Abstract
We have studied long-term optical, ultraviolet (UV) and X-ray observations of OJ 287 collected with the UVOT and XRT instruments mounted on board the Swift satellite to quantify spectral and ...temporal variability patterns observed during different activity states. We characterized the flux variations using the data collected during almost 11 yr of the monitoring of the blazar. Significant variability of the blazar has been detected both in the flux and spectral index from the optical to X-ray regimes. We noted that the variability patterns observed in the optical range are more pronounced than the ones in the X-ray band. There is no clear relation between the optical/UV and X-ray emission, neither during the quiescence state nor during outbursts. The most significant flares in the optical/UV regime were detected in 2015 December–2016 January. The shortest variability time-scale is one day and it is limited by the observation pointing. A low activity state of OJ 287 was observed at the end of 2014, while the beginning of 2015 revealed a flat X-ray spectrum, which has been observed for the first time. On one hand, this can be a spectral upturn where the synchrotron and inverse Compton components meet, but on the other hand, it can be generated by an additional emission component. The spectral studies have not revealed any bluer-when-brighter or redder-when-brighter chromatism in the colour–magnitude diagram for OJ 287 in any state of the source's activity. A harder-when-brighter behaviour was noticed for OJ 287 only in the case of the X-ray observations.
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 Ultraviolet/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 first time, when hard X-ray observations of the source collected with NuSTAR are reported. The studies reveal both low- and high-energy components clearly visible in the energy band, with the break energy of 8 keV, which is the highest break energy ever reported for S5 0716+714. The second part of this work is concentrated on multifrequency observations collected during the flaring activity period. The variability patterns recorded during the period are characterized using a fractional variability amplitude and description of the flare profiles. The correlation studies reveal strong and significant relation between the optical, ultraviolet and γ-ray observations, and no time lag is found for any of the studied relations.
The flat-spectrum radio quasar CTA 102 (redshift 1.037) exhibited a tremendously bright four-month-long outburst from late 2016 to early 2017. In a previous paper, we interpreted the event as the ...ablation of a gas cloud by the relativistic jet. The multiwavelength data have been reproduced very well within this model using a leptonic emission scenario. Here we expand that work by using a hadronic scenario, which gives us greater freedom with respect to the location of the emission region within the jet. This is important, since the inferred gas cloud parameters depend on the distance from the black hole. While the hadronic model faces the problem of invoking super-Eddington jet luminosities, it reproduces well the long-term trend and also days-long subflares. While the latter result in inferred cloud parameters that match those expected for clouds of the broad-line region, the long-term trend is not compatible with such an interpretation. We explore the possibilities that the cloud is from the atmosphere of a red giant star or comes from a star-forming region that passes through the jet. The latter could also explain the much longer-lasting activity phase of CTA 102 from late 2015 until early 2018.
In late 2016 and early 2017, the flat spectrum radio quasar CTA 102 exhibited a very strong and long-lasting outburst. The event can be described by a roughly two-month long increase of the baseline ...flux in the monitored energy bands (optical to γ-rays) by a factor 8, and a subsequent decrease over another two months back to pre-flare levels. The long-term trend was superseded by short but very strong flares, resulting in a peak flux that was a factor 50 above pre-flare levels in the γ-ray domain and almost a factor 100 above pre-flare levels in the optical domain. In this paper, we explain the long-term evolution of the outburst by the ablation of a gas cloud penetrating the relativistic jet. The slice-by-slice ablation results in a gradual increase of the particle injection until the center of the cloud is reached, after which the injected number of particles decreases again. With reasonable cloud parameters, we obtain excellent fits of the long-term trend.
Swift/XRT view of S5 0716+714 during a flare Wierzcholska, A.; Siejkowski, H.
Monthly Notices of the Royal Astronomical Society Letters,
09/2015, Letnik:
452, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The blazar S5 0716+714 has been monitored in its flaring state between 2015 January 19 and February 22 with the Swift X-Ray Telescope (Swift/XRT). During this period, an exceptional flux level was ...observed in the X-ray range as well as at other wavelengths, e.g. optical, near-infrared and very-high-energy γ rays. Here, we report X-ray observations of S5 0716+714 carried out during the outburst. The observed X-ray spectra, well described by a broken power-law model, disentangle both synchrotron and inverse Compton components. The break energy shifts towards higher energies with increasing flux, revealing the dominance of synchrotron radiation in the X-ray spectrum observed. We also report spectrum softening with increasing flux. During the recent flare, significant temporal intranight variability is observed in the X-ray range.
Astrophysical Journal Letters 970, L21 (2024) The Tarantula Nebula in the Large Magellanic Cloud is known for its high star
formation activity. At its center lies the young massive star cluster R136,
...providing a significant amount of the energy that makes the nebula shine so
brightly at many wavelengths. Recently, young massive star clusters have been
suggested to also efficiently produce high-energy cosmic rays, potentially
beyond PeV energies. Here, we report the detection of very-high-energy
$\gamma$-ray emission from the direction of R136 with the High Energy
Stereoscopic System, achieved through a multicomponent, likelihood-based
modeling of the data. This supports the hypothesis that R136 is indeed a very
powerful cosmic-ray accelerator. Moreover, from the same analysis, we provide
an updated measurement of the $\gamma$-ray emission from 30 Dor C, the only
superbubble detected at TeV energies presently. The $\gamma$-ray luminosity
above $0.5\,\mathrm{TeV}$ of both sources is $(2-3)\times
10^{35}\,\mathrm{erg}\,\mathrm{s}^{-1}$. This exceeds by more than a factor of
2 the luminosity of HESS J1646$-$458, which is associated with the most massive
young star cluster in the Milky Way, Westerlund 1. Furthermore, the
$\gamma$-ray emission from each source is extended with a significance of
$>3\sigma$ and a Gaussian width of about $30\,\mathrm{pc}$. For 30 Dor C, a
connection between the $\gamma$-ray emission and the nonthermal X-ray emission
appears likely. Different interpretations of the $\gamma$-ray signal from R136
are discussed.
The FSRQ CTA 102 (z=1.032) has been tremendously active over the last few years. During its peak activity lasting several months in late 2016 and early 2017, the gamma-ray and optical fluxes rose by ...up to a factor 100 above the quiescence level. We have interpreted the peak activity as the ablation of a gas cloud by the relativistic jet, which can nicely account for the months-long lightcurve in 2016 and 2017. The peak activity was in the middle of a 2-year-long high-state, which was characterized by increased fluxes and increased rms variability compared to the previous low-states, and which was flanked by two bright flares. In this presentation, we put the cloud-ablation scenario into the broader context of the 2-year-long high-state.
PSR B1259-63 is a gamma-ray binary system that hosts a pulsar in an eccentric orbit, with a 3.4 year period, around an O9.5Ve star. At orbital phases close to periastron passages, the system radiates ...bright and variable non-thermal emission. We report on an extensive VHE observation campaign conducted with the High Energy Stereoscopic System, comprised of ~100 hours of data taken from \(t_p-24\) days to \(t_p+127\) days around the system's 2021 periastron passage. We also present the timing and spectral analyses of the source. The VHE light curve in 2021 is consistent with the stacked light curve of all previous observations. Within the light curve, we report a VHE maximum at times coincident with the third X-ray peak first detected in the 2021 X-ray light curve. In the light curve -- although sparsely sampled in this time period -- we see no VHE enhancement during the second disc crossing. In addition, we see no correspondence to the 2021 GeV flare in the VHE light curve. The VHE spectrum obtained from the analysis of the 2021 dataset is best described by a power law of spectral index \(\Gamma = 2.65 \pm 0.04_{\text{stat}}\) \(\pm 0.04_{\text{sys}}\), a value consistent with the previous H.E.S.S. observations of the source. We report spectral variability with a difference of \(\Delta \Gamma = 0.56 ~\pm~ 0.18_{\text{stat}}\) \(~\pm~0.10_{\text{sys}}\) at 95% c.l., between sub-periods of the 2021 dataset. We also find a linear correlation between contemporaneous flux values of X-ray and TeV datasets, detected mainly after \(t_p+25\) days, suggesting a change in the available energy for non-thermal radiation processes. We detect no significant correlation between GeV and TeV flux points, within the uncertainties of the measurements, from \(\sim t_p-23\) days to \(\sim t_p+126\) days. This suggests that the GeV and TeV emission originate from different electron populations.
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