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Revisiting the Parallax of the Isolated Neutron Star RX J185635-3754 Using HST/ACS Imaging  [PDF]
F. M. Walter,T. Eisenbeiss,J. M. Lattimer,B. Kim,V. Hambaryan,R. Neuhaeuser
Physics , 2010, DOI: 10.1088/0004-637X/724/1/669
Abstract: We have redetermined the parallax and proper motion of the nearby isolated neutron star RX~J185635-3754. We used eight observations with the high resolution camera of the HST/ACS taken from 2002 through 2004. We performed the astrometric fitting using five independent methods, all of which yielded consistent results. The mean estimate of the distance is 123 (+11, -15) pc (1 sigma), in good agreement with our earlier published determination.
A Revised Parallax and its Implications for RX J185635-3754  [PDF]
Frederick M. Walter,James Lattimer
Physics , 2002, DOI: 10.1086/343850
Abstract: New astrometric analysis of four WFPC2 images of the isolated neutron star RX J185635-3754 show that its distance is 117 +/- 12 pc, nearly double the originally published distance. At the revised distance, the star's age is 5 x 10^5 years, its space velocity is about 185 km/s, and its radiation radius inferred from thermal emission is approximately 15 km, in the range of many equations of state both with and without exotic matter. These measurements remove observational support for an extremely soft equation of state. The star's birthplace is still likely to be in the Upper Sco association, but a connection with zeta Oph is now unlikely.
The Parallax and Proper Motion of RX J1856.5-3754 Revisited  [PDF]
D. L. Kaplan,M. H. van Kerkwijk,J. Anderson
Physics , 2001, DOI: 10.1086/339879
Abstract: RX J1856.5-3754, a bright soft X-ray source believed to be the nearest thermally emitting neutron star, has commanded and continues to command intense interest from X-ray missions. One of the main goals is to determine the radius of this neutron star. An integral part of the determination is an accurate parallax. Walter (2001) analyzed Hubble Space Telescope (HST) data and derived a parallax, pi=16.5+/-2.3 mas. Combining this distance with the angular radius derived from blackbody fits to observations of RX J1856.5-3754 with ROSAT, EUVE, HST, Pons et al. (2001) derived an observed radius ("radiation radius"), R_{infty}= 7 km. This value is smaller than the radii calculated from all proposed equations-of-state (EOS) of dense baryonic matter (Haensel 2001). Here, we have analyzed the same HST data and find pi=7+/-2 mas. We have verified our result using a number of different, independent techniques, and find the result to be robust. Adopting our parallax, the radius of RX J1856.5-3754 is R_{infty}=15+/-6 km. This radius falls squarely in the range of radii, 12--16 km, expected from calculations of neutron star structure for different equations of state. With additional HST observations, the parallax estimate can be improved to the point by which the inferred radius can constrain the choice of EOS.
Towards a Mass and Radius Determination of the Nearby Isolated Neutron Star RX J185635-3754  [PDF]
Jose A. Pons,Frederick M. Walter,James M. Lattimer,Maddapa Prakash,Ralph Neuhaeuser,Penghui An
Physics , 2001, DOI: 10.1086/324296
Abstract: We discuss efforts to determine the mass, radius, and surface composition of the nearby compact object RX J185635-3754 from its multi-wavelength spectral energy distribution. We compute non-magnetized model atmospheres and emergent spectra for selected compositions and gravities, and discuss efforts to fit existing and new observational data from ROSAT, EUVE and the HST. The spectral energy distribution matches that expected from a heavy-element dominated atmosphere, but not from a uniform temperature blackbody. Non-magnetic light element atmospheres cannot be simultaneously reconciled with the optical and X-ray data. We extend previous studies, which were limited to one fixed neutron star mass and radius. For uniform temperature models dominated by heavy elements, the redshift z is constrained to be 0.3 < z < 0.4 and the best-fit mass and radius are M approx 0.9 solar masses and R approx 6 km (for a 61 pc distance). These values for M and R together are not permitted for any plausible equation of state, including that of a self-bound strange quark star. A simplified two-temperature model allows masses and radii up to about 50% larger, or a factor of 2 in the case of a black body. The observed luminosity is consistent with the thermal emission of an isolated neutron star no older than about 1 million years, the age inferred from available proper motion and parallax information
Can HST Measure the Mass of the Isolated Neutron Star RX J185635-3754 ?  [PDF]
Bohdan Paczynski
Physics , 2001,
Abstract: In June 2003 the isolated neutron star RX J185635-3754 will pass within 0.3'' of a 26.5 mag star, changing its position by about 0.6 mas. The displacement, caused by gravitational lensing, will be proportional to the neutron star mass. The total event duration will be approximately 1 year.
Is RX J185635-375 a Quark Star?  [PDF]
J. J. Drake,H. L. Marshall,S. Dreizler,P. E. Freeman,A. Fruscione,M. Juda,V. Kashyap,F. Nicastro,D. O. Pease,B. J. Wargelin,K. Werner
Physics , 2002, DOI: 10.1086/340368
Abstract: Deep Chandra LETG+HRC-S observations of the isolated neutron star candidate RX J1856.5-3754 have been analysed to search for metallic and resonance cyclotron spectral features and for pulsation behaviour. As found from earlier observations, the X-ray spectrum is well-represented by a ~ 60 eV (7e5 K) blackbody. No unequivocal evidence of spectral line or edge features has been found, arguing against metal-dominated models. The data contain no evidence for pulsation and we place a 99% confidence upper limit of 2.7% on the unaccelerated pulse fraction over a wide frequency range from 1e-4 to 100 Hz. We argue that the derived interstellar medium neutral hydrogen column density of 8e19 <= N_H <= 1.1e20 per sq. cm favours the larger distance from two recent HST parallax analyses, placing RX J1856.5-3754 at ~ 140 pc instead of ~ 60 pc, and in the outskirts of the R CrA dark molecular cloud. That such a comparatively rare region of high ISM density is precisely where an isolated neutron star re-heated by accretion of interstellar matter would be expected is either entirely coincidental, or current theoretical arguments excluding this scenario for RX J1856.5-3754 are premature. Taken at face value, the combined observational evidence -- a lack of spectral and temporal features and an implied radius at infinity of 3.8-8.2 km that is too small for current neutron star models -- points to a more compact object, such as allowed for quark matter equations of state.
The Vela Pulsar's proper motion and parallax derived from VLBI observations  [PDF]
R. Dodson,D. Legge,J. E. Reynolds,P. M. McCulloch
Physics , 2003, DOI: 10.1086/378089
Abstract: The Vela pulsar is the brightest pulsar at radio wavelengths. It was the object that told us (via its glitching) that pulsars were solid rotating bodies not oscillating ones. Along with the Crab pulsar is it the source of many of the models of pulsar behavior. Therefore it is of vital importance to know how far away it is, and its origin. The proper motion and parallax for the Vela pulsar have been derived from 2.3 and 8.4 GHz Very Long Baseline Interferometry (VLBI) observations. The data spans 6.8 years and consists of eleven epochs. We find a proper motion of $\mu_{\alpha {\rm cos}\delta}= -49.61 \pm 0.06, \mu_\delta= 29.8 \pm 0.1$ \myr and a parallax of $3.4 \pm 0.2$ mas, which is equivalent to a distance of $293_{-17}^{+19}$ pc. When we subtract out the galactic rotation and solar peculiar velocity we find $\mu_* = 45 \pm 1.3$ \myr with a position angle (PA) of $301^\circ\pm1.8$ which implies that the proper motion has a small but significant offset from the X-ray nebula's symmetry axis.
The proper motion and energy distribution of the isolated neutron star RX J0720.4-3125  [PDF]
C. Motch,V. E. Zavlin,F. Haberl
Physics , 2003, DOI: 10.1051/0004-6361:20030802
Abstract: ESO 4m class telescope and VLT deep imaging of the isolated neutron star RX J0720.4-3125 reveals a proper motion of mu = 97 +/-12 mas/yr and a blue U-B color index. We show that a neutron star atmosphere model modified to account for a limited amount of hydrogen on the star's surface can well represent both the optical and X-ray data without invoking any additional components. The large proper motion almost completely excludes the possibility that accretion from interstellar medium is the powering mechanism of the X-ray emission. It also implies that the proposed spin down is entirely due to magnetic dipole losses. RX J0720.4-3125 is thus a very likely middle aged cooling neutron star. Its overall properties are quite similar to some of the long period radio pulsars recently discovered, giving further support to the idea that RX J0720.4-3125 may be a pulsar whose narrow radio beam does not cross the Earth.
The Proper Motion of the Central Compact Object RX J0822-4300 in the Supernova Remnant Puppis A  [PDF]
Werner Becker,Tobias Prinz,P. Frank Winkler,Robert Petre
Physics , 2012, DOI: 10.1088/0004-637X/755/2/141
Abstract: Using the High Resolution Camera (HRC) aboard the Chandra X-ray Observatory, we have re-examined the proper motion of the central compact object RX J0822-4300 in the supernova remnant Puppis A. New data from 2010 August, combined with three archival data sets from as early as 1999 December, provide a baseline of 3886 days (more than 10 1/2 years) to perform the measurement. Correlating the four positions of RX J0822-4300 measured in each data set implies a projected proper motion of mu 71 \pm 12 masy. For a distance of 2 kpc this proper motion is equivalent to a recoil velocity of 672 \pm 115 km/s. The position angle is found to be 244 \pm 11 degrees. Both the magnitude and direction of the proper motion are in agreement with RX J0822-4300 originating near the optical expansion center of the supernova remnant. For a displacement of 371 \pm 31 arcsec between its birth place and today's position we deduce an age of (5.2 \pm 1.0) 10^3 yrs for RX J0822-4300. The age inferred from the neutron star proper motion and filament motions can be considered as two independent measurements of the same quantity. They average to 4450 \pm 750 yrs for the age of the supernova remnant Puppis A.
Complete Parallax and Proper Motion Solutions For Halo Binary-Lens Microlensing Events  [PDF]
Andrew Gould,Nikolay Andronov
Physics , 1998, DOI: 10.1086/307114
Abstract: A major problem in the interpretation of microlensing events is that the only measured quantity, the Einstein time scale t_E, is a degenerate combination of the three quantities one would like to know, the mass, distance, and speed of the lens. This degeneracy can be partly broken by measuring either a "parallax" or a "proper motion" and completely broken by measuring both. Proper motions can easily be measured for caustic-crossing binary-lens events. Here we examine the possibility (first discussed by Hardy & Walker) that one could also measure a parallax for some of these events by comparing the light curves of the caustic crossing as seen from two observatories on Earth. We derive analytic expressions for the signal-to-noise ratio of the parallax measurement in terms of the characteristics of the source and the geometry of the event. For Galactic halo binary lenses seen toward the LMC, the light curve is delayed from one continent to another by a seemingly minuscule 15 seconds (compared to t_E ~ 40 days). However, this is sufficient to cause a difference in magnification of order 10%. To actually extract complete parallax information (as opposed to merely detecting the effect) requires observations from three non-collinear observatories. Parallaxes cannot be measured for binary lenses in the LMC but they can be measured for Galactic halo binary lenses seen toward M31. Robust measurements are possible for disk binary lenses seen toward the Galactic bulge, but are difficult for bulge binary lenses.
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