Evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star RX J1856.5−3754

The ‘Magnificent Seven’ (M7) are a group of radio-quiet isolated neutron stars discovered in the soft X-rays through their purely thermal surface emission. Owing to the large inferred magnetic fields (B ≈ 1013 G), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the thermal emission. A large observed polarization degree (PD) is, however, expected only if quantum-electrodynamic (QED) polarization effects are present in the magnetized vacuum around the star. The detection of a strong linearly polarized signal would therefore provide the first observational evidence of QED effects in the strong-field regime. While polarization measurements in the soft X-rays are not feasible yet, optical polarization measurements are within reach also for quite faint targets, like the M7 which have optical counterparts with magnitudes ≈26–28. Here, we report on the measurement of optical linear polarization for the prototype, and brightest member, of the class, RX J1856.5−3754 (V ∼ 25.5), the first ever for one of the M7, obtained with the Very Large Telescope. We measured a PD = 16.43 ± 5.26 per cent and a polarization position angle PA = 145°.39 ± 9°.44, computed east of the North Celestial Meridian. The PD that we derive is large enough to support the presence of vacuum birefringence, as predicted by QED

A time-variable, phase-dependent emission line in the X-ray spectrum of the isolated neutron star RXJ0822–4300

RX J0822−4300 is the central compact object associated with the Puppis A supernova remnant. Previous X-ray observations suggested RX J0822−4300 to be a young neutron star with a weak dipole field and a peculiar surface temperature distribution dominated by two antipodal spots with different temperatures and sizes. An emission line at 0.8 keV was also detected. We performed a very deep (130-ks) observation with XMM–Newton, which allowed us to study in detail the phase-resolved properties of RX J0822−4300. Our new data confirm the existence of a narrow spectral feature, best modelled as an emission line, only seen in the ‘soft’-phase interval – when the cooler region is best aligned to the line of sight. Surprisingly, comparison of our recent observations to the older ones yields evidence for a variation in the emission-line component, which can be modelled as a decrease in the central energy from ∼0.80 keV in 2001 to ∼0.73 keV in 2009–10. The line could be generated via cyclotron scattering of thermal photons in an optically-thin layer of gas, or, alternatively, it could originate in low-rate accretion by a debris disc. In any case, a variation in energy, pointing to a variation of the magnetic field in the line-emitting region, cannot be easily accounted for

A high-velocity black hole on a Galactic-halo orbit in the solar neighborhood

Only a few of the dozen or so stellar-mass black holes have been observed away from the plane of the Galaxy$^1$. Those few could have been ejected from the plane as a result of a ``kick'' received during a supernova explosion, or they could be remnants of the population of massive stars formed in the early stages of evolution of the Galaxy. Determining their orbital motion should help to distinguish between these options. Here we report the transverse motion (in the plane of the sky) for the black hole X-ray nova XTE J1118+480 (refs 2-5), from which we derive a large space velocity. This X-ray binary has an eccentric orbit around the Galactic Centre, like most objects in the halo of the Galaxy, such as ancient stars and globular clusters. The properties of the system suggest that its age is comparable to or greater than the age of the Galactic disk. Only an extraordinary ``kick'' from a supernova could have launched the black hole into an orbit like this from a birth place in the disk of the Galaxy.Comment: 8 pages including 2 color figures. Additional figures and animation in http://www.iafe.uba.ar/astronomia/FM/mirabel.htm

Ramipril and Risk of Hyperkalemia in Chronic Hemodialysis Patients

Angiotensin converting enzyme (ACE) inhibitors provide well known cardiorenal-protective benefits added to antihypertensive effects in chronic renal disease. These agents are underused in management of patients receiving hemodialysis (HD) because of common concern of hyperkalemia. However, few studies have investigated effect of renin angiotensin aldosterone system (RAAS) blockade on serum potassium in hemodialysis patients. We assessed the safety of ramipril in patients on maintenance HD. We enrolled 28 adult end stage renal disease (ESRD) patients treated by maintenance HD and prescribed them ramipril in doses of 1.25 to 5 mg per day. They underwent serum potassium concentration measurements before ramipril introduction and in 1 to 3 months afterwards. No significant increase in kalemia was found. Results of our study encourage the use of ACE inhibitors in chronically hemodialyzed patients, but close potassium monitoring is mandatory

Blue stragglers, young white dwarfs, and UV-excess stars in the core of 47 Tucanae

We used a set of archived Hubble Space Telescope/WFPC2 images to probe the stellar population in the core of the nearby galactic globular cluster (GGC) 47 Tuc. From the ultraviolet (UV) color magnitude diagrams (CMDs) obtained for similar to 4000 stars detected within the Planetary Camera (PC) field of view we have pinpointed a number of interesting objects : (1) 43 blue stragglers stars (BSSs), including 20 new candidates; (2) 12 bright (young) cooling white dwarfs (WDs) at the extreme blue region of the UV-CMD; and (3) a large population of UV-excess (UVE) stars, lying between the BSS and the WD sequences. The colors of the WD candidates identified here define a clean pattern in the CMDs, which define the WD cooling sequence. Moreover, both the location on the UV-CMDs and the number of WDs are in excellent agreement with the theoretical expectations. The UVE stars discovered here represent the largest population of anomalous blue objects ever observed in a globular cluster-if the existence of such a large population is confirmed, we have finally found the long-searched-for population of interacting binaries predicted by the theory. Finally, we have investigated the feasibility of the optical identification of the companions of the binary X-ray sources recently detected by Chandra and of binary millisecond pulsars (MSPs) residing in the core of 47 Tuc. Unfortunately, the extreme faintness expected for the MSP companions, together with the huge stellar crowding in the cluster center, prevents statistically reliable identifications based only on positional coincidences

PSR J0357+3205: A fast-moving pulsar with a very unusual X-ray trail

The middle-aged PSR J0357+3205 is a nearby, radio-quiet, bright γ-ray pulsar discovered by the Fermi mission. Our previous Chandra observation revealed a huge, very peculiar structure of diffuse X-ray emission originating at the pulsar position and extending for >9′ on the plane of the sky. To better understand the nature of such a nebula, we have studied the proper motion of the parent pulsar. We performed relative astrometry on Chandra images of the field spanning a time baseline of 2.2 yr, unveiling a significant angular displacement of the pulsar counterpart, corresponding to a proper motion of 0.″165 ± 0.″030 yr-1 at a position angle (P.A.) of 314° ± 8°. At a distance of 500 pc, the space velocity of the pulsar would be of 390 km s-1 assuming no inclination with respect to the plane of the sky. The direction of the pulsar proper motion is aligned very well with the main axis of the X-ray nebula (P.A. = 315.°5 ± 1.°5), pointing to a physical, yet elusive, link between the nebula and the pulsar space velocity. No optical emission in the Hα line is seen in a deep image collected at the Gemini telescope, which implies that the interstellar medium into which the pulsar is moving is fully ionized. © 2013. The American Astronomical Society. All rights reserved..link_to_subscribed_fulltex