R. P. Mignani

Parallax Observations with the Hubble Space Telescope Yield the Distance to Geminga

Using data from Hubble Space Telescope observations, we report the first optical measurement of the annual parallax of a neutron star. This is the counterpart of the X-ray/γ-ray pulsar Geminga, for which strong proper motion has already been reported. Significant displacement from such motion is detected using WFPC2 observations, taken at the dates of the maximum predicted parallactic factor. The resulting distance value is 157 pc (+59, -34), just consistent with the lower end of the wide range derived from X-ray data. This measurement solves the long-standing problem of Gemingas distance and allows for the first precise determination of a neutron star luminosity from electron volts to giga-electron volts. Moreover, the repeated precise positionings substantially improve our knowledge of its proper motion.

Multicolor photometry of the Vela pulsar

Multicolor photometry of the Vela pulsar (PSR B0833 45), updated by recent HST/WFPC2 observa- tions obtained in the 555W , 675W and 814W lters, is presented. The available data provide the best charac- terization so far of the pulsar spectral shape, which is dominated by a flat power law continuum with spectral index = 0:2 0:2, consistent with the extrapolation in the optical domain of the power law component of the X-ray spectrum detected by CHANDRA. In addition, a marginally signicant dip (3) seems to be present at about 6500 A. Spectroscopic observations with the VLT, now in progress, will undoubtly provide a much better assessment of the reality and characteristics of this feature.Multicolor photometry of the Vela pulsar (PSR B0833-45), updated by recent HST/WFPC2 observations obtained in the 555W, 675W and 814W filters, is presented. The available data provide the best characterization so far of the pulsar spectral shape, which is dominated by a flat power law continuum with spectral index \alpha = -0.2 +/- 0.2, consistent with the extrapolation in the optical domain of the power law component of the X-ray spectrum detected by Chandra. In addition, a marginally significant dip (~ 3 sigma) seems to be present at about 6500 AA. Spectroscopic observations with the VLT, now in progress, will undoubtly provide a much better assessment of the reality and characteristics of this feature.

HST observations rule out the association between Cir X-1 and SNR G321.9-0.3

Cir X-1 is one of the most intriguing galactic X-ray sources. It is a16.6 days variable X/radio source, a type-I X-ray burster and a QPO emitter. In spite of an uncertain optical counterpart classication, all these properties identify the source as an LMXB. The morphology of the surrounding radio nebula has suggested an association with the nearby (25 arcmin) SNR G321.9-0.3, implying that Cir X-1 is a runaway binary originated from the supernova explosion 10 5 years ago. To investigate this hypothesis, we carried out a proper motion measurement of the Cir X-1 m 19 optical counterpart using a set of HST/WFC and WFPC2 observations taken 8.6 years apart. We obtained a 3 upper limit of 5m as yr 1 on the source proper motion. Since the runaway hypothesis would have implied a proper motion due North ranging between 15 and 75 mas yr 1 , depending on the actual age of the SNR, our result denitively rules out the association between Cir X-1 and SNR G321.9-0.3.

The proper motion of the isolated neutron star RX J1605.3+3249

Aims. We present a new optical observation of the X-ray dim isolated neutron star RX J1605.3+3249, performed in 2005 with the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) on board Hubble Space Telescope, aimed at measuring the neutron star proper motion.Methods. We have compared the target position with the one measured in a previous HST image of the field obtained in 2001.Results. We infer a proper motion of mu = 155.0 +/- 3.1 mas/yr, corresponding to a star velocity V-T similar to 73 km s(-1) at 100 pc distance. Our measurement refines the result recently reported by Motch et al. ( 2005, A&A, 429, 257), and confirms the optical source as the counterpart of the neutron star.

HST and VLT Observations of Pulsars and Their Environments

The state of the art of optical studies of Isolated Neutron Stars (INSs) and their Pulsar Wind Nebulae (PWNe) is reviewed. In addition, results obtained from recent HST and VLT observations are presented and discussed.

On the accurate positioning of Geminga

Accuracy in the absolute position in the sky is one of the limiting factors for pulsar timing, and timing parameters have a direct impact on the understanding of the physics of Isolated Neutron Stars (INS). This is particularly true for Geminga, the only example of radio silent neutron star known so far. We have combined the Hipparcos and Tycho catalogues, ground-based astrometric data, and Hubble Space Telescope (HST) Wide Field Planetary Camera (WFPC2) images, to yield for the mv=25.5 optical counterpart of Geminga a∼40 mas (per coordinate) uncertainty. Such a positional accuracy, unprecedented for the optical position of an object this faint, is needed to combine in phase γ-ray photons collected over more than 20 years, i.e. over 2.5 billions of the star’s revolutions.

Is the fuzziness of GRB970228 constant

In view of the data gathered in September 1997, we review the flux values collected so far for the “fuzziness” seen in the optical counterpart of GRB970228. Comparison between the ground based data collected in March and the data of September 1997 suggests a fading of the fuzz. Given the diversity of the data in hand, the magnitude of the effect and its significance are not easy to quantify. Only new images, both from the ground and with the Space Telescope, directly comparable to the old ones could settle this problem.