UV Ceti Research Articles

Radio Emission from the Magnetically Active M Dwarf UV Ceti from 1 to 105 GHz

BL and UV Ceti are a nearby (2.7 pc) binary system with similar masses, spectral types, and rapid rotation rates, but very different magnetic activity. UV Ceti’s much stronger large-scale magnetic field may cause this difference, highlighting key unanswered questions about dynamo processes in fully convective objects. Here, we present multiepoch characterization of the radio spectrum of UV Ceti spanning 1–105 GHz, exhibiting flared emission similar to coronal activity, auroral-like emission analogous to planetary magnetospheres, and slowly varying persistent emission. Radio observations are a powerful means to probe the role that the large-scale magnetic field of UV Ceti has in nonthermal particle acceleration because radio-frequency phenomena result from both the activity of small-scale field features as well as large-scale auroral current systems. We find temporal variability at all bands observed, and a hint of rotational modulation in the degree of circular polarization up to 40 GHz. The persistent component of the emission is fairly constant from 1 to 105 GHz, making optically thick emission or optically thin gyrosynchrotron from electrons with an isotropic pitch angle distribution unlikely. We discuss the possibility of emission mechanisms analogous to Jupiter’s radiation belts.

Radio Emission from UV Cet: Auroral Emission from a Stellar Magnetosphere

The archetypical flare star UV Cet was observed by MeerKAT on 2021 October 5–6. A large radio outburst with a duration of ∼2 hr was observed between 886 and 1682 MHz, with a time resolution of 8 s and a frequency resolution of 0.84 MHz, enabling sensitive dynamic spectra to be formed. The emission is characterized by three peaks containing a multitude of broadband arcs or partial arcs in the time-frequency domain. In general, the arcs are highly right-hand circularly polarized. At the end of the third peak, brief bursts occur that are significantly elliptically polarized. We present a simple model that appears to be broadly consistent with the characteristics of the radio emission from UV Cet. Briefly, the stellar magnetic field is modeled as a dipole aligned with the rotational axis of the star. The radio emission mechanism is assumed to be due to the cyclotron maser instability, where x-mode radiation near the electron gyrofrequency is amplified. While the elliptically polarized bursts may be intrinsic to the source, rather stringent limits are imposed on the plasma density in the source and along the propagation path. We suggest that the elliptically polarized radiation may instead be the result of reflection on an overdense plasma structure at some distance from the source. The radio emission from UV Cet shares both stellar and planetary attributes.

CZEV502 – AN M DWARF NEAR THE LEO TRIPLET WITH VERY STRONG FLARES

Discovery of flares in the M dwarf CzeV502 and our follow-up results are presented. We classify it as a dMe eruptive variable of UV Ceti type due to the X-ray activity, measured B − V of 1.5 mag, Hα; emission, and flares. Our monitoring revealed only one reliable and one suspected superflare in 58 nights (210 hrs). The strongest flare with ΔR = 1.5 mag (ΔB ≈ 6-8 mag) could have a total energy of 3E+34 erg. The ASAS-SN data may contain 4 events up to ΔV of 0.43 mag and 12.55 d periodicity corresponding to the rotation or possible binarity. Other brightenings in sky survey (ASAS-3, CRTS, NSVS, and KWS) are doubtful. No event was unveiled on the 1 600 photographic plates. The upper rate limit of 1-2 superflares/1 640 hrs corresponds to activity several orders higher than for other M-dwarfs, especially, for the slow rotators. The low amplitude flares ( ΔB < 0.5 mag) may be common (1 flare/4 hrs).

EBASCS: Disentangling overlapping astronomical sources II, using spatial, spectral, and temporal information

ABSTRACT The analysis of individual X-ray sources that appear in a crowded field can easily be compromised by the misallocation of recorded events to their originating sources. Even with a small number of sources, which none the less have overlapping point spread functions, the allocation of events to sources is a complex task that is subject to uncertainty. We develop a Bayesian method designed to sift high-energy photon events from multiple sources with overlapping point spread functions, leveraging the differences in their spatial, spectral, and temporal signatures. The method probabilistically assigns each event to a given source. Such a disentanglement allows more detailed spectral or temporal analysis to focus on the individual component in isolation, free of contamination from other sources or the background. We are also able to compute source parameters of interest like their locations, relative brightness, and background contamination, while accounting for the uncertainty in event assignments. Simulation studies that include event arrival time information demonstrate that the temporal component improves event disambiguation beyond using only spatial and spectral information. The proposed methods correctly allocate up to 65${{\ \rm per\ cent}}$ more events than the corresponding algorithms that ignore event arrival time information. We apply our methods to two stellar X-ray binaries, UV Cet and HBC 515 A, observed with Chandra. We demonstrate that our methods are capable of removing the contamination due to a strong flare on UV Cet B in its companion ≈40× weaker during that event, and that evidence for spectral variability at times-scales of a few ks can be determined in HBC 515 Aa and HBC 515 Ab.

Statistical studies of flare stars and other flashing objects carried out at the Byurakan Observatory

The review briefly presents the statistical studies of flare stars and related objects carried out at the Byurakan Astrophysical Observatory, in particular: i) determination of the evolutionary status of flare stars, ii) an explanation of the observed difference between flare stars of the galactic field (type UV Cet) and flare stars of systems (open clusters, associations), iii) the connection between flare stars and T Tauri stars, iv) short-term and long-term (evolutionary) variability of flare activity, v) original statistical methods developed for the study of flare stars and their systems, vi) recent advances in research on flashing objects. This review does not present such important areas of research on flare stars as the determination of physical parameters, photometry and colorimetry of stars and their flares, detailed studies of individual stars, theoretical works on possible mechanisms of flares. These areas were well presented in many books and reviews by Ambartsumian and Mirzoyan.

On the mechanism of polarized metre-wave stellar emission

ABSTRACT Two coherent radio emission mechanisms operate in stellar coronae: plasma emission and cyclotron emission. They directly probe the electron density and magnetic field strength respectively. Most stellar radio detections have been made at cm-wavelengths where it is often not possible to uniquely identify the emission mechanism, hindering the utility of radio observations in probing coronal conditions. In anticipation of stellar observations from a suite of sensitive low-frequency ($\nu \sim 10^2\, {\rm MHz}$) radio telescopes, here I apply the general theory of coherent emission in non-relativistic plasma to the low-frequency case. I consider the recently reported low-frequency emission from dMe flare stars AD Leo and UV Ceti and the quiescent star GJ 1151 as test cases. My main conclusion is that unlike the cm-wave regime, for reasonable turbulence saturation regimes, the emission mechanism in metre-wave observations ($\nu \sim 10^2\, {\rm MHz}$) can often be identified based on the observed brightness temperature, emission duration, and polarization fraction. I arrive at the following heuristic: M-dwarf emission that is ≳ hour-long with ${\gtrsim}50{{\ \rm per\ cent}}$ circular polarized fraction at brightness temperatures of ${\gtrsim}10^{12}\,$K at ${\sim}100\, {\rm MHz}$ in M-dwarfs strongly favours a cyclotron maser interpretation.

The flare cumulative frequencies of UV Ceti stars from different spectral types

ABSTRACT In this study, as a result of the photometric observations continued on nine UV Ceti type stars, examining the parameters obtained from the models of distributions of the flare equivalent durations in the logarithmic scale versus flare total durations, it is found that the highest flare equivalent durations occurring on the stars (Plateau values) are varying with the B − V colour indexes of those stars in agreement with the variations of both the magnetic field strength (B) of the magnetic loop and especially the electron density of its plasma (ne). The distributions of flare cumulative frequencies were derived by using the flare equivalent durations, instead of the flare energy. The main result of this study is a secondary variation, which can be just modelled with the damping sine waves, behind the main exponential variation of the flare cumulative frequency. The models indicate that this secondary variation in the flare cumulative frequency distributions is in the same shape with the periodic or quasi-periodic oscillations detected in the light curves of the stellar and solar flares. This caused to ask new questions about the flare mechanism working on stellar atmospheres.

Observing flare stars below 100 MHz with the LWA

ABSTRACT We observed the flare stars AD Leonis, Wolf 424, EQ Pegasi, EV Lacertae, and UV Ceti for nearly 135 h. These stars were observed between 63 and 83 MHz using the interferometry mode of the Long Wavelength Array. Given that emission from flare stars is typically circularly polarized, we used the condition that any significant detection present in Stokes I must also be present in Stokes V at the same time in order for us to consider it a possible flare. Following this, we made one marginal flare detection for the star EQ Pegasi. This flare had a flux density of 5.91 Jy in Stokes I and 5.13 Jy in Stokes V, corresponding to a brightness temperature 1.75 × 1016(r/r*)−2 K.

ASKAP detection of periodic and elliptically polarized radio pulses from UV Ceti

ABSTRACT Active M dwarfs are known to produce bursty radio emission, and multiwavelength studies have shown that solar-like magnetic activity occurs in these stars. However, coherent bursts from active M dwarfs have often been difficult to interpret in the solar activity paradigm. We present Australian Square Array Pathfinder (ASKAP) observations of UV Ceti at a central frequency of 888 MHz. We detect several periodic, coherent pulses occurring over a time-scale consistent with the rotational period of UV Ceti. The properties of the pulsed emission show that they originate from the electron cyclotron maser instability, in a cavity at least 7 orders of magnitude less dense than the mean coronal density at the estimated source altitude. These results confirm that auroral activity can occur in active M dwarfs, suggesting that these stars mark the beginning of the transition from solar-like to auroral magnetospheric behaviour. These results demonstrate the capabilities of ASKAP for detecting polarized, coherent bursts from active stars and other systems.

The Magnetic Binary GJ 65: A Test of Magnetic Diffusivity Effects

Abstract GJ 65 is an M dwarf binary system consisting of the two flare stars BL Cet (GJ 65A) and UV Cet (GJ 65B). Two teams of investigators have recently reported total magnetic fluxes corresponding to fields of 4.5 and 5.2 kG for GJ 65A and 5.8 and 6.7 kG for GJ 65B: for each component, the magnetic results obtained by the two teams agree with each other to within 1σ. For the first time, we can directly compare the predictions of our magneto-convective models, based on fitting observed stellar parameters, with measured field strengths. We find that our models agree with the observed field strengths, provided the effects of finite conductivity are accounted for. Thus, GJ 65 provides us with an opportunity to use observations of field strengths to distinguish between the predictions of our models that assume perfect electrical conductivity and those that allow for finite conductivity.

154 MHz Detection of Faint, Polarized Flares from UV Ceti

Abstract We have detected four flares from UV Ceti at 154 MHz using the Murchison Widefield Array. The flares have flux densities between 10 and 65 mJy—a factor of 100 fainter than most flares in the literature at these frequencies—and are only detected in polarization. The circular polarized fractions are limited to % at 3σ confidence and two of the flares exhibit polarity reversal. We suggest that these flares occur periodically on a timescale consistent with the rotational period of UV Ceti. During the brightest observed flare, we also detect significant linear polarization with a polarization fraction %. Averaging the data in 6 minute, 10 MHz frequency bins we find that the flux density of these flares does not vary over the 30 MHz bandwidth of the Murchison Widefield Array; however, we cannot rule out finer time-frequency structure. Using the measured flux densities for the flares, we estimate brightness temperatures between K, indicative of a coherent emission mechanism. The brightness temperature and polarization characteristics point to the electron cyclotron maser mechanism. We also calculate the flare rates given our four observed flares and compare them to flare rates for the set of M dwarf stars with known 100–200 MHz flares. Our measurement is the first for flares with intensities mJy at 100–200 MHz.

The Global and Small-scale Magnetic Fields of Fully Convective, Rapidly Spinning M Dwarf Pair GJ65 A and B

Abstract The nearby M dwarf binary GJ65 AB, also known as BL Cet and UV Cet, is a unique benchmark for investigation of dynamo-driven activity of low-mass stars. Magnetic activity of GJ65 was repeatedly assessed by indirect means, such as studies of flares, photometric variability, X-ray, and radio emission. Here, we present a direct analysis of large-scale and local surface magnetic fields in both components. Interpreting high-resolution circular polarization spectra (sensitive to a large-scale field geometry) we uncovered a remarkable difference of the global stellar field topologies. Despite nearly identical masses and rotation rates, the secondary exhibits an axisymmetric, dipolar-like global field with an average strength of 1.3 kG while the primary has a much weaker, more complex, and non-axisymmetric 0.3 kG field. On the other hand, an analysis of the differential Zeeman intensification (sensitive to the total magnetic flux) shows the two stars having similar magnetic fluxes of 5.2 and 6.7 kG for GJ65 A and B, respectively, although there is evidence that the field strength distribution in GJ65 B is shifted toward a higher field strength compared to GJ65 A. Based on these complementary magnetic field diagnostic results, we suggest that the dissimilar radio and X-ray variability of GJ65 A and B is linked to their different global magnetic field topologies. However, this difference appears to be restricted to the upper atmospheric layers but does not encompass the bulk of the stars and has no influence on the fundamental stellar properties.

Discovery of the Sub-second Linearly Polarized Spikes of Synchrotron Origin in the UV Ceti Giant Optical Flare

AbstractDuring our optical monitoring of UV Ceti, iconic late-type flaring star, with high temporal resolution using the Russian 6-m telescope in 2008, we detected a giant flare with the amplitude of about 3 magnitudes in U band. Near flare maximum, more than a dozen of spike bursts have been discovered with triangular shapes and durations from 0.6 to 1.2 s and maximal luminosities in the range (1.5–8) × 1027 erg s−1. For the half of these events, the linear polarization exceeds 35% with significance better than 5σ. We argue that these events are synchrotron emission of electron streams with the energies of several hundred MeV moving in the magnetic field of about 1.4 kG. Emission from such ultra-relativistic (with energies far exceeding 10 MeV) particles is being routinely observed in solar flares, but has never been detected from UV Ceti-type stars. This is the first ever detection of linearly polarized optical light from the UV Ceti-type stars which indicates that at least some fraction of the flaring events on these stars is powered by a non-thermal synchrotron emission mechanism.

MOST OBSERVATIONS OF OUR NEAREST NEIGHBOR: FLARES ON PROXIMA CENTAURI

ABSTRACT We present a study of white-light flares from the active M5.5 dwarf Proxima Centauri using the Canadian microsatellite Microvariability and Oscillations of STars. Using 37.6 days of monitoring data from 2014 to 2015, we have detected 66 individual flare events, the largest number of white-light flares observed to date on Proxima Cen. Flare energies in our sample range from 1029 to 1031.5 erg. The flare rate is lower than that of other classic flare stars of a similar spectral type, such as UV Ceti, which may indicate Proxima Cen had a higher flare rate in its youth. Proxima Cen does have an unusually high flare rate given its slow rotation period, however. Extending the observed power-law occurrence distribution down to 1028 erg, we show that flares with flux amplitudes of 0.5% occur 63 times per day, while superflares with energies of 1033 erg occur ∼8 times per year. Small flares may therefore pose a great difficulty in searches for transits from the recently announced 1.27 M ⊕ Proxima b, while frequent large flares could have significant impact on the planetary atmosphere.

Optical microflaring on the nearby flare star binary UV Ceti

We present extremely high time resolution observations of the visual flare star binary UV Cet obtained with the Optical Pulsar Timing Analyzer (OPTIMA) at the 1.3 m telescope at Skinakas Observatory (SKO) in Crete, Greece. OPTIMA is a fiber-fed optical instrument that uses Single Photon Avalanche Diodes to measure the arrival times of individual optical photons. The time resolution of the observations presented here was 4 μs, allowing to resolve the typical millisecond variability time scales associated with stellar flares. We report the detection of very short impulsive bursts in the blue band with well resolved rise and decay time scales of about 2 s. The overall energetics put these flares at the lower end of the known flare distribution of UV Cet.

Optical counterparts of ROSAT X-ray sources in two selected fields at low vs. high Galactic latitudes

The optical identification of large number of X-ray sources such as those from the ROSAT All-Sky Survey is challenging with conventional spectroscopic follow-up observations. We investigate two ROSAT All-Sky Survey fields of size 10 * 10 degrees each, one at galactic latitude b = 83 deg (Com), the other at b = -5 deg (Sge), in order to optically identify the majority of sources. We used optical variability, among other more standard methods, as a means of identifying a large number of ROSAT All-Sky Survey sources. All objects fainter than about 12 mag and brighter than about 17 mag, in or near the error circle of the ROSAT positions, were tested for optical variability on hundreds of archival plates of the Sonneberg field patrol. The present paper contains probable optical identifications of altogether 256 of the 370 ROSAT sources analysed. In particular, we found 126 AGN (some of them may be misclassified CVs), 17 likely clusters of galaxies, 16 eruptive double stars (mostly CVs), 43 chromospherically active stars, 65 stars brighter than about 13 mag, 7 UV Cet stars, 3 semiregular resp. slow irregular variable stars of late spectral type, 2 DA white dwarfs, 1 Am star, 1 supernova remnant and 1 planetary nebula. X-ray emission is, expectedly, tightly correlated with optical variability, and thus our new method for optically identifying X-ray sources is demonstrated to be feasible.

Laboratory astrophysics: Investigation of planetary and astrophysical maser emission

This paper describes a model for cyclotron maser emission applicable to planetary auroral radio emission, the stars UV Ceti and CU Virginus, blazar jets and astrophysical shocks. These emissions may be attributed to energetic electrons moving into convergent magnetic fields that are typically found in association with dipole like planetary magnetospheres or shocks. It is found that magnetic compression leads to the formation of a velocity distribution having a horseshoe shape as a result of conservation of the electron magnetic moment. Under certain plasma conditions where the local electron plasma frequency ω pe is much less than the cyclotron frequency ω ce the distribution is found to be unstable to maser type radiation emission. We have established a laboratory-based facility that has verified many of the details of our original theoretical description and agrees well with numerical simulations. The experiment has demonstrated that the horseshoe distribution produces cyclotron emission at a frequency just below the local electron cyclotron frequency, with polarisation close to X-mode and propagating nearly perpendicularly to the electron beam motion. We discuss recent developments in the theory and simulation of the instability including addressing radiation escape problems, and relate these to the laboratory, space, and astrophysical observations. The experiments showed strong narrow band EM emissions at frequencies just below the cold-plasma cyclotron frequency as predicted by the theory. Measurements of the conversion efficiency, mode and spectral content were in close agreement with the predictions of numerical simulations undertaken using a particle-in-cell code and also with satellite observations confirming the horseshoe maser as an important emission mechanism in geophysical/astrophysical plasmas. In each case we address how the radiation can escape the plasma without suffering strong absorption at the second harmonic layer.

Photometric activity of the Herbig Be star MWC 297 over 25 years

The photometric behavior of the hot, young Herbig Be star MWC 297 on various time scales is studied using published data, as well as new observations. The series of photometric observations covers about 25 years. Over this time, the star showed low-amplitude (ΔV ≈ 0.3 m ) irregular variability modulated by large-scale cyclic variability with an amplitude close to 0.2 m and a period (or quasi-period) of 5.4 ± 0.1 yr. A detailed seasonal analysis of the data shows that the light curve of MWC 297 displays two types of photometric features: low-amplitude Algol-like fading with an amplitude close to 0.2 m and low-amplitude flares resembling the flares of UV Ceti stars, but being more powerful and having longer durations. The variations of the stellar brightness are accompanied by variations of the B-V and V -R colors: when the brightness decreases, B-V decreases, while V -R increases (the star reddens). The reddening law is close to the standard interstellar reddening law. Although the character of the brightness variability of MWC 297 resembles the photometric activity of UX Ori type stars, which is due to variations of their circumstellar extinction, its scale is very far from the scales observed for UX Ori stars. It is difficult to reconcile the level of photometric activity with the idea that MWC 297 is observed through its own gas-dust disk viewed almost edge-on, as has been suggested in several studies.

The Nature of the Light Variation of the Unique Binary DK CVn

AbstractWe present BVR observations of DK CVn from 2007 and 2008. We analysed the BVR light curves of the system and obtained the system's parameters. Using the ‘q-search’ method, we measured the mass ratio of the system (q) as 0.55. Taking the temperature of the primary component as 4040 K, the temperature of the secondary was found to be 3123 K. Several flares were detected, and the distributions of flare equivalent duration versus flare total duration were modelled using the One-Phase Exponential Association Function for these flares. The parameters of the model demonstrated that the flares are the same as those detected from UV Ceti stars. We also demonstrate that the variation at out-of-eclipse must be caused by some cool spot(s) on one of the components. The star is found to show two active longitudes in which the spots are mainly formed. Consequently, this study reveals that DK CVn should be a chromospherically active binary star.

Variation in the flare activity of the star UV Ceti

This is an analysis of the dependence of the flare activity of the well known flare star UV Ceti on the linear distance between the components of this binary system. It is shown that its flaring activity clearly depends on the mutual distance of the components, while this kind of variability is not seen in isolated flare stars.