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X-ray non-detection of the nearest brown dwarf binary WISE 1049--5319

ATel #5012; P. Gandhi, C. Done (Durham), V. D. Ivanov (ESO), N. Huelamo (CAB)
on 26 Apr 2013; 11:58 UT
Credential Certification: Poshak Gandhi (poshak.gandhi@durham.ac.uk)

Subjects: Radio, Infra-Red, X-ray, A Comment, Binary, Star

At a distance of only 2 pc, the recently-discovered brown dwarf (BD) binary WISE 1049--5319 (Luhman 2013) aka Luhman 16 (Mamajek 2013) is the third nearest (sub)stellar system after Alpha Centauri and Barnard's star, and is an object of great interest. Its proximity to us makes it an excellent target for detailed multi-wavelength follow-up, and several optical/infrared observations have already been reported (Kniazev et al. 2013, Burgasser et al. 2013, Gillon et al. 2013).

BDs can also be X-ray sources (e.g. Guedel 2004 and references therein). Persistent coronal X-ray activity is mainly detected in younger (<~10 Myr) systems with strong convection and perhaps ongoing accretion. Activity decreases with age, with older systems being mostly quiescent except for flares (e.g. Rutledge et al. 2000).

The Swift satellite observed the field of Luhman 16 on 2013 March 08 starting UT20:19. We extracted the XRT source, background and point source response data from the UK Swift Science Data Centre reduced products using standard procedures (Photon counting mode, OBSID=00032749001, final combined and extracted exposure time 9877 s).

No X-ray counterpart to Luhman 16 is detected. Only 1 source count was detected in the 0.3-2 keV range (2 counts over the full energy range). The 3-sigma uncertainty on the count rate is 3.7x10^{-4} ct s^{-1}. Assuming an optically-thin thermal plasma with temperature kT=0.3 keV (e.g. Tsuboi et al. 2003) implies a 0.3-2 keV flux limit of 8.8x10^{-15} erg s^{-1} cm^{-2}, or a luminosity upper-limit of 4.3x10^{24} erg s^{-1}. Assuming instead a soft power law with photon-index Gamma=2.5 (say) increases this limit by only 3%.

The above luminosity limit is amongst the most sensitive for X-ray emission from any (sub)stellar system. For comparison, both the Sun and Proxima Centauri display a mean X-ray power output in quiescence of above 10^{26} erg s^{-1} (e.g. Haisch et al. 1990). Barnard's star also has an X-ray luminosity of a few times 10^{25} erg s^{-1} (e.g. Fleming 1998). Current X-ray limits on the luminosities of BDs found in nearby molecular clouds are typically ~10-100 times higher. To our knowledge, only the quiescent upper-limit of X-ray emission from LP 944-20 (at <10^{24} erg s^{-1}) is deeper (Rutledge et al. 2000).

Using the unresolved DENIS i-band, 2MASS JHKs, and WISE W1-W4 fluxes of Luhman 16, we find a bolometric (0.8-22 microns piecewise-integrated) luminosity of 1.9x10^{29} erg s^{-1}, which implies an X-ray (0.3-2) to bolometric luminosity ratio of log(L{xray}/L{bol})<-4.6 for the two components together. This should be regarded as an approximation because spectral features have not been accounted for.

The age of Luhman 16 is ill-constrained though it is likely to be >10^8 years. Non-detection of X-rays also argues against a young system. The low X-ray to bolometric ratio may also be consistent with a slow rotation speed. Longer X-ray monitoring observations of the source are needed to better establish the incidence of any flaring or push the persistent luminosity limit deeper. Radio observations are also encouraged.

The Swift team is acknowledged for carrying out this observation.