X-ray Intraday Variability of the TeV Blazar Mrk 421 with Chandra

Abstract

We present an extensive study of 72 archival Chandra light curves of the highfrequency-peaked type blazar Mrk 421, the first strong extragalactic object to be detected at TeV energies. Between 2000 and 2015 Mrk 421 often displayed intraday variability in the 0.3-10.0 keV energy range, as quantified through fractional variability amplitudes that range up to 21.3 per cent. A variability duty cycle of ∼ 84 per cent is present in these data. Variability timescales, with values ranging from 5.5 to 30.5 ks, appear to be present in seven of these observations. Discrete correlation function analyses show positive correlations between the soft (0.3-2.0 keV) and hard (2.0-10.0 keV) X-ray energy bands with zero time lags, indicating that very similar electron populations are responsible for the emission of all the X-rays observed by Chandra. The hardness ratios of this X-ray emission indicate a general "harder-when-brighter" trend in the spectral behaviour of Mrk 421. Spectral index-flux plots provide model independent indications of the spectral evolution of the source and information on the X-ray emission mechanisms. Brief discussions of theoretical models that are consistent with these observations are given.

NuSTAR View of TeV Blazar Mrk 501

“…We found that for observations on MJD 56,420 and MJD 56,486, the HR increases with increasing flux, that is, the NuSTAR spectra of Mrk 501 became harder with the increasing count rates. Such a "harder-when-brighter" trend has often been noticed in X-ray observations of the HBL-type blazars (e.g., [19,42,43], and references therein). For the other five observations, no strong correlation were seen between HR and count rates.…”

X-ray Flux and Spectral Variability of the TeV Blazars Mrk 421 and PKS 2155-304

“…Hard and soft bands were well correlated with zero time lag, and in general harder-when-brighter trend in the spectral behavior was found [31,60]. In hardness ratio (HR) versus X-ray flux plots, we noticed a clockwise as well as anti-clockwise loop at different epochs of observations which implied that particle acceleration as well as synchrotron cooling both work in the source at different epochs of observations [60]. In a systematic study of the 16 year whole operation period of RXTE, 32 TeV blazar spectra were analyzed.…”

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

“…IDV timescales ranging 2.4-30.0 ks, IDV duty cycle ∼77%, soft (0.3-2.0 keV) and hard (2.0-10.0 keV) LCs well correlated with zero lag were found. IDV timescales were also used to calculate δ the Doppler factor, B the magnetic field, γ the Lorentz factor and R the size of emitting region for the blazar Mrk 421 [43]. For Mrk 421, we found that shortest IDV timescale ≈2.…”

“…A pilot project on searching for IDV in blazars was initiated ( [28,37,42,43,54]; and the references therein). A sample of four HBLs observed on 23 occasions by XMM-Newton was studied; IDV timescales ranging from 15.7-46.8 ks were found on eight occasions; in 13 cases, IDV timescales were longer than the data length, and the hint of weak quasi periodic oscillations (QPOs) was observed on one LC each of blazars ON 231 and PKS 2155-304 [28].…”

“…IDV timescales are used to calculate δ the Doppler factor, B the magnetic field, γ the Lorentz factor and R the size of the emitting region (see Table 4, [42]. Recently, in another search for X-ray IDV, 83 LCs of the TeV blazar Mrk 421 taken during 1999-2015 with Chandra were studied [43]. IDV timescales ranging 2.4-30.0 ks, IDV duty cycle ∼77%, soft (0.3-2.0 keV) and hard (2.0-10.0 keV) LCs well correlated with zero lag were found.…”