Research Note

Search of X-ray emission from roAp stars: the case of γ Equulei

¹ INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: stelzer@astropa.inaf.it
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
³ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1042, USA

Received: 6 December 2010
Accepted: 24 February 2011

Abstract

Context. Rapidly oscillating Ap (roAp) stars represent a subclass of magnetic, chemically peculiar stars. The explanation for their pulsations includes suppressed convection due to the strong magnetic field. These stars rotate slowly such that a solar-like dynamo and ensuing magnetic activity is unlikely to be present. On the other hand, magnetic activity could provide the particle acceleration suspected to be responsible for the presence of short-lived radionuclides on some roAp stars.

Aims. The detection of X-ray emission from Ap stars can be an indicator for the presence of magnetic activity and dynamo action, provided different origins for the emission, such as wind shocks and close late-type companions, can be excluded. Here we report on results for γ Equ, the only roAp star for which an X-ray detection is reported in ROSAT catalogs.

Methods. We use high resolution imaging in X-rays with Chandra and in the near-infrared with NACO/VLT that allow us to spatially resolve companions down to  ≤ 1′′ and  ~0.06′′ separations, respectively.

Results. The bulk of the X-ray emission is associated with a companion of γ Equ identified in our NACO image. Assuming coevality with the primary roAp star (~900 Myr), the available photometry for the companion points at a K-type star with  ~0.6 M_⊙. Its X-ray properties are in agreement with the predictions for its age and mass. An excess of photons with respect to the expected background and contribution from the nearby companion is observed near the optical position of γ Equ. We estimate an X-ray luminosity of log L_x    [erg/s]  = 26.6 and log (L_x/L_bol) = −7.9 for this emission. A small offset between the optical and the X-ray image leaves some doubt on its association with the roAp star.

Conclusions. The faint X-ray emission that we tentatively ascribe to the roAp star is difficult to explain as a solar-like stellar corona due to its very low L_x/L_bol level and the very long rotation period of γ Equ. It could be produced in magnetically confined wind shocks implying a mass loss rate of  ~10^-14 M_⊙/yr or from an additional unknown late-type companion at separation  ≤ 0.4′′. If confirmed by future deeper X-ray observations this emission could point at the origin for the presence of radioactive elements on some roAp stars.