Optical investigations

Since the first optical detection of the brightest M7, RX J1856.5-3754 (RXJ1856 for short), using the Hubble Space Telescope (HST) by Walter, Wolk & Neuhäuser 1996, Nature 379, 233, for six of the M7 faint optical counterparts could be observed with HST or large aperture ground based facilities, such as the Very Large Telescope (VLT) on Cerro Paranal in Chile. The investigation of the optical emission coming from those objects offers a variety of new scientific possibilities.

Photometry: The optical radiation coming from the M7 is in excess compared to the X-ray emission. The most common explanation for that excess is that the X-rays are emitted from hot spots on the neutron star surface, while the optical emission is radiated from the whole surface, or even a thin atmosphere above the surface of the NS. However other explanations are possible, e.g. that the optical emission is coming from surrounding material, heated by the neutron star while it is flying through the medium. Different explanations on the other hand require different models and different shapes of the spectral energy distribution (SED) of the neutron star. If the X-ray emission comes from the surface the radiation would be black-body like. Otherwise the SED would be a power law, if the optical radiation is generated during some accretion process. That is why we observe the M7 in different photometric passbands to constrain the optical excess and the shape of the SED. While the optical SED of RXJ1856 is consistent with a perfect black-body the SED of RX J0720.4-3125 (RXJ0720 for short) has similarities to a power law. We measured the V band magnitude of RXJ0720 with the VLT and published the results and our conclusions in Eisenbeiss et al. 2010, AN 331, 243.

Astrometry - distance and radius: Due to the great spacial resolution of large optical telescopes this wavelength range is perfectly suited for astrometric measurements. Important issues of those measurements are the precise determination of the neutron star position on the sky, the tangential proper motion of the neutron star and the measurement of the trigonometric parallax hence, the distance, the lather being the most challenging, since effects, smaller than the size of a pixel at the detector have to be measured with high accuracy. Using VLT V band measurement from the year 2008, as well as earlier B band measurements from 2000 and 2002 we derived the proper motion of RXJ0720 with high precision. A much better precision is achievable from space, using the HST. With this instrument even the small displacement of the nearby neutron star with respect to farther background stars due to the earths motion around the sun, i.e. the trigonometric parallax can be measured. Unfortunately the neutron stars are very faint in the optical hence, the measurements are difficult. However for the two optically brightest of the M7, RXJ1856 and RXJ0720, observations, scheduled around the extrema of the parallactic ellipse over two years where taken. The most recent reduction of the data is published in Walter, Eisenbeiss, ..., Hambaryan & Neuhäuser 2010, ApJ 724, 669 and yields 123⁺¹¹_-15 pc. The first published value for RXJ0720 is 360⁺¹⁷⁰_-90 pc (Kaplan, van Kerkwijk & Anderson 2007), but more recently a value of 280⁺²¹⁰_-87 pc was derived (Eisenbeiss 2011, PhD thesis). However both values are consistent within the errorbars, illustrating the difficulty of such measurements.

Blackbody radiation, originated from the surface of the neutron star, together with a known distance offers the possibility to determine the radius, successful already in the case of RXJ1856 and leading to a radius of about R_∞ = 17+/-3 km (Trümper et al. 2004) at ~120 pc. This again constrains the equation of state. The same radius is obtained for RXJ0720, but with a larger error bar.