Rachel A. Osten

FROM RADIO TO X-RAY: FLARES ON THE dMe FLARE STAR EV LACERTAE

We present the results of a campaign to observe flares on the M dwarf flare star EV Lacertae over the course of two days in 2001 September, utilizing a combination of radio continuum, optical photometric and spectroscopic, ultraviolet spectroscopic, and X-ray spectroscopic observations to characterize the multiwavelength nature of flares from this active, single, late-type star. We find flares in every wavelength region in which we observed. A large radio flare from the star was observed at both 3.6 and 6 cm and is the most luminous example of a gyrosynchrotron flare yet observed on a dMe flare star. The radio flare can be explained as encompassing a large magnetic volume, comparable to the stellar disk, and involving trapped electrons that decay over timescales of hours. Flux enhancements at 6 cm accompanied by highly negatively circularly polarized emission (?c ? -100%) imply that a coherent emission mechanism is operating in the corona of EV Lac. There are numerous optical white-light flares, and yet no signature of emission-line response from the chromosphere appears. Two small ultraviolet enhancements differ in the amount of nonthermal broadening present. There are numerous X-ray flares occurring throughout the observation, and an analysis of undispersed photons and grating events reveals no evidence for abundance variations. Higher temperatures are present during some flares; however, the maximum temperature achieved varies from flare to flare. There is no evidence for density variations during any flare intervals. In the multiwavelength context, the start of the intense radio flare is coincident with an impulsive optical U-band flare, to within 1 minute, and yet there is no signature of an X-ray response. There are other intervals of time when optical flaring and UV flaring is occurring, but these cannot be related to the contemporaneous X-ray flaring: the time-integrated luminosities do not match the instantaneous X-ray flare luminosity, as one would expect for the Neupert effect. We investigate the probability of chance occurrences of flares from disparate wavelength regions producing temporal coincidences but find that not all the flare associations can be explained by a superposition of flares due to a high flaring rate. We caution against making causal associations of multiwavelength flares based solely on temporal correlations for high flaring rate stars such as EV Lac.

Nonthermal Hard X-Ray Emission and Iron Kα Emission from a Superflare on II Pegasi

We report on an X-ray flare detected on the active binary system II Pegasi with the Swift telescope. The event triggered the Burst Alert Telescope (BAT) in the hard X-ray band on 2005 December 16 at 11:21:52 UT with a 10-200 keV luminosity of 2.2 ? 1032 ergs s-1?a superflare, by comparison with energies of typical stellar flares on active binary systems. The trigger spectrum indicates a hot thermal plasma with T ~ 180 ? 106 K. X-ray spectral analysis from 0.8 to 200 keV with the X-Ray Telescope and BAT in the next two orbits reveals evidence for a thermal component (T > 80 ? 106 K) and Fe K 6.4 keV emission. A tail of emission out to 200 keV can be fit with either an extremely high temperature thermal plasma (T ~ 3 ? 108 K) or power-law emission. Based on analogies with solar flares, we attribute the excess continuum emission to nonthermal thick-target bremsstrahlung emission from a population of accelerated electrons. We estimate the radiated energy from 0.01 to 200 keV to be ~6 ? 1036 ergs, the total radiated energy over all wavelengths ~10 38 ergs, the energy in nonthermal electrons above 20 keV ~3 ? 1040 ergs, and conducted energy 20 keV when compared to the upper and lower bounds on the thermal energy content of the flare. This marks the first occasion in which evidence exists for nonthermal hard X-ray emission from a stellar flare. We investigate the emission mechanism responsible for producing the 6.4 keV feature, and find that collisional ionization from nonthermal electrons appears to be more plausible than the photoionization mechanism usually invoked on the Sun and pre-main-sequence stars.

THE MOUSE THAT ROARED: A SUPERFLARE FROM THE dMe FLARE STAR EV LAC DETECTED BY SWIFT AND KONUS-WIND

We report on a large stellar flare from the nearby dMe flare star EV Lac observed by the Swift and Konus-Wind satellites and the Liverpool Telescope. It is the first large stellar flare from a dMe flare star to result in a Swift trigger based on its hard X-ray intensity. Its peak fX from 0.3 to 100 keV of 5.3 ? 10?8 erg cm?2?s?1 is nearly 7000 times larger than the stars quiescent coronal flux, and the change in magnitude in the white filter is ?4.7. This flare also caused a transient increase in EV Lacs bolometric luminosity (L bol) during the early stages of the flare, with a peak estimated L X /L bol ~ 3.1. We apply flare loop hydrodynamic modeling to the plasma parameter temporal changes to derive a loop semi-length of l/R = 0.37 ? 0.07. The soft X-ray spectrum of the flare reveals evidence of iron K? emission at 6.4 keV. We model the K? emission as fluorescence from the hot flare source irradiating the photospheric iron, and derive loop heights of h/R = 0.1, consistent within factors of a few with the heights inferred from hydrodynamic modeling. The K? emission feature shows variability on timescales of ~200 s which is difficult to interpret using the pure fluorescence hypothesis. We examine K? emission produced by collisional ionization from accelerated particles, and find parameter values for the spectrum of accelerated particles which can accommodate the increased amount of K? flux and the lack of observed nonthermal emission in the 20-50 keV spectral region.

Dissecting Capella's Corona: GHRS Spectra of the Fe XXI λ1354 and He II λ1640 Lines from Each of the Capella Stars*

We report on moderate (λ/Δλ = 20,000) and high (λ/Δλ = 90,000) resolution spectra of the 104-day period Capella binary system (HD 34029) obtained with Hubble Space Telescopes Goddard High-Resolution Spectrometer (GHRS) on 1995 September 9 and 1996 April 9. The observations include a long-duration, moderate-resolution spectrum of the coronal Fe XXI λ1354 line and both moderate- and high-resolution spectra of the He II λ1640 multiplet. Our objective in observing the Fe XXI line formed at T = 1 × 107 K is to determine for the first time the line shape parameters and the contribution of each stars corona. This is feasible because the GHRS can resolve the 53 km s-1 radial velocity separation of the stars. Our analysis led to four surprising results: (1) The contribution of the slowly rotating G8 III star to the total Fe XXI λ1354 flux is similar to that of the more active rapidly rotating G1 III star, in contrast to other UV lines formed at lower temperatures. (2) The centroid velocities of the Fe XXI lines from both stars are near their respective photospheric radial velocities. Thus, there is no evidence for downflows or winds, and the hot coronal plasma must be confined, presumably by strong, closed magnetic fields. This is the first direct kinematic evidence for magnetic confinement in the corona of a giant star. (3) The line widths are thermal, indicating very low turbulence (ξ < 23 km s-1) compared with the 54 km s-1 thermal speed. (4) Our analysis of Extreme-Ultraviolet Explorer (EUVE) spectra that include four Fe XXI lines shows that the volume emission measures of these lines are about a factor of 3 smaller than for the λ1354 line, which was observed several months after the closest EUVE observation. We consider possible explanations for this discrepancy and conclude that variability of the high-temperature coronal emission is the most likely explanation. We observed the He II λ1640 line twice with a time separation of 7 months, corresponding to about 2 orbital periods. The profiles are nearly identical, except for a large difference in flux near the expected radial velocity of the G8 III star. We believe that the broad He II emission produced by the G1 star is formed mostly by collisional excitation in its transition region, while the emission from the G8 star is produced predominantly by a photoionization/recombination process driven by EUV radiation from the G8 stars corona. The He II λ1640 flux that we measure from the G8 star is consistent with predictions based on EUVE flux measurements. The decrease in the λ1640 emission from the G8 star between the phase 0.73 and 0.78 measurements indicates that its EUV radiation is variable, as is observed by EUVE.

The Radio Spectrum of TVLM 513-46546: Constraints on the Coronal Properties of a Late M Dwarf

We explore the radio emission from the M9 dwarf TVLM 513-46546 at multiple radio frequencies, determining the flux spectrum of persistent radio emission, as well as constraining the levels of circular polarization. Detections at both 3.6 and 6 cm provide a spectral index measurement α (where Sν ∝ να) of -0.4 ± 0.1. A detection at 20 cm suggests that the spectral peak is between 1.4 and 5 GHz. The most stringent upper limits on circular polarization are at 3.6 and 6 cm, with V/I < 15%. These characteristics agree well with those of typical parameters for early- to mid-type M dwarfs, confirming that magnetic activity is present at levels comparable with those extrapolated from earlier M dwarfs. We apply analytic models to investigate the coronal properties under simple assumptions of dipole magnetic field geometry and radially varying nonthermal electron density distributions. Requiring the spectrum to be optically thin at frequencies higher than 5 GHz and reproducing the observed 3.6 cm fluxes constrains the magnetic field at the base to be less than about 500 G. There is no statistically significant periodicity in the 3.6 cm light curve, but it is consistent with low-level variability.

Radio, X‐Ray, and Extreme‐Ultraviolet Coronal Variability of the Short‐Period RS Canum Venaticorum Binary σ2 Coronae Borealis

We present the results of a coordinated observing campaign on the short-period (1.14 days) RS CVn binary ?2 Coronae Borealis with the VLA, ASCA, and RXTE. We also discuss earlier observations of the same system obtained by the Extreme Ultraviolet Explorer (EUVE). Dramatic coronal variability is present in all of these observations across the electromagnetic spectrum. ?2 CrB undergoes frequent large flares that occur close together in time. Radio flares are unambiguously correlated with X-ray flares; the X-ray flares peak as much as 1.4 hr before the corresponding radio maxima. Response to flares is more rapid in higher energy X-ray bandpasses, signaling an increase in temperature during the course of the flare. Flares are seen more frequently in the harder RXTE bandpass than in simultaneously taken ASCA observations. There is greater contrast between flaring and nonflaring conditions in the RXTE light curve. Complex flaring is seen in the radio at 3 and 6 cm, consistent with optically thin nonthermal gyrosynchrotron emission for most of the duration of the observation. Bursts of left-circularly polarized emission at 20 cm lasting ?15 minutes appear to be due to a coherent emission process. EUVE spectra reveal coronal material at ne ? 1012 cm-3, with no discernible density differences between flaring and quiescent time intervals. Quiescent ASCA spectra show lower than solar photospheric abundances with iron depleted by a factor of 4 from the solar photospheric value. The abundances increase during a large flare observed with ASCA, with iron enhanced to the solar photospheric value during the rise phase of the flare. Two-temperature fits to extracted spectra show a low-temperature quiescent value of 8 MK and a high-temperature component that varies from 22 MK during quiescence to 50 MK at the peak of the flare. Emission measure distributions measured from the ASCA observations are consistent with the temperatures derived from the discrete two-temperature fits and indicate the presence of very hot (>100 MK) plasma during the rise phase of the ASCA flare. Many of the flares observed with EUVE, ASCA, and RXTE show a double exponential decay phase, further highlighting the importance of this phenomenon in diagnosing flaring conditions. We also find that the observed second decay timescale cannot be explained by some of the currently used flare parameterizations.

Chandra, Extreme Ultraviolet Explorer, and Very Large Array Observations of the Active Binary System σ2 Coronae Borealis

We present the results of a coordinated observing campaign on the short-period RS CVn binary σ2 Coronae Borealis (F6V + G0V; Porb = 1.14 days) with the Very Large Array, the Extreme Ultraviolet Explorer, and the Chandra X-Ray Observatory High-Energy Transmission Grating Spectrometer. The radio emission is consistent with previously determined quiescent gyrosynchrotron properties. Multiple flares were seen with Extreme Ultraviolet Explorer, five occurring within two consecutive orbital periods. The first of these flares was observed with Chandra. The Chandra observations of σ2 CrB showed no systematic variations of line fluxes, widths, or Doppler shifts with orbital phase, nor any response in line width or offset due to the flare. This is consistent with both stars being equally active coronal emitters. We have developed a self-consistent method of spectral analysis to derive information from the line and continuum emissions concerning the distribution of plasma with temperature and elemental abundances. A bimodal temperature distribution is appropriate for both quiescent and flare intervals, with a stable peak at 6-8 MK and another variable enhancement at higher temperatures, with evidence for significant contribution from temperatures up to 50 MK during the flare, compared to 30 MK during quiescence. The iron abundance is subsolar during quiescence but is enriched by about a factor of 2 during a large flare seen with Chandra. The noble gas elements neon and argon show elevated abundances with respect to iron, but there is no clear evidence for any first ionization potential-based abundance pattern during quiescence or the flare. We have determined coronal electron densities from the helium-like ions O VII, Ne IX, Mg XI, and Si XIII, which imply densities ≥1010 cm-3. There is a small enhancement in the electron densities derived for the flare, but it is not statistically significant. We call attention to electron temperature constraints provided by the ratios of 1s2 1S0-1snp 1P1 transitions of the helium-like ions O VII, Ne IX, Mg XI, and Si XIII. The derived coronal electron pressures change by 1-2 orders of magnitude over a 25% change in temperature, implying nonisobaric coronal conditions. We find no evidence for significant departures from the effectively thin coronal assumption. The electron densities inferred from the soft X-ray spectra are inconsistent with cospatial gyrosynchrotron emission; further observations are necessary to discriminate the relative locations of the radio and soft X-ray-emitting plasma.

Discovery of radio emission from the tight M8 binary LP 349-25

We present radio observations of eight ultracool dwarfs with a narrow spectral type range (M8-M9.5) using the Very Large Array at 8.5 GHz. Only the tight M8 binary LP 349-25 was detected. LP 349-25 is the tenth ultracool dwarf system detected in radio wavelengths and its trigonometric parallax π = 67.6 mas, recently measured by Gatewood and coworkers, makes it the furthest ultracool system detected by the Very Large Array to date, and the most radio luminous outside of obvious flaring activity or variability. With a separation of only 1.8 AU, masses of the components of LP 349-25 can be measured precisely without any theoretical assumptions, allowing us to clarify their fully convective status and hence the kind of magnetic dynamo in these components, which may play an important role in explaining our detection of radio emissions from these objects. This also makes LP 349-25 an excellent target for further studies with better constraints on the correlations between X-ray, and radio emission and stellar parameters such as mass, age, temperature, and luminosity in ultracool dwarfs.

Wide-Band Spectroscopy of Two Radio Bursts on AD Leonis

We report high time-resolution broadband spectroscopic observations of two radio bursts on the classical flare star AD Leonis. The observations were acquired by the 305 m telescope at Arecibo Observatory on 2003 June 13-14. Using the Wideband Arecibo Pulsar Processor, these observations sampled a total bandwidth of 400 MHz, distributed over a 500 MHz frequency range, 1120-1620 MHz, with a frequency resolution of 0.78 MHz and a time resolution of 10 ms. The radio burst observed on June 13 is characterized by the presence of multitudes of short-duration (Δt ~ 30 ms), high brightness temperature (Tb > 1014 K), highly circularly polarized, fast-drift radio sub-bursts, with median bandwidths Δν/ν ~ 5%. The inverse drift rates are small and have a symmetric distribution (both positive and negative frequency drifts), with a Gaussian FWHM inverse drift rate of 4.5 × 10-4 s MHz-1. The fast-drift sub-bursts occur at a mean rate of 13 s-1 and show no evidence for periodic recurrence. The fast-drift radio events on AD Leo are highly reminiscent of solar decimetric spike bursts. We suggest that the emission is due to fundamental plasma radiation. A second highly circularly polarized radio burst, recorded June 14, has markedly different properties: a smoothly varying intensity profile characterized by a slow drift in frequency with time (-52 MHz s-1). Under the assumption that the source is due to a disturbance propagating through the low corona, a source size of 0.1-1 R is inferred, implying a brightness temperature range 6 × 1011-6 × 1013 K; another example of a coherent radio burst.

A Chandra X-ray detection of the L dwarf binary Kelu-1 Simultaneous Chandra and Very Large Array observations

Context. Magnetic activity in ultracool dwarfs, as measured in X-rays and Hα, shows a steep decline after spectral type M7-M8. So far, no L dwarf has been detected in X-rays. In contrast, L dwarfs may have higher radio activity than M dwarfs. Aims. We observe L and T dwarfs simultaneously in X-rays and radio to determine their level of magnetic activity in the context of the general decline of magnetic activity with cooler effective temperatures. Methods. The field L dwarf binary Kelu-1 was observed simultaneously with Chandra and the Very Large Array. Results. Kelu-1AB was detected in X-rays with LX = 2.9 +1.8 −1.3 × 10 25 erg s −1 , while it remained undetected in the radio down to a 3σ limit of LR ≤ 1.4 × 10 13 erg s −1 Hz −1 . We argue that, whereas the X-ray and Hα emissions decline in ultracool dwarfs with decreasing effective temperature, the radio luminosity stays (more or less) constant across M and early-L dwarfs. The radio surface flux or the luminosity may better trace magnetic activity in ultracool dwarfs than the ratio of the luminosity to the bolometric luminosity. Conclusions. Deeper radio observations (and at short frequencies) are required to determine if and when the cut-off in radio activity occurs in L and T dwarfs, and what kind of emission mechanism takes place in ultracool dwarfs.