Unprecedented X-ray Flaring Activity of Mrk 421 in 2013 April

Mrk 421 showed an unprecedented X-ray flaring behaviour in 2013 April 10–17 when the con- secutive outbursts by a factor of 8–16 were observed in the 0.3–10 keV and 3–79 keV bands, and it became one of the brightest objects in the X-ray sky. During these events, the source was also very active on intraday timescales with fractional variability amplitude of 2–47 per cent, and showed the flux doubling and halving time scales of 1.16–7.20 hr and 1.04–3.54 hr, respectively. We have revealed up to 20 cases when the flux varied with 3σ significance within 1 ks. In this period, Mrk421 also underwent the most extreme X-ray spectral variability ever reported for BL Lacertae objects. The location of the synchrotron SED peak moved from about 0.1 keV to almost 20 keV. The photon index at 1 keV and curvature parameter showed the ranges a=1.68–2.83 and b=0.09–0.57, respectively, and varied along with the flux on diverse time scales down to the in- tervals shorter than 1 ks. While the X-ray outburst were accompanied by very strong TeV-flares, the source was relatively less active in the radio-UV and GeV energy ranges

Mrk 421 after the Giant X-Ray Outburst in 2013

We present the results of the Swift observations of the nearby BL Lac object Mrk 421 during 2013 November-2015 June. The source exhibited a strong long-term variability in the 0.3-10 keV band, with a maximum-to-minimum flux ratio of 13, and underwent X-ray flares by a factor of 1.8-5.2 on timescales of a few weeks or shorter. The source showed 48 instances of intraday flux variability in this period, which sometimes was observed within the 1 ks observational run. It was characterized by fractional amplitudes of 1.5(0.3)%-38.6(0.4)% and flux doubling/halving times of 2.6-20.1 hr. The X-ray flux showed a lack of correlation with the TeV flux on some occasions (strong TeV flares were not accompanied by comparable X-ray activity and vice versa), indicating that the high-energy emission in Mrk 421 was generated from an emission region more complex than a single zone. The best fits of the 0.3-10 keV spectra were mainly obtained using the log-parabola model, showing a strong spectral variability that generally followed a “harder-when-brighter” trend. The position of the synchrotron spectral energy distribution peak showed an extreme range from a few eV to ∼10 keV that happens rarely in blazars

Strong X-ray flaring activity of the BL Lacertae source OJ 287 in 2016 October-2017 April

We present the results of a detailed X-ray timing and spectral analysis of the BL Lacertae source OJ 287 with X-ray telescope (XRT) onboard Neil Gehrels Swift Observatory, focused on the period of its significantly enhanced X-ray flaring activity during 2016 October-2017 April. In this epoch, the 0.3-10 keV count rate from the XRT observations showed an increase by a factor of ∼10 compared to the quiescent level observed in 2016 April-May, and the mean X-ray flux was a factor of 4.5 higher than in previous years. The source underwent high X-ray flaring activity on weekly time-scales and showed 32 instances of 0.3-10 keV intraday variability (detected within the exposures shorter than 1 ks in the majority of cases) with fractional variability amplitudes of 7-60 per cent. Most of the 0.3-10 keV spectra spectra fitted well with a simple power law, yielding a wide range of the 0.3-10 keV photon index Γ = 1.90-2.90. We found 29 spectra showing an upward curvature due to the significant contribution made by the X-ray photons of inverse Compton origin. The spectral variability of OJ 287 was characterized by the dominance of a `softer-when-brighter' spectral trend, explained by the emergence of a new soft component during X-ray flares. Similar to X-rays, the source underwent a strong outburst by factors of 4.6-6.5 in the optical-ultraviolet energy range which showed a positive correlation with the X-ray emission, indicating its origin to be related to the same electron population, predominantly via the synchrotron mechanism

The second strong X-ray flare and multifrequency variability of 1ES 1959+650 in 2016 January-August

The X-ray variability of the BL Lacertae source 1ES 1959+650 was studied intensively with X-ray telescope (XRT) onboard Swift during 2016 January-August. In this paper, we present the results obtained during this campaign. A long-term high X-ray state was superimposed by shorter-term flares by a factor of 1.9-4.7. We found 35 instances of intra-day variability which showed very fast flux changes by 14-21 per cent occurring within 1 ks and a decline by a factor of 2.3 in 17.2 ks. Similarly to the previous years, this period sometimes was characterized by a lack of correlated X-ray and TeV variability, indicating that the high-energy emission in 1ES 1959+650 was generated in the emission region more complex than a single zone. The source showed a significant X-ray - high-energy flux correlation, while the former was not correlated with the optical-UV fluxes. The best fits of the 0.3-10 keV spectra were mainly obtained using the log-parabola model. Strong spectral variability was detected, shifting the peak of the spectral energy distribution by more than 10 keV that happens rarely in blazars. During some strong short-term flares, the photon index at 1 keV frequently became harder than 1.70, and the spectral evolution was characterized by a harder-when-brighter behaviour

Swift Observations of Mrk 421 in Selected Epochs. I. The Spectral and Flux Variability in 2005-2008

We present detailed results of Swift observations of the nearby TeV-detected blazar Mrk 421, based on the rich archival data obtained during 2005 March-2008 June. The best fits of the 0.3-10 keV spectra were mainly obtained using the log-parabolic model, yielding low spectral curvatures expected in the case of the efficient stochastic acceleration of particles. During strong X-ray flares, the position of the synchrotron spectral energy distribution peak {E}{{p}} was beyond 8 keV for 41 spectra, while it sometimes was situated at the UV frequencies in quiescent states. The photon index at 1 keV exhibited a broad range, and the values a< 1.70 were observed during the strong flares, hinting at the possible presence of a jet hadronic component. The spectral parameters were correlated in some periods, expected in the framework of the first- and second-order Fermi accelerations of X-ray emitting particles, as well as in the case of turbulence spectrum. The 0.3-10 keV flux and spectral parameters sometimes showed very fast variability down to the fluctuations by 6-20% in 180-960 s, possibly related to the small-scale turbulent areas containing strongest magnetic fields. X-ray and very high-energy fluxes often showed correlated variability, although several occurrences of more complicated variability patterns are also revealed, indicating that the multifrequency emission of Mrk 421 could not be generated in a single zone

Swift Observations of Mrk 421 in Selected Epochs. II. An Extreme Spectral Flux Variability in 2009–2012

We present the results from a detailed spectral and timing study of Mrk 421 based on the rich archival Swift data obtained during 2009-2012. Best fits of the 0.3-10 keV spectra were mostly obtained using the log-parabolic model showing the relatively low spectral curvature that is expected in the case of efficient stochastic acceleration of particles. The position of the synchrotron spectral energy density peak E p of 173 spectra is found at energies higher than 2 keV. The photon index at 1 keV exhibited a very broad range of values a = 1.51-3.02, and very hard spectra with a < 1.7 were observed during the strong X-ray flares, hinting at a possible hadronic jet component. The spectral parameters varied on diverse timescales and showed a correlation in some periods, which is expected in the case of first- and second-order Fermi acceleration. The 0.3-10 keV flux showed strong X-ray flaring activity by a factor of 3-17 on timescales of a few days-weeks between the lowest historical state and that corresponding to a rate higher than 100 cts s-1. Moreover, 113 instances of intraday variability were revealed, exhibiting shortest flux-doubling/halving times of about 1.2 hr, as well as brightenings by 7%-24% in 180-720 s and declines by 68%-22% in 180-900 s. The X-ray and very high-energy fluxes generally showed a correlated variability, although one incidence of a more complicated variability was also detected, indicating that the multifrequency emission of Mrk 421 could not be generated in a single zone

Swift Observations of Mrk 421 in Selected Epochs. I. The Spectral and Flux Variability in 2005–2008

We present detailed results of Swift observations of the nearby TeV-detected blazar Mrk 421, based on the rich archival data obtained during 2005 March-2008 June. The best fits of the 0.3-10 keV spectra were mainly obtained using the log-parabolic model, yielding low spectral curvatures expected in the case of the efficient stochastic acceleration of particles. During strong X-ray flares, the position of the synchrotron spectral energy distribution peak {E}{{p}} was beyond 8 keV for 41 spectra, while it sometimes was situated at the UV frequencies in quiescent states. The photon index at 1 keV exhibited a broad range, and the values a< 1.70 were observed during the strong flares, hinting at the possible presence of a jet hadronic component. The spectral parameters were correlated in some periods, expected in the framework of the first- and second-order Fermi accelerations of X-ray emitting particles, as well as in the case of turbulence spectrum. The 0.3-10 keV flux and spectral parameters sometimes showed very fast variability down to the fluctuations by 6-20% in 180-960 s, possibly related to the small-scale turbulent areas containing strongest magnetic fields. X-ray and very high-energy fluxes often showed correlated variability, although several occurrences of more complicated variability patterns are also revealed, indicating that the multifrequency emission of Mrk 421 could not be generated in a single zone

X-Ray Flaring Activity of MRK 421 in the First Half of 2013

We present the results of the Swift and NuSTAR observations of the nearby BL Lac object Mrk 421 during 2013 January-June. The source exhibited a strong long-term variability in the 0.3-10 keV and 3-79 keV bands with the maximum-to-minimum daily-binned flux ratios of 22 and 95, respectively, in about 3 months, mainly due to unprecedented strong X-ray outbursts by more than an order of magnitude in both bands within 2 weeks in 2013 April when the 0.3-10 keV count rate exceeded the level of 200 cts s-1 for the first time, and Mrk 421 became one of the brightest sources in the X-ray sky. The source was also very active on intra-day timescales, and it showed flux doubling and halving timescales of 1.16-7.20 hr and 1.04-3.54 hr, respectively. On some occasions, the flux varied by 4%-23% within 300-840 s. During this period, the source also exhibited some of the most extreme X-ray spectral variability ever reported for BL Lacs—the location of the synchrotron spectral energy distribution peak shifted from a few eV to ∼10 keV, and the photon index at 1 keV and curvature parameter varied on timescales from a few weeks down to intervals shorter than 1 ks. MAGIC and First G-APD Cherenkov Telescope observations also revealed a very strong very high energy (VHE) flare during April 11-17. The UV and HE γ-ray flares were much weaker compared to their X-ray counterparts, and they generally showed significantly stronger correlation with each other than with the X-ray fluxes

Strong X-Ray and Multiwavelength Flaring Activity for 1ES 1959+650, 2016 August-2017 November

We present the detailed timing and spectral results obtained during the Swift observations of the TeV-detected blazar 1ES 1959+650 in 2016 August-2017 November. The source continued its trend of gradually enhancing X-ray flaring activity in the 0.3-10 keV energy range, starting in 2015 August, and a new highest historical brightness state was recorded twice. A long-term high state was superimposed by both weak and strong flares, and we detected 32 instances of intraday X-ray flux variability, including several occasions of extremely fast fluctuations with fractional amplitudes of 5.4%-7.5% within 1 ks exposures. The 0.3-10 keV spectra generally showed a best fit with the log-parabolic model, yielding a very wide range of the curvature parameter b and the photon index at 1 keV. The position of the synchrotron SED peak E p showed an extreme variability on various timescales between energies less than 0.1 keV and 7.7 ± 0.7 keV, with 25% of the spectra peaking at hard X-rays. In 2017 May-November, the source mostly showed lower spectral curvature and an anticorrelation b-E p, expected in the case of efficient stochastic acceleration of X-ray-emitting electrons. The 0.3-100 GeV and optical-UV fluxes also attained their highest historical values in this period, although the latter showed an anticorrelation with the 0.3-10 keV emission, which is explained by the stochastic acceleration of electrons with a narrow initial energy distribution, having an average energy significantly higher than the equilibrium energy

The prolonged X-ray flaring activity of Mrk 501 in 2014

The X-ray variability of the BL Lacertae source Mrk 501 was studied during 11.5 yr of monitoring with Swift. Here, we report the results of this study pertaining to the epoch of 2014 March-October, when our target showed the most powerful and long-lasting X-ray flaring activity. This epoch was characterized by X-ray flares varying in amplitude by factors of 2-5 on time-scales of a few weeks or shorter. We detected 35 instances of the intraday variability, sometimes occurring within the 1 ks observational runs. The X-ray flux was generally correlated with the TeV flux, while the 0.3-300 GeV and optical-UV fluxes did not show a significant correlation. Some notable incidences of more complicated variability patterns could also be recognized, indicating that the high-energy emission in Mrk 501 arose from an emission region more complex than a single zone. The best fits of the 0.3-10 keV spectra were mainly obtained using the logparabola model. Strong spectral variability was detected, affecting the slope but not the curvature of the spectrum. In strong flares, the spectral index became harder than 1.70. The spectral evolution was characterized by a harder-when-brighter behaviour, shifting the peak of the spectral energy distribution by about 20 keV that happens rarely in blazars