Radio Images of a Large Stellar Coronal Loop on Algol

Radio Images of A Large Stellar Coronal Loop on Algol

W. M. Peterson*, R. L. Mutel*, M. Gudel**,¨ W. M. Goss†

*Department of Physics and Astronomy, University of Iowa, Van Allen Hall, Iowa City, Iowa

52240, USA

**ETH Zurich, Institute of Astronomy, 8093 Zurich, Switzerland

†National Radio Astronomy Observatory, Pete V. Domenici Science Operations Center, 1003

Lopezville Road, Socorro, New Mexico 87801, USA

The close binary Algol contains a radio-bright K subgiant star in a very close (0.062 AU), rapid (2.86 day) orbit with a main sequence B8 star. Since the rotation periods of the two stars are tidally locked to the orbital period, the consequent rapid rotation drives a robust magnetic dynamo. A large body of evidence points to the existence of an extended, complex coronal magnetosphere originating at the cooler K subgiant1–4. The detailed morphology of the subgiant’s corona and its possible interaction with its companion are unknown, though theory predicts that the coronal plasma should be conﬁned in magnetic loop structure5, as seen on the Sun. Here we report multi-epoch radio imaging of the Algol system, in which we see a large, persistent coronal loop approximately one subgiant diameter in height, whose base is straddling the subgiant and whose apex is oriented toward the B star. This strongly suggests that a persistent asymmetric magnetic ﬁeld structure is aligned between the two stars. The loop is larger than anticipated theoretically6, 7, but the size may be a result of a magnetic interaction between the two stars.

1 We made six twelve-hour observations of Algol during a period from 6 April to 17 August

2008 with the High Sensitivity Array (HSA), a global very long baseline interferometer array.

For these observations, the HSA consisted of the ten 25-meter very long baseline array (VLBA)

antennas, the 100-meter Green Bank Telescope, the 100-meter Effelsberg radiotelescope, and the

Very Large Array in phased array mode. We observed at a frequency of 15 GHz, a crucial choice

since it provided a resolving beam size (0.4 mas x 0.6 mas) which was half the size of most previous

VLBI observations2, 8–12 and much smaller than the projected orbital separation (2.3 mas). This made it possible to to discern the detailed radio morphology of the system and its association with the individual components.

The angular motion of the less-massive KIV subgiant (here denoted as Algol B) during a twelve-hour period is signiﬁcant compared with the resolving beam, particularly for eclipse observations when the star moves by as much as 2 milliarcseconds, crossing several beamwidths.

To compensate for this, we applied a small, time-based position correction to the data to cancel out the motion of Algol B so that the phase ﬁtting algorithm tracked the K star center position.

We determined a global astrometric solution for proper motion, parallax, and ﬁducial reference position by combining the centroid position from all six epochs with twelve additional radio positions from previous VLBI observations2, 9, 13, 14 and unpublished data from the VLBA archive. All

observations serendipitously used the same phase reference source J0313+412. We determined

the proper motion, parallax and ﬁducial position of the barycenter of the Algol using orbital pa-

rameters well-determined by spectroscopy15 and optical interferometry16. Fig. 1 shows that the

radio centroid closely tracks the position of Algol B, conﬁrming the previous result of Lestrade

2 et al.14. This is also supported by X-ray eclipse observations in which strong ﬂares have been

occulted during secondary eclipse17, 18. More importantly, by explicitly assuming that there is no systematic offset (constant at all epochs) between Algol B and the radio centroid position, the radio centroid represents the position of Algol B in the International Celestial Reference Frame (ICRF) to ±0.38 mas accuracy. This is signiﬁcantly more accurate than recent astrometric surveys of radio star positions13, 19 and may be the most accurate determination of a stellar position ever made.

For all epochs, the radio morphology consisted of either an elongated double structure or an entire loop, with angular scale comparable to the diameter of Algol B. We interpret these images as compelling evidence for a single large coronal loop structure with feet ﬁxed on the polar regions of Algol B and its apex co-moving with and approximately oriented along the line of sight to the main sequence companion (Algol A). We note that the term ’single loop’ is resolution-dependent:

At higher angular resolution, the physical conﬁguration may have a more complicated topology e.g., ﬁlamentary structures, such as suggested for solar loops20.

In order to support the large coronal loop interpretation, we modeled the emission that would be produced by a theoretical coronal loop filled with energetic electrons emitting gyrosynchrotron radiation. Assuming a power-law distribution of electron energies, we determined the flux density at each point on the loop by solving for the specific intensity using approximate expressions21

for gyrosynchrotron emission and absorption coefﬁcient. We also assumed a uniform electron

density, since the scale height for free electrons on Algol B is several stellar radii. We were able

to best match the observed emission patterns with a surface magnetic ﬁeld strength on the order of

3 103 Gauss and electron densities of ∼ 103 cm−3, as shown for a sample epoch in Fig. 3.

A large, quasi-stable, pole-oriented coronal loop structure anchored on Algol B appears to

be the dominant feature of the magnetosphere of this star. This diverges from what we know of the

Sun, where most ﬂaring and coronal loop activity is seen in relatively compact magnetic structures

close to the equatorial regions22. It is more reminiscent of global magnetospheres as seen on planets

and some low-mass cool stars23. This large coronal loop, co-rotating and oriented toward the inner binary’s center of mass, may provide a natural explanation for a number of previous observations of coronal emission from Algol and perhaps other similar active binaries. For example, observations of Algol’s quiescent X-ray light curve over three consecutive binary orbits24 showed a repeating periodic pattern with a phase indicating that the hemisphere of Algol B facing Algol A is more active than the other. This is expected if the X-ray corona is conﬁned to a large loop oriented toward Algol A. Periodic Doppler shifts in several X-ray lines were observed that were smaller than expected from a source located very near Algol B1, suggesting an asymmetric corona biased toward

the system center of mass. In addition, the line widths were larger than expected from rotation and

thermal broadening, suggesting a radially extended corona on the order of a stellar radius. A giant

X-ray ﬂare17 was occulted during secondary eclipse, allowing an accurate determination of its

location, which was found to be just above the south pole of Algol B. This is also consistent with

a coronal loop scenario in which the coronal feet are located near the poles of Algol B. Finally,

the orientation of the coronal loop toward the center of mass may be a result of the magnetic

topology in which a magnetically threaded accretion disk around Algol A25 interacts with Algol B’s

magnetic ﬁeld, causing reconnection and consequent electron acceleration toward the polar regions

4 at the feet of the coronal loop. There is some evidence for this scenario based on observations of periodic ’super humps’ in the long-term radio light curves4, 26.

Coronal loop models with sizes several time the the active star’s radius have been proposed to explain coronal radio emission from active binary systems27, 28. If these structures are bright emission features of a dipolar magnetic structure, it suggests the presence of a globally operating magnetic dynamo. The similarities in non-ﬂare radio and X-ray emission, including overall size24 and orientation toward Algol A1 suggest that both plasmas are driven by a common energy source, and may be largely co-spatial. A major future challenge is the construction of a self-consistent model to test whether both non-thermal radio and possibly thermal X-ray plasma may co-exist in large loops with feet tied to the poles of the active star.

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Competing Interests The authors declare that they have no competing ﬁnancial interests.

Supplementary Information is linked to the online version of the paper at www.nature.com/nature.

Acknowledgements The National Radio Astronomy Observatory is a facility of the National Science

Foundation operated under cooperative agreement by Associated Universities, Inc. Support for this work was provided by the NSF through award GSSP xxxxxx from the NRAO. RLM is grateful to Institute of

Astronomy at ETH, Zurich for ...

Author Contributions All authors contributed substantially to this work. R.L.M. and M.G. did the obser- vation planning. W.M.P and W.M.G. analyzed the observations. R.L.M., M.G. and W.M.P. wrote and edited the manuscript.

Author Information Reprints and permissions information is available at www.nature.com/reprints. Cor- respondence and requests for materials should be addressed to W.M.P. ([email protected]).

9 Figure 1 Algol radio centroid positions for six observing epochs in the co-moving frame

of Algol-B (panel a) and Algol-A (panel b). The close association of radio positions

with Algol-B clearly demonstrates that the radio source co-moves with the KIV subgiant.

Observing epochs are indicated by symbols: 2008.27 (red pentagram), 2008.39 (violet

square), 2008.47 (orange circle), 2008.51 (cyan triangle), 2008.57 (green inverted trian-

gle), 2008.63 (blue diamond). The error bars indicate one standard deviation position

uncertainties. The blue and red centered circles show the angular size of Algol A and B,

respectively.

Figure 2 Algol radio images at three different orbital phases. Contour levels are: (a) 1,

2, 3, 4, 5 mJy beam−1,(b) 10, 20, 30 ... 100 mJy beam−1 and (c) 1, 2, 3 ... 10 mJy beam−1.

The positions and sizes of Algol A (blue) and B (red) are shown. The one standard deviation uncertainty in position is indicated by the black cross in each panel. Note the position and orientation of the coronal loop structure, which straddles Algol B and is oriented in the direction of Algol A. Data were calibrated and imaged using the Astronomical Image

Processing Software (AIPS) package. Standard corrections were applied to compensate for gain and phase offsets in antenna electronics and the correlator, for slight errors in the correlator’s predicted model for the orientation of the Earth’s rotation axis, and for

ﬂuctuations due to the Earth’s ionosphere. Precise astrometric positions were computed using a standard phase-referencing scheme29, switching between Algol and the angu- larly nearby (1.0◦) extragalactic radio source J0313+412 every three minutes, then using

J0313’s known ICRF coordinates13 when solving for Algol’s centroid position.

10 Figure 3 Comparison of the radio emission from a gyrosynchrotron ﬁlled-loop model to

the Algol radio image at epoch 2008.57 (phase 0.5). Panel (a) shows the radio image

with positions of Algol B (red circle) and Algol A (blue circle) overlaid and one standard

deviation position uncertainty shown by the black cross. Contours are 1, 5, 9, 13, and

17 mJy beam−1. Panel (b) shows a model radio brightness distribution derived from a

theoretical coronal loop bounded by dipolar ﬁeld lines ﬁlling L-shells between 2 < L <

4, having a circular footprint and a surface magnetic ﬁeld intensity of B = 103 Gauss,

3 −3 with a uniform density of electrons ne = 10 cm having an energy distribution given by

−1.5 2 n(E) = ne(E/Emin) where Emin = 0.16 mec . The resulting brightness distribution was convolved with a 0.4 mas beam to mimic the angular resolution of the array. Contour levels are the same as in (a). The brightness asymmetry, evident in both the observed brightness image and the model, arises from tilting the loop model by 82◦ to the observer’s

line of sight, consistent with the inner binary’s orbital inclination.