Abstract
Studies and constraints on the emission region are crucial to the blazar radiation mechanism. Yet previous works have mainly focused on individual sources. In this work, we make use of the ...largest and the latest spectral energy distribution fitting results in the literature to statistically study the blazar emission region properties in the framework of a one-zone leptonic model. Our results reveal: (1) that flat-spectrum radio quasars (FSRQs) show lower electron energy (
γ
p
≲ 1.6 × 10
3
) than BL Lacertae objects (BL Lacs) and tend to have a stronger magnetic field (
B
) and smaller electron-to-magnetic energy ratio (
U
e
/
U
B
) than BL Lacs; (2) we find that the electromagnetic equipartition would rather happen in the jets of BL Lacs than happen in the jets of FSRQs; (3) there are 682 blazars with a magnetic field weaker than the critical value for generating the Kelvin–Helmholtz instability, thus one-third of the blazars in our sample are able to produce this instability; and (4) the distance (
d
em
) between the emission region and the central black hole is on the scale of ∼0.1 pc, so the location of the emission region may be evenly distributed inside and outside the broad-line region.
Abstract
BL Lacertae underwent a series of historical high flux activity over a year from 2020 August in the optical to VHE
γ
-rays. In this paper, we report on optical flux and spectral variability ...of the first historical maxima outburst event during October–November in the
g
,
r
, and
i
bands with the 1.26 m telescope at the Xinglong Observatory, China. We detected significant intranight variations with amplitude rising up to ∼30%, where the fastest variability timescale was found to be a few tens of minutes, giving an emitting region size of the order 10
−3
pc, which corresponds to ∼100 Schwarzschild radius of the central black hole, likely coming from some jet mini-structures. Unlike on the intranight timescale, a clear frequency-dependent pattern along symmetric timescales (∼11 days) of flux variation is detected on a long timescale. The spectral evolution was predominated by flattening of the spectra with increasing brightness i.e., a bluer-when-brighter trend in 96% of the cases. On the night before the outburst peak, the color indices shown in a color–magnitude diagram, clustered into two distinct branches, within a period of ∼6 hr, which is connected to a hard-soft-hard spectral evolution trend extracted from time-resolved spectra. To the best of our knowledge, such a trend has never been seen in BL Lac or any other blazars before. The results obtained in this study can be explained in the context of shock-induced particle acceleration or magnetic reconnection in the jet where turbulent processes most likely resulted in the asymmetric flux variation on a nightly timescale.
The “Blazar Sequence” in the TeV Band Ouyang, Zhihao; Xiao, Hubing; Chen, Jianzhen ...
The Astrophysical journal,
06/2023, Letnik:
949, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
The “blazar sequence” has been proposed for more than 20 yr, yet its nature is still unclear. In this work, for the first time we expand this topic to the TeV band by using a sample of 58 ...TeV blazars, including 48 blazars in the quiescent state and 21 blazars in the flaring state (11 blazars show in both quiescent and flaring state). We investigate the correlation between the TeV luminosity, which has been compensated for attenuation from extragalactic background light, and the synchrotron peak frequency. We note that there is no correlation between TeV luminosity and peak frequency in the quiescent state and a strong anticorrelation in the flaring state for the observed value. However, there is a strong positive correlation in both the quiescent state and the flaring state for the intrinsic value. This indicates that the blazar sequence is shown in the flaring state rather than in the quiescent state for the observed value and the blazar sequence is not present in both states after removing the beaming effect. In addition, to confirm whether the beaming effect results in the blazar sequence, we compare the Fermi
γ
-ray luminosity between the quiescent state and the flaring state. We find the Fermi
γ
-ray luminosity in the flaring state is greater than that in the quiescent state, and the Doppler factor in the flaring state is greater. We suggest the blazar sequence in the flaring state may be due to a stronger beaming effect.
ABSTRACT
Here we report on the results of the Whole Earth Blazar Telescope photo-polarimetric campaign targeting the blazar S5 0716+71, organized in 2014 March to monitor the source simultaneously in
...BVRI
and near-IR filters. The campaign resulted in an unprecedented data set spanning ∼110 hr of nearly continuous, multiband observations, including two sets of densely sampled polarimetric data mainly in the
R
filter. During the campaign, the source displayed pronounced variability with peak-to-peak variations of about 30% and “bluer-when-brighter” spectral evolution, consisting of a day-timescale modulation with superimposed hour-long microflares characterized by ∼0.1 mag flux changes. We performed an in-depth search for quasi-periodicities in the source light curve; hints for the presence of oscillations on timescales of ∼3 and ∼5 hr do not represent highly significant departures from a pure red-noise power spectrum. We observed that, at a certain configuration of the optical polarization angle (PA) relative to the PA of the innermost radio jet in the source, changes in the polarization degree (PD) led the total flux variability by about 2 hr; meanwhile, when the relative configuration of the polarization and jet angles altered, no such lag could be noted. The microflaring events, when analyzed as separate pulse emission components, were found to be characterized by a very high PD (>30%) and PAs that differed substantially from the PA of the underlying background component, or from the radio jet positional angle. We discuss the results in the general context of blazar emission and energy dissipation models.
ABSTRACT
We present the results of photometric observations of three TeV blazars, 3C 66A, S4 0954+658, and BL Lacertae (BL Lac), during the period 2013–2017. Our extensive observations were performed ...in a total of 360 nights which produced ∼6820 image frames in BVRI bands. We study flux and spectral variability of these blazars on these lengthy time-scales. We also examine the optical spectral energy distributions of these blazars, which are crucial in understanding the emission mechanism of long-term variability in blazars. All three TeV blazars exhibited strong flux variability during our observations. The colour variations are mildly chromatic on long time-scales for two of them. The nature of the long-term variability of 3C 66A and S4 0954+658 is consistent with a model of a non-thermal variable component that has a continuous injection of relativistic electrons with power-law distributions around 4.3 and 4.6, respectively. However, the long-term flux and colour variability of BL Lac suggests that these can arise from modest changes in velocities or viewing angle toward the emission region, leading to variations in the Doppler boosting of the radiation by a factor of ∼1.2 over the period of these observations.
We analysed the multiband optical behaviour of the BL Lacertae object, S5 0716+714, during its outburst state from 2014 November to 2015 March. We took data on 23 nights at three observatories, one ...in India and two in Bulgaria, making quasi-simultaneous observations in B, V, R, and I bands. We measured multiband optical fluxes, colour, and spectral variations for this blazar on intraday and short time-scales. The source was in a flaring state during the period analysed and displayed intense variability in all wavelengths. R-band magnitude of 11.6 was attained by the target on 2015 January 18, which is the brightest value ever recorded for S5 0716+714. The discrete correlation function method yielded good correlation between the bands with no measurable time lags, implying that radiation in these bands originate from the same region and by the same mechanism. We also used the structure function technique to look for characteristic time-scales in the light curves. During the times of rapid variability, no evidence for the source to display spectral changes with magnitude was found on either of the time-scales. The amplitude of variations tends to increase with increasing frequency with a maximum of ∼22 per cent seen during flaring states in B band. A mild trend of larger variability amplitude as the source brightens was also found. We found the duty cycle of our source during the analysed period to be ∼90 per cent. We also investigated the optical spectral energy distribution of S5 0716+714 using B, V, R, and I data points for 21 nights. We briefly discuss physical mechanisms most likely responsible for its flux and spectral variations.
We present the results of extensive multiband intranight optical monitoring of BL Lacertae during 2010–2012. BL Lacertae was very active in this period and showed intense variability in almost all ...wavelengths. We extensively observed it for a total for 38 nights; on 26 of them, observations were done quasi-simultaneously in B, V, R and I bands (totalling 113 light curves), with an average sampling interval of around 8 min. BL Lacertae showed significant variations on hour-like time-scales in a total of 19 nights in different optical bands. We did not find any evidence for periodicities or characteristic variability time-scales in the light curves. The intranight variability amplitude is generally greater at higher frequencies and decreases as the source flux increases. We found spectral variations in BL Lacertae in the sense that the optical spectrum becomes flatter as the flux increases but in several flaring states, deviates from the linear trend suggesting different jet components contributing to the emission at different times.
We examine the 2008–2016 γ-ray and optical light curves of three bright BL Lac objects, 0716+714, MRK 421, BL Lac, which exhibit large structured variability. We searched for periodicities by using a ...fully Bayesian approach. For two out of three sources investigated, no significant periodic variability was found. In the case of BL Lac, we detected a periodicity of ~680 days. Although the signal related to this is modest, the coincidence of the periods in both gamma and optical bands is indicative of a physical relevance. Taking into consideration previous literature results, possibly related γ-ray and optical periodicities of about one year time scale are proposed in four bright γ-ray blazars out of the ten examined in detail. Compared with results from periodicity search of optical archives of quasars, the presence of quasi-periodicities in blazars may be more frequent by a large factor. This suggests the intriguing possibility that the basic conditions for their observability are related to the relativistic jet in the observer direction, but the overall picture remains uncertain.
In an effort to locate the sites of emission at different frequencies and physical processes causing variability in blazar jets, we have obtained high time-resolution observations of BL Lacertae over ...a wide wavelength range: with the Transiting Exoplanet Survey Satellite (TESS) at 6000-10000 with 2 minute cadence; with the Neil Gehrels Swift satellite at optical, UV, and X-ray bands; with the Nuclear Spectroscopic Telescope Array at hard X-ray bands; with the Fermi Large Area Telescope at γ-ray energies; and with the Whole Earth Blazar Telescope for measurement of the optical flux density and polarization. All light curves are correlated, with similar structure on timescales from hours to days. The shortest timescale of variability at optical frequencies observed with TESS is ∼0.5 hr. The most common timescale is 13 1 hr, comparable with the minimum timescale of X-ray variability, 14.5 hr. The multiwavelength variability properties cannot be explained by a change solely in the Doppler factor of the emitting plasma. The polarization behavior implies that there are both ordered and turbulent components to the magnetic field in the jet. Correlation analysis indicates that the X-ray variations lag behind the γ-ray and optical light curves by up to ∼0.4 day. The timescales of variability, cross-frequency lags, and polarization properties can be explained by turbulent plasma that is energized by a shock in the jet and subsequently loses energy to synchrotron and inverse Compton radiation in a magnetic field of strength ∼3 G.
We report on a recent multiband optical photometric and polarimetric observational campaign of the blazar OJ 287 that was carried out during 2016 September-2017 December. We employed nine telescopes ...in Bulgaria, China, Georgia, Japan, Serbia, Spain, and the United States. We collected over 1800 photometric image frames in BVRI bands and over 100 polarimetric measurements over ∼175 nights. In 11 nights with many quasi-simultaneous multiband (V, R, I) observations, we did not detect any genuine intraday variability in flux or color. On longer timescales, multiple flaring events were seen. Large changes in color with respect to time and in a color-magnitude diagram were seen, and while only a weak systematic variability trend was noticed in color with respect to time, the color-magnitude diagram shows a bluer-when-brighter trend. Large changes in the degree of polarization and substantial swings in the polarization angle were detected. The fractional Stokes parameters of the polarization showed a systematic trend with time in the beginning of these observations, followed by chaotic changes and then an apparently systematic variation at the end. These polarization changes coincide with the detection and duration of the source at very high energies as seen by VERITAS. The spectral index shows a systematic variation with time and V-band magnitude. We briefly discuss possible physical mechanisms that could explain the observed flux, color, polarization, and spectral variability.
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