J/A+A/688/A49 M dwarfs observed with eROSITA and TESS (Joseph+, 2024)
Simultaneous X-ray and optical variability of M dwarfs observed
with eROSITA and TESS.
Joseph W.M., Stelzer B., Magaudda E., Vicanek Martinez T.
<Astron. Astrophys. 688, A49 (2024)>
=2024A&A...688A..49J 2024A&A...688A..49J (SIMBAD/NED BibCode)
ADC_Keywords: Stars, M-type ; X-ray sources ; Photometry ; Optical ;
Photometry, infrared ; Effective temperatures
Keywords: stars: activity - stars: atmospheres - stars: coronae -
stars: flare - stars: rotation
Abstract:
M-dwarf stars are the most numerous stars in the Galaxy, and are
highly magnetically active. They exhibit bursts of radiation and
matter, called flares and coronal mass ejections which have the
potential to strongly affect the habitability of their planets.
We investigate variability through simultaneous optical and X-ray
observations, forming the largest statistical sample of M dwarfs
observed in this way so far. Such simultaneous observations at
different wavelengths, which correspond to emissions from different
layers of the stellar atmosphere, are required to constrain the flare
frequency and energetics and to understand the physics of flares.
We used light curves from the extended ROentgen Survey with an Imaging
Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma
mission (SRG) and the Transiting Exoplanet Survey Satellite (TESS) for
a sample of M dwarfs observed simultaneously with both instruments.
Specifically, we identified 256 M dwarfs in the TESS Southern
Continuous Viewing Zone (SCVZ), which corresponds to a sky area of
452.39deg2, with simultaneous TESS and eROSITA detection. For this
work, we selected the 25 X-ray brightest or most X-ray variable stars.
We used photometric data from Gaia and 2MASS to obtain stellar
parameters such as distances, colours, masses, radii, and bolometric
luminosities. X-ray fluxes and luminosities were determined from
observed eROSITA count rates using appropriate rate-to-flux conversion
factors. We defined and examined various variability diagnostics in
both wavebands and how these parameters are related to each other.
Our stars are nearby (mostly within ∼100pc ), rotating fast (Prot<9d),
and display a high optical flare frequency, as expected from the
selection of particularly X-ray-active objects. The optical duty cycle
- defined as the fraction of observing time in which the stars were
in a high activity state - is well correlated with the optical flare
rate and was therefore used as proxy for optical variability. The
X-ray and optical duty cycles are positively correlated, and there is
a trend of faster rotators tending to have higher X-ray and optical
variability. For stars with many X-ray flaring events, the chances of
these events being found together with optical flares are high. A
quantitative variability study of individual flares in the X-ray light
curves is severely affected by data gaps due to the low (4h) cadence
during the eROSITA all-sky survey. To mitigate this, we made use of
the optical flares observed with TESS combined with knowledge
accumulated from solar flares to put additional constraints on the
peak flux and timing of X-ray events. With this method we could
perform an exponential fit to 17 X-ray light curves in the aftermath
of an optical flare, and we find that the energies for these X-ray
flares are well correlated with the corresponding optical flare
energy. We also found two peculiar flaring events with
uncharacteristically long duration and high energies observed in both
their X-ray and optical light curves.
Despite the substantial uncertainties associated with our analysis,
which are mostly related to the poor sampling of the eROSITA light
curves, our results showcase in an exemplary way the relevance of
simultaneous all-sky surveys in different wavebands for obtaining
unprecedented quantitative information on stellar variability.
Description:
Stellar parameters of the 25 X-ray bright or variable stars in the
TESS Southern Continuous Viewing Zone from the Gaia, 2MASS and TESS
are presented. For each TIC or Gaia DR2 identifier the corresponding
2MASS ID, distance, proper motion corrections, Gaia magnitudes in the
BP,RP and G bands along with the derived Johnson-Kron-Cousin V
band magnitude, 2MASS J,H and Ks band magnitudes, stellar radius,
mass, and effective temperatures derived from J,H and V band
magnitudes as well as those derived from BP,RP, J and H magnitudes,
bolometric luminosity derived from bolometric corrections to Gaia
magnitudes, detection information (detected [1] or not detected [0])
for individual TESS sectors are. The following, which were obtained
from eRASS2 and eRASS3, are also presented: proper-motion corrected
X-ray coordinates (RA CORR and DEC CORR), separation between Gaia
coordinates and the matched eRASS coordinates, detection likelihood
and mean count rate in 0.2-5keV range, and X-ray flux and luminosity
derived from the count rate.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 303 25 Stellar parameters from Gaia, 2MASS, and TESS
tableb1.dat 231 25 Stellar parameters from eRASS
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See also:
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
IV/38 : TESS Input Catalog - v8.0 (TIC-8) (Stassun+, 2019)
II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 12 A12 --- TIC TESS Input Catalogue Identifier (TIC_ID)
14- 32 I19 --- GaiaDR2 Gaia ID from data release 2 (GaiaDR2)
34- 41 I8 --- 2MASS 2MASS ID (2MASS)
43- 61 F19.15 pc Dist Distance to the star from Gaia parallax
(Distance)
63- 83 F21.16 mas/yr pmRA Gaia proper motion right ascension direction
(PMRA)
85-104 F20.15 mas/yr pmDE Gaia proper motion declination direction
(PMDEC)
106-115 F10.7 mag BPmag Magnitude in Gaia BP band (GaiaBP)
117-125 F9.6 mag RPmag Magnitude in Gaia RP band (GaiaRP)
127-136 F10.7 mag Gmag Magnitude in Gaia G band. (GaiaG)
138-155 F18.15 mag Vmag Johnson-Kron-Cousin V from Gaia magnitudes
GBP and GRP (V)
157-162 F6.3 mag Jmag Magnitude in 2MASS J band (J)
164-169 F6.3 mag Hmag Magnitude in 2MASS H band (H)
171-176 F6.3 mag Ksmag Magnitude in 2MASS K s band (K_s)
178-196 F19.17 Rsun Rad Stellar radius based on radius empirical
relations from (Mann et al.,
2015ApJ...804...64M 2015ApJ...804...64M) using MKs (Radius)
198-216 F19.17 Msun Mass Stellar mass based on mass empirical relations
from (Mann et al., 2015ApJ...804...64M 2015ApJ...804...64M)
using MKs (Mass)
218-235 F18.13 K Teffvjjh Stellar effective temperatures based on
empirical relations from (Mann et al.,
2015ApJ...804...64M 2015ApJ...804...64M) using J, H and V
(Teffvjjh)
237-254 F18.13 K Teffbprpjh Stellar effective temperatures based on
empirical relations from (Mann et al.,
2015ApJ...804...64M 2015ApJ...804...64M) using BP,RP,J & H
(Teffbprpjh)
256-277 E22.17 10-7W Lbol Bolometric luminosities calculated from
Bolometric corrections to Gaia magnitudes
(L_Bol)
279 I1 --- TS27 Boolean value to indicate whether the star was
observed in TESS sector 27 (TESSsector27)
281 I1 --- TS28 Boolean value to indicate whether the star was
observed in TESS sector 28 (TESSsector28)
283 I1 --- TS29 Boolean value to indicate whether the star was
observed in TESS sector 29 (TESSsector29)
285 I1 --- TS30 Boolean value to indicate whether the star was
observed in TESS sector 30 (TESSsector30)
287 I1 --- TS31 Boolean value to indicate whether the star was
observed in TESS sector 31 (TESSsector31)
289 I1 --- TS32 Boolean value to indicate whether the star was
observed in TESS sector 32 (TESSsector32)
291 I1 --- TS33 Boolean value to indicate whether the star was
observed in TESS sector 33 (TESSsector33)
293 I1 --- TS34 Boolean value to indicate whether the star was
observed in TESS sector 34 (TESSsector34)
295 I1 --- TS35 Boolean value to indicate whether the star was
observed in TESS sector 35 (TESSsector35)
297 I1 --- TS36 Boolean value to indicate whether the star was
observed in TESS sector 36 (TESSsector36)
299 I1 --- TS37 Boolean value to indicate whether the star was
observed in TESS sector 37 (TESSsector37)
301 I1 --- TS38 Boolean value to indicate whether the star was
observed in TESS sector 38 (TESSsector38)
303 I1 --- TS39 Boolean value to indicate whether the star was
observed in TESS sector 39 (TESSsector39)
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Byte-by-byte Description of file: tableb1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 19 I19 --- GaiaDR2 Gaia ID from data release 2 (GaiaDR2)
21- 38 F18.14 deg RA2deg Proper motion corrected X-ray right ascension
during eRASS 2 (J2000) (RACORReRASS2)
40- 57 F18.14 deg RA3deg Proper motion corrected X-ray right ascension
during eRASS 3 (J2000) (RACORReRASS3)
59- 77 F19.15 deg DE2deg Proper motion corrected X-ray declination
during eRASS 2 (J2000) (DECCORReRASS2)
79- 97 F19.15 deg DE3deg Proper motion corrected X-ray declination
during eRASS 3 (J2000) (DECCORReRASS3)
99-118 F20.17 arcsec Sep2 Separation along a great circle between the
proper motion corrected Gaia-DR2 coordinates
and the matched eRASS coordinates RA CORR
eRASS2 and DEC CORR eRASS2
(Separation_eRASS2)
120-138 F19.16 arcsec Sep3 Separation along a great circle between the
proper motion corrected Gaia-DR2 coordinates
and the matched eRASS coordinates RA CORR
eRASS3 and DEC CORR eRASS3
(separation_eRASS3)
140-151 F12.6 --- DetLike2 Detection likelihood in energy band 0.2-5 KeV
during eRASS2 (DETLIKEeRASS2)
153-164 F12.6 --- DetLike3 Detection likelihood in energy band 0.2-5 KeV
during eRASS2 (DETLIKEeRASS3)
166-176 F11.9 ct/s MLrate02 Count rate in energy band 0.2-5 KeV during
eRASS2 (MLRATE0_eRASS2)
178-188 F11.9 --- MLrate03 Count rate in energy band 0.2-5 KeV during
eRASS2 (MLRATE0_eRASS3)
190-211 E22.17 10-7W Lx X-ray luminosity from the average MLRATE0
from eRASS 2 and eRASS 3 and Gaia distances
(L_x)
213-231 F19.10 mW/m2 Fxsurf X-ray surface flux from the average MLRATE0
from eRASS 2 and eRASS3 (Fxsurf)
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Acknowledgements:
Wilhelmina Joseph, joseph(at)astro.uni-tuebingen.de
(End) Patricia Vannier [CDS] 09-Jul-2024