ABSTRACT
In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration. We present the WEBT optical data in ...the BVRI bands acquired at 36 observatories around the world. In mid-2021 the source showed its historical maximum, with R = 11.14. The light curves display many episodes of intraday variability, whose amplitude increases with source brightness, in agreement with a geometrical interpretation of the long-term flux behaviour. This is also supported by the long-term spectral variability, with an almost achromatic trend with brightness. In contrast, short-term variations are found to be strongly chromatic and are ascribed to energetic processes in the jet. We also analyse the optical polarimetric behaviour, finding evidence of a strong correlation between the intrinsic fast variations in flux density and those in polarization degree, with a time delay of about 13 h. This suggests a common physical origin. The overall behaviour of the source can be interpreted as the result of two mechanisms: variability on time-scales greater than several days is likely produced by orientation effects, while either shock waves propagating in the jet, or magnetic reconnection, possibly induced by kink instabilities in the jet, can explain variability on shorter time-scales. The latter scenario could also account for the appearance of quasi-periodic oscillations, with periods from a few days to a few hours, during outbursts, when the jet is more closely aligned with our line of sight and the time-scales are shortened by relativistic effects.
Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales
. This variability seems mostly random, although some ...quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare
in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet
, plasma instabilities
or orbital motion in an accretion disc
. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref.
). BL Lac, the prototype of a subclass of blazars
, is powered by a 1.7 × 10
M
(ref.
) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref.
)). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities
near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image.
Abstract
We study the broadband emission of Mrk 501 using multiwavelength observations from 2017 to 2020 performed with a multitude of instruments, involving, among others, MAGIC, Fermi's Large Area ...Telescope (LAT), NuSTAR, Swift, GASP-WEBT, and the Owens Valley Radio Observatory. Mrk 501 showed an extremely low broadband activity, which may help to unravel its baseline emission. Nonetheless, significant flux variations are detected at all wave bands, with the highest occurring at X-rays and very-high-energy (VHE)
γ
-rays. A significant correlation (>3
σ
) between X-rays and VHE
γ
-rays is measured, supporting leptonic scenarios to explain the variable parts of the emission, also during low activity. This is further supported when we extend our data from 2008 to 2020, and identify, for the first time, significant correlations between the Swift X-Ray Telescope and Fermi-LAT. We additionally find correlations between high-energy
γ
-rays and radio, with the radio lagging by more than 100 days, placing the
γ
-ray emission zone upstream of the radio-bright regions in the jet. Furthermore, Mrk 501 showed a historically low activity in X-rays and VHE
γ
-rays from mid-2017 to mid-2019 with a stable VHE flux (>0.2 TeV) of 5% the emission of the Crab Nebula. The broadband spectral energy distribution (SED) of this 2 yr long low state, the potential baseline emission of Mrk 501, can be characterized with one-zone leptonic models, and with (lepto)-hadronic models fulfilling neutrino flux constraints from IceCube. We explore the time evolution of the SED toward the low state, revealing that the stable baseline emission may be ascribed to a standing shock, and the variable emission to an additional expanding or traveling shock.
OBJECTIVE Augmented reality (AR) has the potential to improve the accuracy and efficiency of instrumentation placement in spinal fusion surgery, increasing patient safety and outcomes, optimizing ...ergonomics in the surgical suite, and ultimately lowering procedural costs. The authors sought to describe the use of a commercial prototype Spine AR platform (SpineAR) that provides a commercial AR head-mounted display (ARHMD) user interface for navigation-guided spine surgery incorporating real-time navigation images from intraoperative imaging with a 3D-reconstructed model in the surgeon's field of view, and to assess screw placement accuracy via this method. METHODS Pedicle screw placement accuracy was assessed and compared with literature-reported data of the freehand (FH) technique. Accuracy with SpineAR was also compared between participants of varying spine surgical experience. Eleven operators without prior experience with AR-assisted pedicle screw placement took part in the study: 5 attending neurosurgeons and 6 trainees (1 neurosurgical fellow, 1 senior orthopedic resident, 3 neurosurgical residents, and 1 medical student). Commercially available 3D-printed lumbar spine models were utilized as surrogates of human anatomy. Among the operators, a total of 192 screws were instrumented bilaterally from L2–5 using SpineAR in 24 lumbar spine models. All but one trainee also inserted 8 screws using the FH method. In addition to accuracy scoring using the Gertzbein-Robbins grading scale, axial trajectory was assessed, and user feedback on experience with SpineAR was collected. RESULTS Based on the Gertzbein-Robbins grading scale, the overall screw placement accuracy using SpineAR among all users was 98.4% (192 screws). Accuracy for attendings and trainees was 99.1% (112 screws) and 97.5% (80 screws), respectively. Accuracy rates were higher compared with literature-reported lumbar screw placement accuracy using FH for attendings (99.1% vs 94.32%; p = 0.0212) and all users (98.4% vs 94.32%; p = 0.0099). The percentage of total inserted screws with a minimum of 5° medial angulation was 100%. No differences were observed between attendings and trainees or between the two methods. User feedback on SpineAR was generally positive. CONCLUSIONS Screw placement was feasible and accurate using SpineAR, an ARHMD platform with real-time navigation guidance that provided a favorable surgeon-user experience.
The Jurassic system as developed along a strike of some 500 miles in the Jebel Tuwaiq, central Saudi Arabia (Nejd), is described and divided into formations, of which the names are here published for ...the first time, with a synopsis of their lithology and fossil contents. Apart from small collections,'including only one ammonite species, made on camel journeys by H. St J. B. Philby, nothing was hitherto known of these formations; their age (except for the Middle Callovian date given by the ammonite), and the existence of most of them, were unknown until the area began to be mapped and explored geologically about 1940 by geologists of the Arabian American Oil Company, whose results are here summarized. The total thickness of marine Jurassic rocks described exceeds 1000 m., all of it in neritic facies. This sequence is divided, in ascending order, into Marrat, Dhruma, Tuwaiq Mountain, Hanifa, Jubaila and Riyadh formations, above which follows the Cretaceous system. Below the marine Marrat is the Minjur Sandstone (315 m.), of 'continental’ facies, with only obscure plant remains, which possibly corresponds to the supposedly Rhaetic-Lower Lias plant-bearing Kohlan Sandstones of the Yemen. The underlying Jilh formation of Saudi Arabia contains marine Middle Triassic fossils near the top. Collections of ammonites establish the presence of marine Jurassic stages from Lower Toarcian (Lower Marrat) to Lower Kimeridgian (Jubaila); the Riyadh has not yet yielded ammonites at outcrop. The ammonites described are of extraordinary interest from the points of view of palaeogeography, correlation, phylogeny and systematics. While most of the faunas contain just enough links with other parts of the world to establish broad correlations, many are entirely new, or confined, outside Arabia, to Sinai. In the Toarcian there was faunal continuity with Madagascar and Baluchistan, in the Bajocian only with Sinai, from the Middle Bathonian onwards with western Europe. Most of the commonest cosmopolitan genera are missing from central Arabia, where there lived a succession of highly peculiar forms characterized at several successive horizons by unstable suture-lines. It has been found necessary to make eight new genera, besides three already named but hitherto not found outside Sinai, Madagascar and Baluchistan, although employing a taxonomic scale so large that some palaeontologists will consider it old-fashioned. This is believed, however, to be more useful at this stage of our knowledge than a proliferation of generic names based on inadequate material; for the area from which the collections were made is about equal to that of all the Jurassic outcrops of England. The paper concludes with a discussion of problems of evolution, speciation, correlation, and palaeogeography raised by the material. In an appendix are described relevant Bajocian ammonites from Sinai, with a hitherto unpublished section of the strata.
Jurassic Ammonites from Jebel Tuwaiq, Central Arabia Arkell, W. J.; Bramkamp, R. A.; Steineke, M.
Philosophical transactions of the Royal Society of London. Series B, Biological sciences,
03/1952, Letnik:
236, Številka:
633
Journal Article
The Jurassic system as developed along a strike of some 500 miles in the Jebel Tuwaiq, central Saudi Arabia (Nejd), is described and divided into formations, of which the names are here published for ...the first time, with a synopsis of their lithology and fossil contents. Apart from small collections, including only one ammonite species, made on camel journeys by H. St J. B. Philby, nothing was hitherto known of these formations; their age (except for the Middle Callovian date given by the ammonite), and the existence of most of them, were unknown until the area began to be mapped and explored geologically about 1940 by geologists of the Arabian American Oil Company, whose results are here summarized. The total thickness of marine Jurassic rocks described exceeds 1000 m., all of it in neritic facies. This sequence is divided, in ascending order, into Marrat, Dhruma, Tuwaiq Mountain, Hanifa, Jubaila and Riyadh formations, above which follows the Cretaceous system. Below the marine Marrat is the Minjur Sandstone (315 m.), of 'continental' facies, with only obscure plant remains, which possibly corresponds to the supposedly Rhaetic-Lower Lias plant-bearing Kohlan Sandstones of the Yemen. The underlying Jilh formation of Saudi Arabia contains marine Middle Triassic fossils near the top. Collections of ammonites establish the presence of marine Jurassic stages from Lower Toarcian (Lower Marrat) to Lower Kimeridgian (Jubaila); the Riyadh has not yet yielded ammonites at outcrop. The ammonites described are of extraordinary interest from the points of view of palaeogeography, correlation, phylogeny and systematics. While most of the faunas contain just enough links with other parts of the world to establish broad correlations, many are entirely new, or confined, outside Arabia, to Sinai. In the Toarcian there was faunal continuity with Madagascar and Baluchistan, in the Bajocian only with Sinai, from the Middle Bathonian onwards with western Europe. Most of the commonest cosmopolitan genera are missing from central Arabia, where there lived a succession of highly peculiar forms characterized at several successive horizons by unstable suture-lines. It has been found necessary to make eight new genera, besides three already named but hitherto not found outside Sinai, Madagascar and Baluchistan, although employing a taxonomic scale so large that some palaeontologists will consider it old-fashioned. This is believed, however, to be more useful at this stage of our knowledge than a proliferation of generic names based on inadequate material; for the area from which the collections were made is about equal to that of all the Jurassic outcrops of England. The paper concludes with a discussion of problems of evolution, speciation, correlation, and palaeogeography raised by the material. In an appendix are described relevant Bajocian ammonites from Sinai, with a hitherto unpublished section of the strata.
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