An Optical Counterpart Candidate for the Isolated Neutron Star RBS1774 - AstrophysicsReport as inadecuate

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Abstract: Multiwavelength studies of the seven identified X-ray dim isolated neutronstars XDINSs offer a unique opportunity to investigate their surface thermaland magnetic structure and the matter-radiation interaction in presence ofstrong gravitational and magnetic fields. As a part of an ongoing campaignaimed at a complete identification and spectral characterization of XDINSs inthe optical band, we performed deep imaging with the ESO Very Large TelescopeVLT of the field of the XDINS RBS1774 1RXS J214303.7 +065419. The recentlyupgraded FORS1 instrument mounted on the VLT provided the very first detectionof a candidate optical counterpart in the B band. The identification is basedon a very good positional coincidence with the X-ray source chance probability~2E-3. The source has B=27.4 +- 0.2 1 sigma confidence level, and theoptical flux exceeds the extrapolation of the X-ray blackbody at opticalwavelengths by a factor ~35 +- 20 at 3sigma confidence level. This is barelycompatible with thermal emission from the neutron star surface, unless thesource distance is d~200-300 pc, and the star is an almost aligned rotator orits spin axis is nearly aligned with the line of sight. At the same time, sucha large optical excess appears difficult to reconcile with rotation-poweredmagnetospheric emission, unless the source has an extremely large opticalemission efficiency. The implications and possible similarities with theoptical spectra of other isolated NSs are discussed.

Author: S. Zane, R. Mignani, R. Turolla, A. Treves, F. Haberl, C. Motch, L. Zampieri, M. Cropper


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