791 research outputs found
Flares observed with XMM-Newton and the VLA
We present lightcurves obtained in X-ray by the XMM-Newton EPIC cameras and
simultaneous radio lightcurves obtained with the VLA for five active M-type
flare stars. A number of flare events were observed, and by comparing radio
with X-ray data, we consider various possible flare mechanisms. In cases where
there seems to be a clear correlation between radio and X-ray activity, we use
an energy budget argument to show that the heating which leads to the X-ray
emission could be due to the same particles emitting in the radio. In cases
where there is radio activity without corresponding X-ray activity, we argue
that the radio emission is likely to arise from coherent processes involving
comparatively few particles. In one case, we are able to show from polarization
of the radio emission that this is almost certainly the case. Cases for which
X-ray activity is seen without corresponding radio activity are more difficult
to explain. We suggest that the heating particles may be accelerated to very
high energy, and the resulting synchrotron radio emission may be beamed in
directions other than the line of sight.Comment: 12 pages, 10 figure
X-rays from T Tau: A test case for accreting T Tauri stars
We test models for the generation of X-rays in accreting T Tauri stars (TTS),
using X-ray data from the classical TTS T Tau. High-resolution spectroscopy
from the Reflection Grating Spectrometers on XMM-Newton is used to infer
electron densities, element abundances and the thermal structure of the X-ray
source. We also discuss the ultraviolet light curve obtained by the Optical
Monitor, and complementary ground-based photometry. A high-resolution image
from Chandra constrains contributions from the two companions of T Tau N. The
X-ray grating spectrum is rich in emission lines, but shows an unusual mixture
of features from very hot (~30 MK) and very cool (1-3 MK) plasma, both emitted
by similar amounts of emission measure. The cool plasma confirms the picture of
a soft excess in the form of an enhanced OVII/OVIII Lya flux ratio, similar to
that previously reported for other accreting TTS. Diagnostics from lines formed
by this plasma indicate low electron densities (<~ 1E10 cm-3). The Ne/Fe
abundance ratio is consistent with a trend in pre-main sequence stars in which
this ratio depends on spectral type, but not on accretion. On the basis of line
density diagnostics, we conclude that the density of the cool ``soft-excess''
plasma is orders of magnitude below that predicted for an accretion shock,
assuming previously determined accretion rates of (3-6)E-8 M_sun/y. We argue
that loading of magnetic field lines with infalling material suppresses the
heating process in a part of the corona. We thus suggest that the X-ray
production of T Tau is influenced by the accretion process although the X-rays
may not form in the bulk of the accretion footpoints.Comment: 12 pages, 7 figures, A&A style. Accepted by A&A, to appear in a
special section/issue dedicated to the XMM-Newton Extended Survey of the
Taurus Molecular Cloud (XEST). See also
http://www.issibern.ch/teams/Taurus/papers.htm
A Chandra X-ray detection of the L dwarf binary Kelu-1: Simultaneous Chandra and Very Large Array observations
Magnetic activity in ultracool dwarfs, as measured in X-rays and H,
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. 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. The field L
dwarf binary Kelu-1 was observed simultaneously with Chandra and the Very Large
Array. Kelu-1AB was detected in X-rays with erg/s, while it remained undetected in the radio down to a limit of erg/s/Hz. We argue that,
whereas the X-ray and H 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. 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.Comment: Accepted for publication as a Letter in Astronomy & Astrophysic
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