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 The long-term trend was superseded by short but very strong flares, resulting in a peak flux that was a factor 50 above pre-flare levels in the γ-ray domain and almost a factor 100 above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.
Context.
After the discovery of rings around the largest known Centaur object, (10199) Chariklo, we carried out observation campaigns of stellar occultations produced by the second-largest known ...Centaur object, (2060) Chiron, to better characterize its physical properties and presence of material on its surroundings.
Aims.
We aim to provide constraints on (2060) Chiron’s shape for the first time using stellar occultations. We investigate the detectability of material previously observed in its vicinity using the 2018 occultation data obtained from South Africa Astronomical Observatory (SAAO).
Methods.
We predicted and successfully observed two stellar occultations by Chiron. These observations were used to constrain its size and shape by fitting elliptical limbs with equivalent surface radii in agreement with radiometric measurements. We also obtained the properties of the material observed in 2011 with the same technique used to derive Chariklo’s ring properties in our previous works, used to obtain limits on the detection of secondary events in our 2018 observation.
Results.
Constraints on the (2060) Chiron shape are reported for the first time. Assuming an equivalent radius of
R
equiv
= 105
−7
+6
km, we obtained a semi-major axis of
a
= 126 ± 22 km. Considering Chiron’s true rotational light curve amplitude and assuming it has a Jacobi equilibrium shape, we were able to derive a 3D shape with a semi-axis of
a
= 126 ± 22 km,
b
= 109 ± 19 km, and c = 68 ± 13 km, implying in a volume-equivalent radius of
R
vol
= 98 ± 17 km. We determined the physical properties of the 2011 secondary events around Chiron, which may then be directly compared with those of Chariklo rings, as the same method was used. Data obtained from SAAO in 2018 do not show unambiguous evidence of the proposed rings, mainly due to the large sampling time. Meanwhile, we discarded the possible presence of a permanent ring similar to (10199) Chariklo’s C1R in optical depth and extension.
Conclusions.
Using the first multi-chord stellar occultation by (2060) Chiron and considering it to have a Jacobi equilibrium shape, we derived its 3D shape, implying a density of 1119 ± 4 kg m
−3
. New observations of a stellar occultation by (2060) Chiron are needed to further investigate the material’s properties around Chiron, such as the occultation predicted for September 10, 2023.
Blazars are the most common class of TeV extragalactic emitters. In the framework of the AGN unified model, they are understood as AGNs with a relativistic jet pointing close the line of sight. They ...are characterized by extreme variability, observed to be as fast as minutes. These flares are usually observed at multiple wavelengths and their study require fast reaction and coordination among multiwavelength observatories. An important part of blazars observations with the H.E.S.S. array of Cherenkov telescopes is thus in the form of Target of Opportunity (ToO) observations. In this contribution the H.E.S.S. blazar ToO program is presented, with a focus on recent results.
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