We report the results of flux and spectral variability studies of all seven {\it Nuclear Spectroscopic Telescope Array (NuSTAR)} observations of TeV $γ-$ray emitting blazar Markarian (or Mrk) 501. We found strong evidence of intraday variability in 3-79 keV X-ray light curves (LCs) of Mrk 501 during four out of these seven observations. We examined spectral variability using a model-independent hardness-ratio analysis and found a general "harder-when-brighter" behaviour in two observations. We also investigated the nature of 3-79 keV X-ray spectra of TeV blazar Mrk 501 and found that five out of seven spectra are well described by the curved log-parabola models with photon indices (at 10 keV) $α\sim$ 2.12-2.32 and a curvature $β\sim$ 0.15-0.28. The two other spectra are somewhat better represented by simple power-law models with photon indices 2.70 and 2.75. We briefly discuss available physical models to explain our results.
We present torus modelling for the X-ray spectra of a nearby narrow-line Seyfert 1 galaxy Mrk 1239 ($z=0.0199$), based on archival Suzaku, NuSTAR and Swift observations. Our model suggests very soft intrinsic power-law continuum emission of $Γ\approx2.57$ in 2019 and $Γ\approx2.98$ in 2007. By applying a correction factor to the unabsorbed X-ray luminosity, we find that Mrk 1239 is accreting near or around the Eddington limit. Our best-fit spectral model also suggests a torus with a column density of $\log(N_{\rm H, ave}/$cm$^{-2})=25.0\pm0.2$ and a high covering factor of $0.90$ in Mrk 1239, …
The well studied blazar Markarian 421 (Mrk 421, $z$=0.031) was the subject of an intensive multi-wavelength campaign when it flared in 2013 April. The recorded X-ray and very high energy (VHE, E$>$100 GeV) $γ$-ray fluxes are the highest ever measured from this object. At the peak of the activity, it was monitored by the hard X-ray focusing telescope {\it Nuclear Spectroscopic Telescope Array} ({\it NuSTAR}) and {\it Swift} X-Ray Telescope (XRT). In this work, we present a detailed variability analysis of {\it NuSTAR} and {\it Swift}-XRT observations of Mrk 421 during this flaring episode. We o…
We reviewed X-ray flux and spectral variability properties studied to date by various X-ray satellites for Mrk 421 and PKS 2155-304, which are TeV emitting blazars. Mrk 421 and PKS 2155-304 are the most X-ray luminous blazars in the northern and southern hemispheres, respectively. Blazars show flux and spectral variabilities in the complete electromagnetic spectrum on diverse timescales ranging from a few minutes to hours, days, weeks, months and even several years. The flux and spectral variability on different timescales can be used to constrain the size of the emitting region, estimate the …
We report on the detection of excess hard X-ray emission from the TeV BL Lac object Mrk421 during the historical low-flux state of the source in January 2013. NuSTAR observations were conducted four times between MJD56294 and MJD56312 with a total exposure of 80.9 ksec. The source flux in the 3-40 keV range was nearly constant except for MJD56307, when the average flux level increased by a factor of three. Throughout the exposure, the X-ray spectra of Mrk421 were well represented by a steep power-law model with a photon index of 3.1, although a significant excess was noted above 20 keV in the …
The jet properties of Mrk 501 and 3C 279 are derived by fitting the broadband spectral energy distributions with the lepton models. The derived gamma_b (the break Lorenz factor of the electron distribution) are 10^4-10^6 for Mrk 501 and 200~600 for 3C 279 and the magnetic field strength (B) of Mrk 501 is usually one order of magnitude lower than that of 3C 279, but their Doppler factors (delta) are comparable. A spectral variation feature that the peak luminosity is correlated with the peak frequency, which is opposite to the blazar sequence, is observed in the two sources. We find that (1) th…
The blazar Mrk 501 is among the brightest X-ray and TeV sources in the sky, and among the few sources whose spectral energy distributions can be characterized by current instruments with relatively short observations (minutes to hours). In 2013, we organized an extensive multi-instrument campaign including Fermi-LAT, MAGIC, VERITAS, F-GAMMA, Swift, GASP-WEBT, and other groups and instruments which provided the most detailed temporal and energy coverage on Mrk 501 to date. This campaign included, for the first time, observations with the Nuclear Stereoscopic Telescope Array (NuSTAR), a satellit…
We observed with the NuSTAR satellite 3 blazars at z>2, detected in gamma-rays by Fermi/LAT and in the soft X-rays, but not yet observed above 10 keV. The flux and slope of their X-ray continuum, together with Fermi/LAT data allows us to estimate their total electromagnetic output and peak frequency. For some of them we can study the source in different states, and investigate the main cause of the observed different spectral energy distribution. We then collected all blazars at redshift greater than 2 observed by NuSTAR, and confirm that these hard and luminous X-ray blazars are among the mos…
We investigate the position of the radio core in a blazar by multi-epoch astrometric observations at 43 GHz. Using the VLBI Exploration of Radio Astrometry (VERA), we have conducted four adjacent observations in February 2011 and another four in October 2011, and succeeded in measuring the position of the radio core in the TeV blazar Mrk 501 relative to a distant compact quasar NRAO 512. During our observations, we find that (1) there is no positional change within ~0.2 mas or ~2.0 pc de-projected with $\pm1σ$ error for the weighted-mean phase-referenced positions of Mrk 501 core relative to N…
We present long-term observations of several VHE (E > 100 GeV) blazars with VERITAS, together with contemporaneous Swift and RXTE X-ray data. The observed targets include Mrk 421, Mrk 501, 1ES 2344+514. Strong flux and spectral variability is seen in Mrk 421 on nightly time-scales between January and June 2008, revealing a highly correlated X-ray to GeV/TeV connection. Modest X-ray variability is evident in Mrk 501. Observations of 1ES 2344+514 in December 2007 show VHE gamma-ray and X-ray flux doubling on nightly time-scales.
The radiation mechanism of very high energy $γ$-ray emission from blazars and crucial parameters like magnetic field, and size of the emitting region are not well understood yet. To understand the above mentioned properties of blazars, we observed five nearby TeV $γ$-ray emitting blazars (Mrk421, Mrk501, 1ES2344+514, 1ES1218+304 and 3C454.3) and one radio galaxy (M87) using the High Altitude GAmma Ray (HAGAR) telescope. HAGAR is an array of seven telescopes located at Hanle, India to detect Cherenkov light caused by extensive air showers initiated by $γ$-rays. Mrk421 was observed to undergo on…
We present the results of X-ray spectral analysis of the Seyfert type 2 galaxy Mrk 417 observed by the NuSTAR (3-60 keV) and the Swift/BAT (14-150) observatories. The NuSTAR data shows a good fit with a baseline model which composed of an absorbed cutoff power-law component and reflected emission from the cold neutral medium. We found that the spectrum is steep $Γ= 1.63_{-0.11}^{+0.10}$ and obscured by hydrogen column density of $N_{H} = 3.22_{-0.39}^{+0.41} \cdot 10^{23} cm^{-2}$, narrow $FeK_α$ emission line has an equivalent width of $EW = 115_{-1}^{+2}$ eV that suggests this feature is ori…
We have examined 40 NuSTAR light curves (LCs) of five TeV emitting high synchrotron peaked blazars: 1ES 0229+200, Mrk 421, Mrk 501, 1ES 1959+650 and PKS 2155-304. Four of the blazars showed intraday variability in the NuSTAR energy range of 3-79 keV. Using an auto correlation function analysis we searched for intraday variability timescales in these LCs and found indications of several between 2.5 and 32.8 ks in eight LCs of Mrk 421, a timescale around 8.0 ks for one LC of Mrk 501, and timescales of 29.6 ks and 57.4 ks in two LCs of PKS 2155-304. The other two blazars' LCs do not show any evid…
Hard X-ray observations are crucial to study the non-thermal jet emission from high-redshift, powerful blazars. We observed two bright z>2 flat spectrum radio quasars (FSRQs) in hard X-rays to explore the details of their relativistic jets and their possible variability. S5 0014+81 (at z=3.366) and B0222+185 (at z=2.690) have been observed twice by the Nuclear Spectroscopic Telescope Array (NuSTAR) simultaneously with Swift/XRT, showing different variability behaviours. We found that NuSTAR is instrumental to explore the variability of powerful high-redshift blazars, even when no gamma-ray emi…
We present a detailed spectral study of the narrow-line Seyfert~1 galaxy, Markarian~335, using eight epoch observations made between 2013 and 2020 with the Nuclear Spectroscopic Telescope Array. The source was variable during this period both in spectral flux and flow geometry. We estimated the height of the Compton cloud from the model fitted parameters for the whole observation period. This allowed us to investigate the underlying physical processes that drive the variability in X-rays. Our model fitted mass varies in a narrow range, between $(2.44\pm0.45-3.04\pm0.56)\times10^{7}M_\odot$, ho…
Flux distribution is an important tool to understand the variability processes in active galactic nuclei. We now have available a great deal of observational evidences pointing towards the presence of log-normal components in the high energy light curves, and different models have been proposed to explain these data. Here, we collect some of the recent developments on this topic using the well-known blazar Mrk 501 as example of complex and interesting aspects coming from its flux distribution in different energy ranges and at different timescales. The observational data we refer to are those c…
Here we report our recent study on the spectral energy distribution (SED) of the high frequency BL Lac object Mrk 421 in different luminosity states. We used a full-fledged chi2-minimization procedure instead of more commonly used "eyeball" fit to model the observed flux of the source (from optical to very high energy), with a Synchrotron-Self-Compton (SSC) emission mechanism. Our study shows that the synchrotron power and peak frequency remain constant with varying source activity, and the magnetic field decreases with the source activity while the break energy of electron spectrum and the Do…
Monitoring of the narrow line Seyfert 1 galaxy Markarian 335 (Mrk 335) with the Swift satellite discovered an X-ray flare beginning 2014 August 29. At the peak, the 0.5-5keV count rate had increased from that in the low flux state by a factor of 10. A target of opportunity observation was triggered with NuSTAR, catching the decline of the flare on 2014 September 20. We present a joint analysis of Swift and NuSTAR observations to understand the cause of this flare. The X-ray spectrum shows an increase in directly observed continuum flux and the softening of the continuum spectrum to a photon in…
Various attempts have been made in the literature at describing the origin and the physical mechanisms behind flaring events in blazars with radiative emission models, but detailed properties of multi-wavelength (MWL) light curves still remain difficult to reproduce. We have developed a versatile radiative code, based on a time-dependent treatment of particle acceleration, escape and radiative cooling, allowing us to test different scenarios to connect the continuous low-state emission self-consistently with that during flaring states. We consider flares as weak perturbations of the quiescent …
Context: The nearby (z=0.031) TeV blazar Mrk421 was reported to be in a high state of flux activity since November, 2009. Aims: To investigate possible changes in the physical parameters of Mrk421 during its high state of activity using multiwavelength data. Methods: We have observed this source in bright state using High Altitude GAmma Ray (HAGAR) telescope array at energies above 250 GeV during February 13 - 19, 2010. Optical, X-ray and gamma-ray archival data are also used to obtain the SEDs and light curves. Results: Mrk421 was found to undergo one of its brightest flaring episodes on Febr…
Some extensions to the Standard Model lead to the introduction of Lorentz symmetry breaking terms, expected to induce deviations from Lorentz symmetry around the Planck scale. A parameterization of effects due to Lorentz invariance violation (LIV) can be introduced by adding an effective term to the photon dispersion relation. This affects the kinematics of electron-positron pair creation by TeV $γ$ rays on the extragalactic background light (EBL) and translates into modifications of the standard EBL opacity for the TeV photon spectra of extragalactic sources. Exclusion limits are presented, o…
There is an increasing number of observational evidence that very high energy gamma-rays in radio-loud active galactic nuclei are produced in the direct vicinity of a supermassive black hole (SMBH), close to the base of a relativistic jet. In the case of some blazars, the angle between the jet axis and the observer's line of sight is smaller than the angular extent of the jet. Gamma-rays that are produced close to SMBH therefore have to propagate in the nonthermal radiation of the extended jet before reaching the observer. This gamma-ray emission can be strongly absorbed in the extended jet ra…
The X-ray emission in BL Lac objects is believed to be dominated by synchrotron emission from their relativistic jets. However, when the jet emission is not strong, one could expect signatures of X-ray emission from inverse Compton scattering of accretion disc photons by the corona. Moreover, the observed X-ray variability can also originate in the disc, and gets propagated and amplified by the jet. Here, we present results on the BL Lac object Mrk 421 using the NuSTAR data acquired during 2017 when the source was in a moderate X-ray brightness state. For comparison with high jet activity, we …
We report on the X-ray monitoring programme (covering slightly more than 11 days) carried out jointly by XMM-Newton and NuSTAR on the intermediate Seyfert galaxy Mrk 915. The light curves extracted in different energy ranges show a variation in intensity but not a significant change in spectral shape. The X-ray spectra reveal the presence of a two-phase warm absorber: a fully covering mildly ionized structure [log xi/(erg cm/s)~2.3, NH~1.3x10^21 cm-2] and a partial covering (~90 per cent) lower ionized one [log xi/(erg cm/s)~0.6, NH~2x10^22 cm-2]. A reflection component from distant matter is …
Quasi-periodic oscillations (QPO) originating from innermost regions of blazars can provide unique perspective of some of the burning issues in blazar studies including disk-jet connection, launch of relativistic jets from the central engine, and other extreme conditions near the fast rotating supermassive black holes. However, a number of hurdles associated with searching QPOs in blazars e.g., red-noise dominance, modest significance of the detection and periodic modulation lasting for only a couple of cycles, make it difficult to estimate the true significance of the detection. In this work,…
We present an extensive study of 72 archival Chandra light curves of the high-frequency-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 s…
The most powerful sources among the blazar family are MeV blazars. Often detected at $z>2$, they usually display high X- and \gm-ray luminosities, larger-than-average jet powers and black hole masses $\gtrsim 10^9 M_{\odot}$. In the present work we perform a multiwavelength study of three high redshift blazars: 3FGL J0325.5+2223 ($z=2.06$), 3FGL J0449.0+1121 ($z= 2.15$), and 3FGL J0453.2$-$2808 ($z=2.56$), analysing quasi simultaneous data from GROND, \swift-UVOT and XRT, \nustar, and \fermi-LAT. Our main focus is on the hard X-ray band recently unveiled by \nustar~(3$-$79 keV) where these obj…
Probing the high energy emission processes of blazars through their variability relies crucially on long-term monitoring. We present unprecedented light curves from unbiased observations of very high energy fluxes from the blazars Mrk 421 and Mrk 501 based on a joint analysis of data from the First G-APD Cherenkov Telescope (FACT) and the High Altitude Water Cherenkov (HAWC) Observatory. Thanks to an offset of 5.3 hours of the geographic locations, a complementary coverage of up to 12 hours of observation per day allows us to track variability on time scales of hours to days in more detail tha…
We present 3-50 keV NuSTAR observations of the AGN Mrk 335 in a very low flux state. The spectrum is dominated by very strong features at the energies of the iron line at 5-7 keV and Compton hump from 10-30 keV. The source is variable during the observation, with the variability concentrated at low energies, which suggesting either a relativistic reflection or a variable absorption scenario. In this work we focus on the reflection interpretation, making use of new relativistic reflection models that self consistently calculate the reflection fraction, relativistic blurring and angle-dependent …
Extremely High energy peaked BL Lac (EHBL) objects are a special class of blazars with peculiar observational properties at X-ray and $γ$--ray energies. The observations of these sources indicate hard X-ray and $γ$--ray spectra and absence of rapid flux variations in the multi-wavelength light curves. These observational features challenge the leptonic models for blazars due to unusually hard particle spectrum in the emission region of the blazar jet and provide a strong motivation for exploring alternative scenarios to interpret the broad-band emission from blazars. At present, only few TeV b…
We report the first results of timing and spectral studies of \textit{Nuclear Spectroscopic Telescope Array (NuSTAR)} observations of six TeV emitting high-frequency peaked blazars: 1ES 0347$-$121, 1ES 0414$+$009, RGB J0710$+$591, 1ES 1101$-$232, 1ES 1218$+$304 and H 2356-309. Two out of these six TeV blazars, 1ES 1101$-$232 and 1ES 1218$+$304, showed strong evidence of intraday variations in the 3--79 keV energy range during those observations. We also found a hint of an intraday variability timescale of 23.5 ks in the light curve of 1ES 1218$+$304 using an autocorrelation function analysis. …
The archetypical very-high-energy gamma-ray blazar Mrk 421 was monitored for more than 3 years with the Gas Slit Camera onboard Monitor of All Sky X-ray Image (MAXI), and its longterm X-ray variability was investigated. The MAXI lightcurve in the 3 -- 10 keV range was transformed to the periodogram in the frequency range $f = 1 \times 10^{-8}$ -- $2 \times 10^{-6}$ Hz. The artifacts on the periodogram, resulting from data gaps in the observed lightcurve, were extensively simulated for variations with a power-law like Power Spectrum Density (PSD). By comparing the observed and simulated periodo…
We present two new NuSTAR observations of the narrow line Seyfert 1 (NLS1) galaxy Mrk 766 and give constraints on the two scenarios previously proposed to explain its spectrum and that of other NLS1s: relativistic reflection and partial covering. The NuSTAR spectra show a strong hard (>15 keV) X-ray excess, while simultaneous soft X-ray coverage of one of the observations provided by XMM-Newton constrains the ionised absorption in the source. The pure reflection model requires a black hole of high spin ($a>0.92$) viewed at a moderate inclination ($i=46^{+1}_{-4}$ degrees). The pure partial cov…
The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory surveys the very high energy sky in the 300 GeV to $>100$ TeV energy range. HAWC has detected two blazars above $11σ$, Markarian 421 (Mrk 421) and Markarian 501 (Mrk 501). The observations are comprised of data taken in the period between June 2015 and July 2018, resulting in a $\sim 1038$ days of exposure. In this work we report the time-averaged spectral analysis for both sources above 0.5 TeV. Taking into account the flux attenuation due to the extragalactic background light (EBL), the intrinsic spectrum of Mrk 421 is described …
With bolometric luminosities exceeding $10^{48}$ erg s$^{-1}$, powerful jets and supermassive black holes at their center, MeV blazars are some of the most extreme sources in the Universe. Recently, the Fermi-Large Area Telescope detected five new $γ$-ray emitting MeV blazars beyond redshift $z=3.1$. With the goal of precisely characterizing the jet properties of these extreme sources, we started a multiwavelength campaign to follow them up with joint NuSTAR, Swift and SARA observations. We observe six high-redshift quasars, four of them belonging to the new $γ$-ray emitting MeV blazars. Thoro…
Context. Investigation of the hard X-ray emission properties of blazars is key to the understanding of the central engine of the sources and associated jet process. In particular, simultaneous spectral and timing analyses of the intra-day hard X-ray observations provide us a means to peer into the compact innermost blazar regions, not accessible to our current instruments. Aims. The primary objective of the work is to associate the observed hard X-ray variability properties in the blazars to their flux and spectral states, thereby, based on the correlation among them, extract the details about…
XMM-Newton and NuSTAR multiple exposures allow us to disentangle the different emission components of active galactic nuclei (AGNs) and to study the evolution of their different spectral features. In this work, we present the timing and spectral properties of five simultaneous XMM-Newton and NuSTAR observations of the Narrow Line Seyfert 1 galaxy Mrk 359. We aim to provide the first broadband spectral modeling of Mrk 359 describing its emission spectrum from the UV up to the hard X-rays. To do this, we performed temporal and spectral data analysis, characterising the amplitude and spectral cha…
Radio monitoring of the broad absorption line quasar (BALQSO) Mrk 231 from 13.9 GHz to 17.6 GHz detected a strong flat spectrum flare. Even though BALQSOs are typically weak radio sources, the 17.6 GHz flux density doubled in ~150 days, from ~135 mJy to ~270 mJy. It is demonstrated that the elapsed rise time in the quasar rest frame and the relative magnitude of the flare is typical of some of the stronger flares in blazars that are associated with the ejection of discrete components on parsec scales. The decay of a similar flare was found in a previous monitoring campaign at 22 GHz. We conclu…
We report on long-term multi-wavelength monitoring of blazar Mrk~421 from 2010 to 2011. The source exhibited extreme X-ray flares in 2010. Our research group performed optical photopolarimetric follow-up observations using the Kanata telescope. In 2010, the variability in the X-ray band was significant, while the optical and ultraviolet (UV) flux decreased gradually. Polarization properties also exhibited unique variability in 2010, suggesting the presence of systematic component of polarization and magnetic field alignment for the emergence of a new polarized emission region. In contrast, in …
We present the results of the power spectral density (PSD) analysis for the blazars Mrk\,421 and PKS\,2155$-$304, using good-quality, densely sampled light curves at multiple frequencies, covering 17 decades of the electromagnetic spectrum, and variability timescales from weeks up to a decade. The data were collected from publicly available archives of observatories at radio from OVRO, optical and infrared (B, V, R, I, J, H, and K-bands), X-rays from the {\it Swift} and the {\it Rossi} X-ray Timing Explorer, high and very high energy $γ-$rays from the {\it Fermi} and Very Energetic Radiation I…
We report the results of a multifrequency campaign targeting S5 0716+714 in the flaring state of the source observed in 2015 January and February. The observations have been performed using the following instruments: Fermi/Large Area Telescope (LAT), Nuclear Spectroscopic Telescope Array, X-ray Telescope and Ultraciolet/Optical Telescope. The elevated flux level was visible in all frequencies and the outburst consists of five sub-flares. In this paper we focus on the analysis of the X-ray observations both in the soft and hard regimes for data collected with NuStar and Swift/XRT. This is the f…
The X-ray emission from blazars has been widely investigated using several space telescopes. In this work, we explored statistical properties of the X-ray variability in the blazars S5 0716+714, OJ 287, Mkn 501 and RBS 2070 using the archival observations from the XMM-Newton telescope between the period 2002-2020. Several methods of timing and spectral analyses including fractional variability, minimum variability timescale, power spectral density analyses and countrate distribution were performed. In addition, we fitted various spectral models to the observations as well as estimated hardness…
Self-consistent modeling of energy-dependent synchrotron polarization in blazars with application to Mrk 421
Centre for Space Research, North-West University（北威特大学空间研究中心）；Instituto de Física, Universidade Federal de Goiás（戈亚斯联邦大学物理研究所）；Université Paris Cité, CNRS, Astroparticule et Cosmologie（巴黎西岱大学，法国国家科学研究中心，亚原子粒子与宇宙学）；Ruhr Astroparticle and Plasma Physics Center (RAPP Center)（鲁尔天体粒子与等离子体物理中心（RAPP中心））；Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB)（波鸿鲁尔大学，物理与天文学院，天文研究所（AIRUB））
Blazars are a peculiar class of active galactic nuclei (AGNs) that enlighten the sky at all wavelengths. The electromagnetic emission of these sources is jet-dominated resulting in a spectral energy distribution (SED) that has a typical double-humped shape. X-ray photons provide a wealth of information on the physics of each source as in the X-ray band we can observe the tail of SED first peak, the rise of the second one or the transition between the two. NuSTAR, thanks to its capability of focusing X-rays up to 79 keV provides broadband data particularly suitable to compute SEDs in a still po…
In this work we interpret and discuss the time variable rotation measure (RM) found, for the first time over a 1-yr period, in the core region of a blazar. These results are based on a one-year, multi-frequency (15, 24, and 43 GHz) Very Long Baseline Array (VLBA) monitoring of the TeV blazar Markarian 421 (Mrk 421). We investigate the Faraday screen properties and its location with respect to the jet emitting region. Given that the 43 GHz radio core flux density and the RM time evolution suggest a similar trend, we explore the possible connection between the RM and the accretion rate. Among th…
(Abridged) Soft and hard X-ray excesses, compared to the continuum power-law shape between ~2-10 keV, are common features observed in the spectra of active galactic nuclei (AGN) and are associated with the accretion disc-corona system around the supermassive black hole. However, the dominant process at work is still highly debated and has been proposed to be either relativistic reflection or Comptonisation. We aim to characterise the main X-ray spectral physical components from the bright bare Broad Line Seyfert 1 AGN Mrk 110, and the physical process(es) at work in its disc-corona system view…
Because of the short duty-cycles and observation-time constraints, studies of bright TeV (E>100 GeV) blazars are mostly restricted to flaring episodes or rather short (days to few weeks) multiwavelength campaigns. At the same time, long-term studies of these objects are essential to gain a more complete understanding of the blazar phenomenon and to constrain theoretical models concerning jet physics. Only unbiased long-term studies are adequate for the determination of flaring state probabilities and for estimating the statistical significance of possible correlations between TeV flaring state…
Blazars, a class of Active Galactic Nuclei (AGN) characterized by a close orientation of their relativistic outflows (jets) towards the line of sight, are a well established extragalactic TeV $γ$-ray emitters. Since 2006, three nearby and TeV bright blazars, Markarian (Mrk) 421, Mrk 501 and 1ES 1959+650, are regularly observed by the MAGIC telescope with single exposures of 30 to 60 minutes. The sensitivity of MAGIC allows to establish a flux level of 30% of the Crab flux for each such observation. In a case of Mrk 421 strong flux variability in different time scales and a high correlation bet…
The results of 8-year R-band photopolarimetric data of blazar Mrk 421 collected from February 2008 to May 2016 are presented, along with extensive multiwavelength observations covering from radio to TeV $γ$-rays around the flares observed in May 2008, March 2010, and April 2013. The most important results are found in 2013 when the source displayed in the R-band a very high brightness state of $11.29\pm 0.03$ mag ($93.60\pm 1.53$ mJy) on April 10th and a polarization degree of ($11.00\pm0.44$) on May 13th. The analysis of the optical data shows that the polarization variability is due to the s…
We derive lower bounds on intergalactic magnetic fields (IGMFs) from upper limits on the pair echo emission from the blazar Mrk 501, that is, delayed GeV emission from secondary $e^{-}e^{+}$ pairs produced via interactions of primary TeV gamma rays with the cosmic infrared background. Utilizing only simultaneous GeV-TeV light curves observed by VERITAS, MAGIC and {\it Fermi}-LAT during a multiwavelength campaign in 2009 that included a TeV flare, bounds are deduced on the IGMF strength of $B \gtrsim 10^{-20} {\rm G}$ at 90% confidence level for a field coherence length of 1 kpc. Since our anal…
We present a broadband (0.4-70 keV) X-ray spectral analysis of the luminous infrared galaxy (LIRG) system Mrk 463 observed with Nuclear Spectroscopic Telescope Array (NuSTAR), Chandra, and XMM-Newton, which contains double active galactic nuclei (AGNs) (Mrk 463E and Mrk 463W) with a separation of $\sim$ 3.8 kpc. Detecting their transmitted hard X-ray $>10$ keV continua with NuSTAR, we confirm that Mrk 463E and Mrk 463W have AGNs with intrinsic luminosities of (1.6-2.2) $\times 10^{43}$ and (0.5-0.6) $\times 10^{43}$ erg s$^{-1}$ (2-10 keV) obscured by hydrogen column densities of 8 $\times 10^…
The X-ray spectra of some active galactic nuclei (AGN) show a soft X-ray excess, emission in excess to the extrapolated primary X-ray continuum below 2 keV. Recent studies have shown that this soft excess can be described well as originating from either a relativistic ionized reflection, the extreme blurring of the reprocessed emission from the innermost region of the accretion disk, or Comptonization from an optically thick and warm region called the 'warm corona', in which electron scattering is the dominant source of opacity. To constrain the origin of the soft excess in the Seyfert 1 galax…
MeV blazars are a sub--population of the blazar family, exhibiting larger--than--average jet powers, accretion luminosities and black hole masses. Because of their extremely hard X--ray continua, these objects are best studied in the X-ray domain. Here, we report on the discovery by the $Fermi$ Large Area Telescope and subsequent follow-up observations with $NuSTAR$, $Swift$ and GROND of a new member of the MeV blazar family: PMN J0641$-$0320. Our optical spectroscopy provides confirmation that this is a flat--spectrum radio quasar located at a redshift of $z=1.196$. Its very hard $NuSTAR$ spe…
We present Nustar 3-40 keV observations of the optically selected Type 2 quasar (QSO2) SDSS J1034+6001 or Mrk 34. The high-quality hard X-ray spectrum and archival XMM-Newton data can be fitted self-consistently with a reflection-dominated continuum and strong Fe Kalpha fluorescence line with equivalent-width >1 keV. Prior X-ray spectral fitting below 10 keV showed the source to be consistent with being obscured by Compton-thin column densities of gas along the line-of-sight, despite evidence for much higher columns from multiwavelength data. NuSTAR now enables a direct measurement of this col…
Since August 2009, MAXI experiment on the ISS has been performing all-sky X-ray monitoring. With MAXI, we detected flaring activities of some blazers, including Mrk 421, Mrk 501, and 3C 273. Recently, new X-ray flaring activities were detected from two blazers, MAXI J1930+093 = 2FGL J1931.1+0938 (Atel#5943) and 2MAXI J0243-582 = BZB J0244-5819 (Atel#6012). The MAXI monitoring also covers black hole binaries, including Cyg X-1 and Cyg X-3 which emit GeV gamma-rays. Their gamma-ray emission was found to coincide with their X-ray state transitions. We present light curves and outstanding events o…
