current advanced

γ

[...] Read more.

Among the highly dynamical non-thermal universe, flashes of

γ

-ray photons from jets in active galactic nuclei (AGNs) are always attractive. Not only are they extraordinary observational phenomena, but they also become powerful probes of the jets. Benefiting from the current advanced

γ

-ray observational facilities, especially the Large Area Telescope aboard the Fermi space

γ

-ray observatory and Imaging Atmospheric Cherenkov Telescope arrays, fast (intraday)

γ

-ray variability has become a common feature detected in various subtypes of jetted AGNs. Moreover, extreme events with variability timescales down to a few minutes have been occasionally detected, which put a severe constraint on the classical jet model. Herein, recent studies on the detection of fast

γ

-ray variability in jetted AGNs are summarized, and corresponding implications are discussed. Scenarios proposed to explain the minute-scale

γ

-ray variability as well as future observational opportunities are also briefly summarized. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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22 pages, 743 KiB

Open AccessFeature PaperArticle

Effelsberg Monitoring of a Sample of RadioAstron Blazars: Analysis of Intra-Day Variability

by Jun Liu, Hayley Bignall, Thomas P. Krichbaum, Xiang Liu, Alex Kraus, Yuri Y. Kovalev, Kirill V. Sokolovsky, Emmanouil Angelakis and J. Anton Zensus

Galaxies 2018, 6(2), 49; https://doi.org/10.3390/galaxies6020049 - 18 Apr 2018

Cited by 9 | Viewed by 4468

Abstract

We present the first results of an ongoing intra-day variability (IDV) flux density monitoring program of 107 blazars, which were selected from a sample of RadioAstron space very long baseline interferometry (VLBI) targets. The IDV observations were performed with the Effelsberg 100-m radio [...] Read more.

We present the first results of an ongoing intra-day variability (IDV) flux density monitoring program of 107 blazars, which were selected from a sample of RadioAstron space very long baseline interferometry (VLBI) targets. The IDV observations were performed with the Effelsberg 100-m radio telescope at 4.8 GHz, focusing on the statistical properties of IDV in a relatively large sample of compact active galactic nuclei (AGN). We investigated the dependence of rapid (<3 day) variability on various source properties through a likelihood approach. We found that the IDV amplitude depends on flux density and that fainter sources vary by about a factor of 3 more than their brighter counterparts. We also found a significant difference in the variability amplitude between inverted- and flat-spectrum radio sources, with the former exhibiting stronger variations.

γ

-ray loud sources were found to vary by up to a factor 4 more than

γ

-ray quiet ones, with 4

σ

significance. However a galactic latitude dependence was barely observed, which suggests that it is predominantly the intrinsic properties (e.g., angular size, core-dominance) of the blazars that determine how they scintillate, rather than the directional dependence in the interstellar medium (ISM). We showed that the uncertainty in the VLBI brightness temperatures obtained from the space VLBI data of the RadioAstron satellite can be as high as ~70% due to the presence of the rapid flux density variations. Our statistical results support the view that IDV at centimeter wavelengths is predominantly caused by interstellar scintillation (ISS) of the emission from the most compact, core-dominant region in an AGN. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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17 pages, 784 KiB

Open AccessFeature PaperArticle

Sub-Hour X-Ray Variability of High-Energy Peaked BL Lacertae Objects

by Bidzina Kapanadze

Galaxies 2018, 6(1), 37; https://doi.org/10.3390/galaxies6010037 - 15 Mar 2018

Viewed by 2956

Abstract

The study of multi-wavelength flux variability in BL Lacertae objects is very important to discern unstable processes and emission mechanisms underlying their extreme observational features. While the innermost regions of these objects are not accessible from direct observations, we may draw conclusions about [...] Read more.

The study of multi-wavelength flux variability in BL Lacertae objects is very important to discern unstable processes and emission mechanisms underlying their extreme observational features. While the innermost regions of these objects are not accessible from direct observations, we may draw conclusions about their internal structure via the detection of flux variations on various timescales, based on the light-travel argument. In this paper, we review the sub-hour X-ray variability in high-energy peaked BL Lacertae sources (HBLs) that are bright at X-rays and provide us with an effective tool to study the details related to the physics of the emitting particles. The X-ray emission of these sources is widely accepted to be a synchrotron radiation from the highest-energy electrons, and the complex spectral variability observed in this band reflects the injection and radiative evolution of freshly-accelerated particles. The detection of sub-hour X-ray flux variability is very important since it can be related to the small-scale jet turbulent structures or triggered by unstable processes occurring in the vicinity of a central supermassive black hole. We summarize the fastest X-ray variability instances detected in bright HBLs and discuss their physical implications. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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7 pages, 1027 KiB

Open AccessFeature PaperArticle

Statistical Analysis of the Microvariable AGN Source Mrk 501

by Alberto C. Sadun, Masoud Asadi-Zeydabadi, Brian Mills and Joseph Ward Moody

Galaxies 2018, 6(1), 20; https://doi.org/10.3390/galaxies6010020 - 05 Feb 2018

Cited by 3 | Viewed by 3102

Abstract

We report on the optical observations and analysis of the high-energy peaked BL Lac object (HBL), Mrk 501, at redshift z = 0.033. We can confirm microvariable behavior over the course of minutes on several occasions per night. As an alternative to the [...] Read more.

We report on the optical observations and analysis of the high-energy peaked BL Lac object (HBL), Mrk 501, at redshift z = 0.033. We can confirm microvariable behavior over the course of minutes on several occasions per night. As an alternative to the commonly understood dynamical model of random variations in intensity of the AGN, we develop a relativistic beaming model with a minimum of free parameters, which allows us to infer changes in the line of sight angles for the motion of the different relativistic components. We hope our methods can be used in future studies of beamed emission in other active microvariable sources, similar to the one we explored. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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14 pages, 1281 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Microvariability in BL Lacertae: “Zooming” into the Innermost Blazar Regions

by Gopal Bhatta and James R. Webb

Galaxies 2018, 6(1), 2; https://doi.org/10.3390/galaxies6010002 - 03 Jan 2018

Cited by 25 | Viewed by 3923

Abstract

In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for four nights in 2016. We studied the intranight flux and spectral variability [...] Read more.

In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for four nights in 2016. We studied the intranight flux and spectral variability of the source in detail with an objective to characterize microvariability in the blazars, a frequently observed phenomenon in blazars. The results show that the source often displays a fast flux variability with an amplitude as large as ~0.2 magnitude within a few hours, and that the color variability in the similar time scales can be characterized as “bluer-when-brighter” trend. We also observed markedly curved optical spectrum during one of the nights. Furthermore, the correlation between multi-band emission shows that in general the emission in all the bands are highly correlated; and in one of the nights V band emission was found to lead the I band emission by ~13 min. The search for characteristic timescale using z-transformed auto-correlation function and the structure function analyses reveals characteristic timescale of ~50 min in one of the R band observations. We try to explain the observed results in the context of the passage of shock waves through the relativistic outflows in blazars. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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229 KiB

Open AccessFeature PaperArticle

Multi-Wavelength Intra-Day Variability and Quasi-Periodic Oscillation in Blazars

by Alok C. Gupta

Galaxies 2018, 6(1), 1; https://doi.org/10.3390/galaxies6010001 - 25 Dec 2017

Cited by 30 | Viewed by 3546

Abstract

We reviewed multi-wavelength blazars variability and detection of quasi-periodic oscillations on intra-day timescales. The variability timescale from a 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 [...] Read more.

We reviewed multi-wavelength blazars variability and detection of quasi-periodic oscillations on intra-day timescales. The variability timescale from a 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 the emitting region, black hole mass estimation, etc. It is noticed that in general, blazars show intra-day variability in the complete electromagnetic spectrum. However, some classes of blazars either do not show or show very little intra-day variability in a specific band of the electromagnetic spectrum. Blazars show rarely quasi-periodic oscillations in time series data in optical and X-ray bands. Other properties and emission mechanisms of blazars are also briefly discussed. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

978 KiB

Open AccessFeature PaperArticle

Suborbital Fermi/LAT Analysis of the Brightest Gamma-Ray Flare of Blazar 3C 454.3

by Krzysztof Nalewajko

Galaxies 2017, 5(4), 100; https://doi.org/10.3390/galaxies5040100 - 12 Dec 2017

Cited by 3 | Viewed by 3063

Abstract

Recent detection of suborbital gamma-ray variability of Flat Spectrum Radio Quasar (FSRQ) 3C 279 by Fermi Large Area Telescope (LAT) is in severe conflict with established models of blazar emission. This paper presents the results of suborbital analysis of the Fermi/LAT data for [...] Read more.

Recent detection of suborbital gamma-ray variability of Flat Spectrum Radio Quasar (FSRQ) 3C 279 by Fermi Large Area Telescope (LAT) is in severe conflict with established models of blazar emission. This paper presents the results of suborbital analysis of the Fermi/LAT data for the brightest gamma-ray flare of another FSRQ blazar 3C 454.3 in November 2010 (Modified Julian Date; MJD 55516-22). Gamma-ray light curves are calculated for characteristic time bin lengths as short as 3 min. The measured variations of the 0.1–10 GeV photon flux are tested against the hypothesis of steady intraorbit flux. In addition, the structure function is calculated for absolute photon flux differences and for their significances. Significant gamma-ray flux variations are measured only over time scales longer than ∼5 h, which is consistent with the standard blazar models. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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820 KiB

Open AccessFeature PaperArticle

Multi-Band Intra-Night Optical Variability of BL Lacertae

by Haritma Gaur, Alok C. Gupta, Rumen Bachev, Anton Strigachev, Evgeni Semkov, Paul J. Wiita, Minfeng Gu and Sunay Ibryamov

Galaxies 2017, 5(4), 94; https://doi.org/10.3390/galaxies5040094 - 08 Dec 2017

Cited by 5 | Viewed by 3177

Abstract

We monitored BL Lacertae frequently during 2014–2016 when it was generally in a high state. We searched for intra-day variability for 43 nights using quasi-simultaneous measurements in the B, V, R, and I bands (totaling 143 light curves); the typical sampling interval was [...] Read more.

We monitored BL Lacertae frequently during 2014–2016 when it was generally in a high state. We searched for intra-day variability for 43 nights using quasi-simultaneous measurements in the B, V, R, and I bands (totaling 143 light curves); the typical sampling interval was about eight minutes. On hour-like timescales, BL Lac exhibited significant variations during 13 nights in various optical bands. Significant spectral variations are seen during most of these nights such that the optical spectrum becomes bluer when brighter. The amplitude of variability is usually greater for longer observations but is lower when BL Lac is brighter. No evidence for periodicities or characteristic variability time-scales in the light curves was found. The color variations are mildly chromatic on long timescales. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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1723 KiB

Open AccessFeature PaperArticle

Intra-Night Variability of OJ 287 with Long-Term Multiband Optical Monitoring

by Wei Zeng, Qing-Jiang Zhao, Ze-Jun Jiang, Zhi-Hui Kong, Zhen Liu, Dong-Dong Wang, Xiong-Fei Geng, Shen-Bang Yang and Ben-Zhong Dai

Galaxies 2017, 5(4), 85; https://doi.org/10.3390/galaxies5040085 - 22 Nov 2017

Cited by 6 | Viewed by 3846

Abstract

We present long-term optical multi-band photometric monitoring of the blazar OJ 287 from 6 March 2010 to 3 April 2016, with high temporal resolution in the

V R I

-bands. The flux variations and colour-magnitude variations on long and short timescales were investigated [...] Read more.

We present long-term optical multi-band photometric monitoring of the blazar OJ 287 from 6 March 2010 to 3 April 2016, with high temporal resolution in the

V R I

-bands. The flux variations and colour-magnitude variations on long and short timescales were investigated to understand the emission mechanisms. In our observation, the major outbursts occurred in January 2016, as predicted by the binary pair of black holes model for OJ 287, with

F_{v a r}

of 1.3∼2.1%, and variability amplitude (Amp) of 5.8∼9.0%. The intra-night variability (IDV) durations were from 18.5 to 51.3 min, and the minimal variability timescale was about 4.7 min. The colour-magnitude variation showed a weak positive correlation on the long timescale with Pearson’s

r = 0.450

, while a negative correlation was found on intra-night timescales. We briefly discuss the possible physical mechanisms that are most likely to be responsible for the observed flux and colour-magnitude correlation variability. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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2841 KiB

Open AccessArticle

Coordinated Micro-Variability CIRCE Polarimetry and SARA JKT Multi-Frequency Photometry Observations of the Blazar S5 0716+71

by James R. Webb, Stephen Eikenberry, Yigit Dalliar, Nankun Meng and Douglas H. Laurence

Galaxies 2017, 5(4), 77; https://doi.org/10.3390/galaxies5040077 - 13 Nov 2017

Cited by 3 | Viewed by 3338

Abstract

A critical observation for testing the KRM jet model, in development at FIU, is to observe high time resolution, high accuracy photometry and polarimetry over a wide range of frequencies in the optically thin portion of the synchrotron spectrum. The detection of micro-variability [...] Read more.

A critical observation for testing the KRM jet model, in development at FIU, is to observe high time resolution, high accuracy photometry and polarimetry over a wide range of frequencies in the optically thin portion of the synchrotron spectrum. The detection of micro-variability during these observations would be ideal so the background and flaring components could be separated. Target of opportunity H-band photometry and polarimetry observations on the Gran Telescopio Canarias (GTC) 10.4-m with the Canarias InfraRed Camera Experiment (CIRCE) instrument were made in conjunction with the Southeastern Association for Research in Astronomy (SARA) JKT observing in the optical VRI bands in order to test the model. Here we present simultaneous micro-variability observations of Blazar S5 0716+71 made on 14 April 2017, with the CIRCE instrument on the GTC 10.4-m telescope and optical observations made with the 1.0-m SARA JKT in La Palma. The CIRCE observations consisted of high time resolution polarimetric observations in the H band over a period of 2.4 h on source, measuring both the H-band flux and the polarization degree and angle. Simultaneous observations with the SARA JKT 1.0-m yielded VRI light curves with about three minute time resolution over ∼4 h. 0716+71 showed only small amounts of variability during the observation. We present here the resulting data and a comparison to previous observations. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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