Tokamak turbulence, driven by the ion-temperature gradient and occurring in the presence of flow shear, is investigated by means of local, ion-scale, electrostatic gyrokinetic simulations (with both ...kinetic ions and electrons) of the conditions in the outer core of the Mega-Ampere Spherical Tokamak (MAST). A parameter scan in the local values of the ion-temperature gradient and flow shear is performed. It is demonstrated that the experimentally observed state is near the stability threshold and that this stability threshold is nonlinear: sheared turbulence is subcritical, i.e. the system is formally stable to small perturbations, but, given a large enough initial perturbation, it transitions to a turbulent state. A scenario for such a transition is proposed and supported by numerical results: close to threshold, the nonlinear saturated state and the associated anomalous heat transport are dominated by long-lived coherent structures, which drift across the domain, have finite amplitudes, but are not volume filling; as the system is taken away from the threshold into the more unstable regime, the number of these structures increases until they overlap and a more conventional chaotic state emerges. Whereas this appears to represent a new scenario for transition to turbulence in tokamak plasmas, it is reminiscent of the behaviour of other subcritically turbulent systems, e.g. pipe flows and Keplerian magnetorotational accretion flows.
We present homogeneous, standardized UBV(RI)C photometry for over 700 nearby stars selected on the basis of Hipparcos parallaxes. Additionally, we list JHK photometry for about half of these stars, ...as well as L photometry for 86 of the brightest. A number of stars with peculiar colours or anomalous locations in various colour–magnitude diagrams are discussed.
New changing look case in NGC 1566 Oknyansky, V L; Winkler, H; Tsygankov, S S ...
Monthly Notices of the Royal Astronomical Society,
02/2019, Letnik:
483, Številka:
1
Journal Article
ABSTRACT
We present results of the long-term multiwavelength study of optical, UV, and X-ray variability of the nearby changing-look (CL) Seyfert NGC 1566 observed with the Swift Observatory and the ...MASTER Global Robotic Network from 2007 to 2019. We started spectral observations with South African Astronomical Observatory 1.9-m telescope soon after the brightening was discovered in July 2018 and present here the data for the interval between 2018 August and 2019 September. This paper concentrates on the remarkable post-maximum behaviour after 2018 July when all bands decreased with some fluctuations. We observed three significant re-brightenings in the post-maximum period during 2018 November 17–2019 January 10, 2019 April 29–2019 June 19, and 2019 July 27–2019 August 6. An X-ray flux minimum occurred in 2019 March. The UV minimum occurred about 3 months later. It was accompanied by a decrease of the LUV/LX-ray ratio. New post-maximum spectra covering (2018 November 31–2019 September 23) show dramatic changes compared to 2018 August 2, with fading of the broad lines and Fe X λ6374 until 2019 March. These lines became somewhat brighter in 2019 August–September. Effectively, two CL states were observed for this object: changing to type 1.2 and then returning to the low state as a type 1.8 Sy. We suggest that the changes are due mostly to fluctuations in the energy generation. The estimated Eddington ratios are about 0.055 ${{\ \rm per\ cent}}$ for minimum in 2014 and 2.8 ${{\ \rm per\ cent}}$ for maximum in 2018.
We present photometric and high-dispersion spectroscopic measurements that show HR 7920 is a periodic variable. The photometry reveals at least four frequencies higher that 10 d super( -1), two of ...which are also probably present in the radial velocity variations. The frequencies are in a range typical of delta Scuti star pulsations. A further low frequency of about 2.8 d super( -1) may be present in both radial velocities and photometry; if real, this points to gamma Doradus variability, which would make HR 7920 a hybrid pulsator. An attempt is made to identify the modes of the delta Scuti pulsations, which include both radial and non-radial modes. A new rotational velocity of 75 km s super( -1) is derived from co-added spectra, contrasting with published values in the range 128-150 km s super( -1).
We present homogeneous and standardized UBV(RI)CJHK photometry for over 100 M stars selected from an earlier paper on the basis of apparent photometric constancy. L photometry has been obtained for ...stars brighter than about L= 6. Most of the stars have a substantial number of UBV(RI)C observations and, it is hoped, will prove useful as red supplementary standards. Additionally, we list JHK photometry for nearly 300 Hipparcos red stars not selected as standards, as well as L photometry for the brightest stars.
Recently, we discovered an entirely new type of variability in the upper atmospheres of rapidly oscillating Ap (roAp) stars. This manifests itself in amplitude modulation of the radial velocities ...that has not been previously detected in photometric studies of the same stars. To study this new variability further we obtained a full night, 8.85 h, of high time resolution (70 s), high spectral resolution (R= 105 000), high signal‐to‐noise ratio (on average S/N ∼ 130) data with Ultraviolet and Visual Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) for the roAp star HD 134214. We also obtained 4.2 h of new photometric data in Johnson B with the South African Astronomical Observatory (SAAO) 0.5‐m telescope 2 d later. HD 134214 has been known for years to be singly periodic with a relatively stable amplitude in photometry; it has the highest pulsation frequency of any roAp star of 2.949 mHz (P= 5.65 min). Our new UVES data show this principal frequency, plus five other frequencies in amplitude spectra of rare earth elements lines and the Hα line. The new frequencies are stable over the 8.85 h of observation, and the highest of them ν2= 2.782 mHz is the same as found in data taken 2 yr earlier with UVES presented in the discovery paper. The amplitudes of the new frequencies drop faster with atmospheric depth than does the amplitude of the principal frequency, hence explaining why they are generally not seen in broad‐band photometric measurements that sample on average more deeply in the atmosphere. Our new photometric measurements also detect ν2 for the first time in photometric data. Our analysis suggests that the new frequencies are associated with pulsation modes, but the nature of those modes and why they increase in amplitude with atmospheric height more strongly than the principal frequencies is not yet known.
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