Issue |
A&A
Volume 516, June-July 2010
|
|
---|---|---|
Article Number | A58 | |
Number of page(s) | 7 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361/201014088 | |
Published online | 28 June 2010 |
Doppler tomography of the black hole
binary A0620-00 and the origin
of chromospheric emission in quiescent X-ray binaries![[*]](/icons/foot_motif.png)
J. I. González Hernández1 - J. Casares2
1 - Dpto. de Astrofísica y Ciencias de la Atmósfera, Facultad de
Ciencias Físicas, Universidad Complutense de Madrid, 28040
Madrid, Spain
2 - Instituto de Astrofísica de Canarias, C/ Via Láctea s/n, 38200 La
Laguna, Spain
Received 18 January 2010 / Accepted 28 March 2010
Abstract
Context. Doppler tomography of emission line
profiles in low-mass X-ray binaries allows us to disentangle the
different emission sites and study the structure and variability of
accretion disks.
Aims. We present UVES high-resolution spectroscopic
observations of the black hole binary A0620-00 at quiescence.
Methods. These spectroscopic data constrain the
orbital parameters
d and
.
These values, together with the mass ratio
,
imply a minimum mass for the compact object of
,
consistent with previous works.
Results. The H
emission from the accretion disk is much weaker than in previous
studies, possibly because of a decrease in disk activity. Doppler
imaging of the H
line shows for the first time a narrow component coming from the
secondary star, with an observed equivalent width of
Å,
perhaps associated to chromospheric activity. Subtracting a K-type
template star and correcting for the veiling of the accretion disk
yields an equivalent width of
Å.
A bright hot spot is also detected at the position where the gas stream
trajectory intercepts the accretion disk.
Conclusions. The H
flux associated to the secondary star is too large to be powered by
X-ray irradiation. It is comparable to those observed in RS CVn
binaries with similar orbital periods and is probably triggered by the
rapid stellar rotation.
Key words: black hole physics - accretion, accretion disks - binaries: close - stars: individual: V616 Mon - X-rays: binaries - stars: activity
1 Introduction
The black hole binary A0620-00 (V616 Mon) is one of the most studied
low-mass X-ray binaries (LMXBs) and is considered as the prototype soft
X-ray
transient (SXT). It was discovered in 1975 by the Ariel 5
(Elvis et al. 1975)
satellite during an X-ray outburst where the optical
brightness of the system increased by roughly 6 mag in a few
days.
One year and several months later the system returned to its
quiescent state at mag.
The spectrum of a stellar
counterpart was then identified and classified as a K5V-K7V star
(Murdin
et al. 1980; Oke 1977). Later spectroscopic
observations allowed determination of the orbital period of the
secondary star at
7.75 h
(McClintock & Remillard 1986),
which implied the presence of a black hole of minimun mass
3.1
(Marsh et al. 1994).
The orbital inclination of the system was later estimated from
IR
light curves at
,
implying a primary black hole mass
of
(Gelino et al. 2001; see also Shahbaz et al. 1994).
However, this study adopts a K4V
stellar component with
K,
300 K cooler than the
effective temperature
K derived by González Hernández et al. (2004)
from
high-resolution optical spectroscopic observations. This may affect the
required veiling in the IR, hence the
derived inclination and black hole mass (Hynes
et al. 2005). Furthermore, Cantrell
et al. (2010) present evidence of substantial disk
contamination in their IR light curves and find
,
which
translates into a lower black hole mass of
.
On the other hand, contradictory results have been reported using
low-resolution IR spectra. Harrison
et al. (2007) find a very small or negligible disk
veiling at IR
wavelengths, whereas Froning
et al. (2007) conclude that it can be 18% in the H
band. The latter also argue that
K is needed to fit
their observations;
however, we must note here that most studies just adopt a
based
on spectral classification, derived through comparison
with low-resolution template spectra, without determining
the true
of the
template (Hynes et al. 2005).
A similar
inconsistency has been found for the black hole binary XTE
J1118+480 for which Gelino
et al. (2006) adopted a
=4250 K
where
González
Hernández et al. (2006,2008b) derive a spectroscopic
of
K.
The accretion disk of A0620-00 has been studied in the UV (McClintock et al. 1995) and the optical (Marsh et al. 1994; Orosz et al. 1994), allowing investigation of the inner and outer disk. Both works in the UV and optical seem to agree that the accretion disk is in a true quiescent state; however, this does not mean that the disk is inactive and probably its variability appears to be relevant (Shahbaz et al. 2004; Cantrell et al. 2010,2008). Shahbaz et al. (2004) also suggest that the accretion disk in A0620-00 could be eccentric which may have been confirmed by more recent observations reported by Neilsen et al. (2008). As pointed out by these authors, to determine the definite mass of the compact object, it is very important to understand the structure and variability of the accretion disk as exemplified by Cantrell et al. (2010).
Here we present high-resolution spectroscopy of A0620-00 where
we detect clear emission arising from the secondary star in the
H
Doppler map. This feature has only been observed
before in the systems GU Mus (Casares
et al. 1997), Nova Oph 77 (Harlaftis
et al. 1997), Cen X-4 (D'Avanzo et al. 2005; Torres
et al. 2002) and Nova
Scorpii 1994 (Shahbaz et al. 1999).
These data also allow us to revisit the orbital parameters of the
system, which we find to be consistent with previous studies.
2 Observations
We obtained 20 spectra of A0620-00 with the UV-Visual Echelle
Spectrograph (UVES) at the European Southern Observatory (ESO),
Observatorio Cerro Paranal (Chile), using the
8.2 m Very Large Telescope (VLT) on 5, 17, and 21 December
2000, covering the
spectral regions 4800-5800 Å,
and
5800-6800 Å at resolving power
,
with a dispersion of 0.029 and
0.035 Å pixel-1 for the blue and red
arms, respectively. The total exposure time was 2.9 h. The
spectra were reduced in a standard
manner using the UVES reduction package within the MIDAS environment.
The exposure time was fixed at
500 s to minimize the effects of
orbital smearing which, for the orbital parameters of A0620-00, is in
the
range 2-54
,
so in some cases larger than the
instrumental resolution of
7
.
The signal-to-noise
ratio per pixel in the individual spectra is
4 and 8 in
continuum regions close to the H
and H
lines,
respectively. Thus the spectra were binned in wavelength with steps of
0.1 Å pixel-1, increasing the
and 12 at H
and
H
,
respectively.
In this paper, we also use the K3-K4.5V template star
HD 209100,
observed on 12 November 2000 with the Coralie spectrograph,
installed
on the 1.2 m Euler Swiss Telescope at the ESO La Silla
Observatory
(Chile), with a spectral resolution of
.
This spectrum was rebinned to the
same step and degraded to the same resolution of the spectra of
A0620-00.
2.1 Revised orbital parameters
We extracted the radial velocities by cross-correlating each UVES
spectrum of the target with the K3-4.5V template spectrum of HD209100,
using the software MOLLY developed by Marsh. Here we only concentrated
on the H
spectra, which cover
the spectral regions
4800-5800 Å,
because
they contain a larger number of metallic absorption lines for the
cross-correlation so provide smaller errorbars by a factor
of 2. In any case, the result of cross-correlating the H
spectra
yields identical results. Prior to the cross-correlation, the template
spectrum was rotationally broadened by 96
to
match the rotational
velocity of the donor star (see Sect. 2.2).
A
sine wave fit,
,
to the obtained
velocities yields the following orbital solution (see
Fig. 1):
,
,
and
d, where T0 is defined as
the heliocentric time of the inferior conjunction of the companion
star. The orbital period was initially set to the value reported by McClintock & Remillard (1986)
and subsequently refined to
d after dividing the difference between our T0
and the one quoted by Orosz
et al. (1994) by an integer number of cycles
(17957). The quoted uncertainties are at 1
,
and we have rescaled the errors by a factor 1.4 so that the minimum
reduced
is 1.0.
This orbital period, P, together with the
velocity amplitude of the orbital motion of the secondary star, K2,
leads to
a mass function of
.
Our value
is consistent at the 1
level with previous results
by Marsh et al. (1994,
) and Neilsen et al. (2008,
).
The derived radial velocity of the centre of mass of the
system also agrees at the 1
with previous studies
(e.g.
,
Marsh et al. 1994).
Our
high-resolution data has better spectral
resolution than all previous data by a factor greater than 10. For
instance, the studies presented
by Marsh et al. (1994)
and Neilsen et al. (2008)
used spectra with a resolving power
of 70 and 130
,
respectively. Despite this, Neilsen
et al. (2008)
provides a significantly lower uncertainty for the semiamplitude
velocity of 0.5
given
their resolving power. This is because our orbital coverage is much
less complete than
previous studies, especially around orbital phase 0.75.
![]() |
Figure 1:
Top panel:
radial velocities of the
secondary star in A0620-00 folded on the orbital solution of the data
with best fitting sinusoid. Individual velocity errors are |
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2.2 Rotational velocity
Following Marsh et al. (1994),
we also computed the optimal
by
subtracting broadened versions of the template star, in
steps of 1
,
and minimizing the residual. We used a spherical
rotational profile with linearized limb darkening
,
which is appropriate for the stellar parameters of the donor star (González Hernández et al. 2004)
and our wavelength range
5250 Å
(Al-Naimiy 1978). The best fit
corresponds to a
,
but the error is purely formal since it does not account for
systematics due to our choice of
,
which is adequate for the
continuum but likely overestimated for the absorption lines (Collins & Truax 1995). In an
attempt to derive a more
realistic error, we also computed the rotational broadening for
the extreme case
and find
.
A more conservative value is then provided by the mean of the
two determinations, i.e.,
.
As a result of
the optimal subtraction we also find that the donor star contributes
80% to the
total light in the H
region and
85%
in the H
region. At this point we note that the absorption
features of the secondary star can be smeared by as much as
53
according to the length of the exposure times and the orbital phase of
the observations. We also tried to correct for this by simulating
the smearing in the template spectrum according to Casares
et al. (1997) but
found that the effect is completely negligible.
In the case of tidally locked Roche-lobe filling stars, the
rotational velocity relates to the velocity semiamplitude, K2,
and
the mass ratio, q=M2/M1=K1/K2,
through the expression
(e.g. Wade & Horne 1988).
Our derived rotational velocity, combined with our value of K2,
implies a binary mass ratio
.
However, because of the Roche lobe geometry,
displays a
10%
modulation with maxima at the orbital quadratures
(Casares et al. 1996).
Most of our spectra are located at orbital phases around 0.25, so our
previous determination is likely to
be overestimated by a factor
5%, which would make it consistent
with Marsh et al. (1994)
and Neilsen et al. (2008).
Therefore,
we think that the value provided by Marsh
et al. (1994),
,
is more realistic. Using this result and our value for
K2, we obtain
,
which is the same value given in
Neilsen et al. (2008).
This, combined with the mass function, provides a minimum mass for the
compact object of
.
![]() |
Figure 2:
Equivalent width of H |
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3 Ellipsoidal variations in the H
line
We determined the equivalent width (EW) of H
profiles for
the 20 UVES spectra of A0620-00 by adding numerically the normalized
fluxes multiplied by the wavelength step. In Fig. 2 we depict the
evolution of the EW of H
with orbital phase. The error bars were determined by measuring the
change in EW from varying the continuum location
according to the S/Nin the
continuum regions.
Despite our limited phase coverage, the orbital variation in the EWs
hints at the ellipsoidal varibility seen by Marsh
et al. (1994) and
Neilsen et al. (2008)
due to the dominance of the donor star's light.
We have also displayed for comparison the solution of the fit
to a
sinusoid plus a constant to the data of Marsh
et al. (1994), .
Our mean EW is 43 Å, significantly lower than in Marsh et al. (1994), whose
spectroscopic data were obtained in December
1991, and much lower than that in Neilsen
et al. (2008), whose data were
obtained in December 2006. This may come from an increase in the
continuum from the accretion disk, a decrease in the H
flux or
a combination of the two. Despite being in quiescence,
A0620-00 shows significant
variability and different states associated with changes in disk
activity (e.g. Cantrell et al.
2008). Therefore, the lower EW seen in
our
spectra over previous studies could well be associated to a
different level of accretion disk activity. The relative contribution
of the accretion disk to the
continuum in the H
region is
15%
in our data and
6% in Marsh et al. (1994). This,
combined with the variation in EWs
between the two data sets, implies that the H
flux has dropped
33% in our
data with respect to Marsh
et al. (1994). In fact,
Cantrell et al. (2008)
find that A0620-00 was in a lower level of activity (what
they called ``passive'' state) most of the time between December 1999
and December 2003. Our observations were taken in December 2000,
when the system was in the ``passive'' state, and this seems to be the
reason for the low level of H
flux that we see.
4 Doppler
images of H
and H
![]() |
Figure 3:
Doppler images of H |
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We have used Doppler tomography (Marsh
& Horne 1988) to map the H
and
H
emission in A0620-00. By combining the orbitally-resolved line
profiles we were able to reconstruct the brightness distribution of
the system in velocity space. The result is displayed in
Fig. 3.
The Doppler maps were built by combining the 20
profiles. The spectra of H
and H
were continuum
subtracted, leaving the pure line emission, and rebinned to a velocity
scale of 9
per
pixel. The top panels show trailed spectra and
the bottom panels the corresponding Doppler images. The location of
the main components in the system, such as the Roche lobe of the
secondary star and the predicted gas stream trajectory and the
Keplerian trajectory of the disk along the stream are indicated. These
tracks were calculated by adopting the values
and
.
Although the disk activity seems to be lower than in previous studies
(see Sect. 3),
we can see a clear detection of the bright
spot between the velocity trajectories of the gas stream and of the
disk along the stream, as previously reported in Neilsen
et al. (2008); Marsh et al. (1994)
and in other systems.
The Doppler maps shown in Fig. 3 were
constructed without subtracting any K-type template spectrum from the
original data. We note here an intense H
emission
exactly at the position of the secondary star. The emission coming from
the secondary star can also be identified
through the short tracks of the expected S-wave emission on the
trailed spectrogram. The H
Doppler map also shows emission around the position of
the Roche lobe but it is less clear. The S/N
of the spectra at
H
is significantly lower and the line is also close the edge of
the CCD, which makes this map somewhat uncertain.
5 Narrow H
component emission from the secondary
star
![]() |
Figure 4:
Isolating the H |
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Companion stars of X-ray transients are tidally locked, with short
orbital periods, hence high rotational velocities. Thus, it
is not surprinsing that the secondary star in A0620-00, with
,
is chromospherically active. However, this is the
first time that chromospheric emission from the secondary star has
been clearly detected in this system before subtracting the spectrum of
a
template K-type star. Marsh
et al. (1994) do detect a narrow
S-wave in their trailed spectra but only after subtracting the
template star, which, as a matter of fact, provides a limit on the
chromospheric emission set by the equivalent width of the H
absorption
in the template star. The reason this chromospheric component has not
been detected in
previous studies seems to hold on variability arguments, perhaps
related to the low level of accretion disk activity seen in our data
(which otherwise might somehow dilute the chromospheric
component) or simply caused by variations in the donor star's activity
(magnetic cycles).
We attempted to detrend the narrow H
component from the broad
underlying accretion disk contribution. The K3-4.5V template spectrum
used in this work shows an H
line in absorption of
1.3-1.4 Å.
This could be considered as a lower limit to
the EW of the chromospheric emission of the
secondary star in A0620-00. In fact, the derived effective temperature
of
K by
González Hernández et al. (2004)
favours a slightly earlier spectral type, i.e. K2V,
which would shift this limit up to 1.4-1.5 Å.
In the Doppler-corrected average of the 20 spectra of the
system, this
narrow component appears as a narrow feature on top of the broad
doubled-peak H
emission produced by the accretion disk (see
spectrum (c) of Fig. 4).
The emission of the accretion disk
can be approximated by a Doppler-shifted average (in the rest frame of
system centre-of-mass) of the double-peaked profile (spectrum (a) of
Fig. 4).
This is not exactly the real accretion disk
emission profile since it also contains the smeared S-wave component
of the secondary star. We then shifted in velocity 20 versions of
spectrum (a) to the rest frame of the secondary star at the given
phase of each individual spectrum and again computed the average using
the same weights as before. The resulting profile (spectrum (b) of
Fig. 4)
was subsequently subtracted from the original Doppler-corrected
average, i.e., spectrum (c), leaving the narrow H
component associated with the secondary
star completely isolated (spectrum (d) of Fig. 4). After this
subtraction,
we added a constant value equal to 1 since the continuum level of the
resulting spectrum was at zero, in order to avoid to work with
negative spectral points. We performed a second
iteration of the whole process by first subtracting each
original spectrum by spectrum (d), properly corrected by the star's
velocity at each orbital phase, in order to eliminate the smeared
narrow component of the secondary star from the profiles (a) and (b).
In fact, the spectra displayed in
Fig. 4
shows the results of this last iteration.
The spectrum (d) of Fig. 4 is the normalized
spectrum of the secondary star without the accretion disk contribution.
We measured the equivalent width of the narrow H
component from the secondary star in this spectrum to be
Å. The error bar has been estimated by changing the position of the
continuum, taking the S/Nof the
spectrum of the secondary star into account. In order to measure the
real chromospheric emission H
line,
one needs to subtract the photospheric component using a template star
with the same spectral type (see e.g. Montes
et al. 1995).
We then subtracted a K-type template star properly broadened (spectrum
(e)) and scaled by a veiling factor of 0.76, and find an EW
of
Å (spectrum (f)), which has to be corrected for the same veiling
factor, leading to a final value of EW(H
Å.
Table 1:
H
fluxes, equivalent widths, and Rossby numbers of the LMXBs displayed in
Fig. 5.
6 Discussion
Strassmeier et al. (1990)
measured the HEWs
of a sample of F6-M2 single and binary stars, showing that the
H
EW increases towards shorter rotation periods, P.
Extrapolation of this trend EW vs. P
at the orbital period of A0620-00 provides an expected EW
of
2.3 Å,
assuming that the
secondary star is a dwarf main-sequence star and that its rotation is
sychronized with the orbital motion. This EW is
marginally consistent (at 1.5
)
with the observed EW of the secondary star.
However, the extrapolation might
not be adequate since chromospheric features powered by rotation
typically saturates for period shorter than 1-3 days
(e.g. Cardini & Cassatella 2007,
for chromospheric Mg II lines).
We can also compare the H
EW, converted into flux following
the approach of Soderblom
et al. (1993), with the observations of field stars
and binaries. Thus, the H
flux at the stellar surface,
is
erg cm-2 s-1
(see Table 1).
We use a similar prescription than that
of Eq. (2) in Soderblom
et al. (1993), i.e.
,
where
is the continuum flux at H
and is derived using the flux calibration of Hall
(1996),
.
Here we used (B-V)0=0.965
estimated from theoretical colours
(Bessell et al. 1998)
according to the stellar parameters of the secondary
star (González Hernández et al.
2004). The surface flux is usually normalized to the
bolometric flux, i.e.
.
The connection between chromospheric activity and rotation is
obtained by studying the correlation
with the Rossby number
,
where
is the convective turnover time, P, the rotation
period of the secondary star and
its radius. Using the Rossby number is usually
preferred over P and
since it does not depend on the mass of the star.
![]() |
Figure 5: Chromospheric flux ratios vs. Rossby numbers of chromospherically active single stars from López-Santiago et al. (2010, filled circles) and chromospherically active binary systems (RS CVn and BY Dra classes) from Montes et al. (1995, open circles). Low-mass X-ray binaries are also depicted: A0620-00 (diamond), Nova Muscae 1991 (square), Cen X-4 (filled triangle, from Torres et al. 2002; open triangle, from D'avanzo et al. 2005), and Nova Scorpii 1994 (inverted triangle). |
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For the case of A0620-00, we adopt K (González Hernández et al. 2004),
d and
d,
computed from Eq. (4) of
Noyes et al. (1984).
We derive
and
,
and this is listed in Table 1,
together with values derived for other quiescent X-ray transients. In
Fig. 5
we compare these values with those of
chromospherically active single stars (López-Santiago
et al. 2010)
and binary systems (RS CVn and BY Dra classes) from Montes
et al. (1995).
Our value of the Rossby number places the
secondary star of A0620-00 in the region of activity saturation, where
all measurements
tend to the same average value of
,
and
is consistent with the general trend. The other X-ray binaries show
similar results, except for the black
hole X-ray binary Nova Scorpii 1994, which displays too high an
value for its relatively low Rossby number. This is even more evident
when comparing the H
fluxes, with
for Nova Scorpii 1994, significantly higher,
by almost one order of magnitude, than the saturation level
at
.
In addition, D'Avanzo et al.
(2005) also
suggest that the H
EW of the secondary star in
Cen X-4 is correlated with the veiling of the accretion disk (see
Table 1),
by comparing their values with those given by
Torres et al. (2002).
Although this is not expected in a chromospheric activiy scenario, the
two Cen X-4 points in Fig. 5 fall in the
saturation
region, together with other X-ray and chromospherically active
binaries. The behaviour seen in Nova Scorpii 1994 and Cen X-4 suggests
that rapid rotation might not be the only explanation for
the narrow H
feature, at least in these LMXBs, but perhaps a combination of rotation
and reprocessing of X-ray flux from
the accretion disk into H
photons in the secondary star.
It is therefore worth investigating in A0620-00 if X-ray heating could
be an alternative explanation for this feature.
The system A0620-00 has been observed in quiescence with the Chandra
X-ray satellite, providing a 0.5-10 keV unabsorbed flux
erg cm-2 s-1(Gallo et al. 2006). The
X-ray flux at the stellar surface can be computed as
erg cm-2 s-1,
where we have adopted an orbital separation a=4.47
and a
distance d=1.2 kpc. This means that almost
92% of the incident
X-ray radiation would have to be reprocessed to H
photons in order to power the observed H
emission.
Following Hynes et al. (2002),
,
where f1 is the fraction of
X-ray emission intercepted by the
companion; i.e., the solid angle subtended by the companion from the
compact object (
),
and
is the fraction of input energy emitted in H
(Hynes et al. 2002,
and references therein). Adopting
from González Hernández et al. (2004),
we obtain
.
This number is significantly lower than the observed value, which
indicates that the incident X-ray irradiation is not enough to produce
the narrow H
line in the secondary star. D'Avanzo
et al. (2005) have also derived these quantities for
the case of the neutron star binary Cen X-4 and find both estimates to
be consistent, because the X-ray flux in Cen X-4 is
erg cm-2 s-1,
i.e. almost two orders of magnitude higher than in A0620-00.
There is still a remote posibility that the source of
irradiating photons is hidden away in the EUV (extreme UV) energy
range, between 100-1200 Å. Although this energy range is not
directly observed, we can guess roughly how much flux is involved
through interpolating the nearby soft X-ray and the far-UV (FUV)
emission.
The FUV flux (in the range 1350-2200 Å) has been determined at
erg cm-2 s-1
(McClintock et al. 1995),
i.e. similar to the X-ray flux.
This and the absence of He II
4686 Å
line emission in the optical spectrum (Marsh
et al. 1994) suggest that flux in the EUV should be
about
10-13 erg cm-2 s-1.
Even if we consider all the ionizing photons (X-ray+EUV+FUV), the total
flux would be roughly three times the X-ray flux, i.e.
erg cm-2 s-1.
Therefore, if we assume that the incident irradiation is
,
then the 31% of the incident radiation would have to be reprocessed to H
photons in order to power the observed H
emission,
which still is too high a fraction compared to the
fraction previously estimated as below 1%.
7 Summary
We have presented high-resolution UVES/VLT spectroscopy of the black
hole binary A0620-00 at quiescence. Our orbital parameters are
consistent with previous studies by Marsh
et al. (1994) and Neilsen
et al. (2008). In particular, we derived
d and
.
These values, together with the mass ratio
,
imply a minimum mass for the
compact object of
.
We also performed Doppler tomography of the accretion disk
emission
and discovered emission at the position of the
secondary star in the Doppler maps of H
and H
,
which were not detected in previous studies. We isolated the
chromospheric H
emission
from the secondary star and measured an equivalent width of
Å. This equivalent
width is too large to be
explained by X-ray and/or UV irradiation from the inner accretion disk;
therefore, chromospheric activity, induced by rapid rotation, seems the
most likely origin of this feature in the black hole binary A0620-00.
J.I.G.H. acknowledges support from the project AYA2008-00695 of the Spanish Ministry of Education and Science. J.C. acknowledges support from the Spanish Ministry of Science and Technology through the project AYA2007-66887. This work has been partially funded by the Spanish MICINN under the Consolider-Ingenio 2010 Program grant CSD2006-00070: First Science with the GTC (http://www.iac.es/consolider-ingenio-gtc). We are grateful to Tom Marsh for the use of the MOLLY analysis package. J.I.G.H. is grateful to Javier López Santiago for providing us with the data of chromospherically active single and binary stars. J.I.G.H. acknowledges thanks helpful discussions with Javier López Santiago, David Montes and Raquel Martínez Arnáiz.
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Footnotes
- ... binaries
- Based on observations obtained with UVES at VLT Kueyen 8.2 m telescope in programme 66.D-0157(A).
All Tables
Table 1:
H
fluxes, equivalent widths, and Rossby numbers of the LMXBs displayed in
Fig. 5.
All Figures
![]() |
Figure 1:
Top panel:
radial velocities of the
secondary star in A0620-00 folded on the orbital solution of the data
with best fitting sinusoid. Individual velocity errors are |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Equivalent width of H |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
Doppler images of H |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Isolating the H |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Chromospheric flux ratios vs. Rossby numbers of chromospherically active single stars from López-Santiago et al. (2010, filled circles) and chromospherically active binary systems (RS CVn and BY Dra classes) from Montes et al. (1995, open circles). Low-mass X-ray binaries are also depicted: A0620-00 (diamond), Nova Muscae 1991 (square), Cen X-4 (filled triangle, from Torres et al. 2002; open triangle, from D'avanzo et al. 2005), and Nova Scorpii 1994 (inverted triangle). |
Open with DEXTER | |
In the text |
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