1,720,997 research outputs found
The bright optical companion to the eclipsing millisecond pulsar in NGC 6397
No full textWe report the optical identification of the companion to the eclipsing millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397. A bright variable star with an anomalous red color and optical variability (similar to0.2 mag) that nicely correlates to the orbital period of the pulsar (similar to1.35 days) has been found close to the pulsar position. The peculiar shape of the optical light curves is unprecedented for a millisecond pulsar companion and is a clear signature of tidal distortions
The helium white dwarf orbiting the millisecond pulsar in the halo of the globular cluster NGC 6752
No full textWe have used deep high-resolution multiband images taken at the ESO Very Large Telescope to identify the optical binary companion to the millisecond pulsar (PSR J1911 - 5958A) located in the halo of the Galactic globular cluster NGC 6752. The object turns out to be a blue star whose position in the color-magnitude diagram is consistent with the cooling sequence of a low-mass (M similar to 0.17-0.20 M-.), low-metallicity helium white dwarf (He WD) at the cluster distance. This is the second He WD, with this mass, that has been found to orbit a millisecond pulsar in Galactic globular clusters. The anomalous position of PSR J1911 - 5958A with respect to the globular cluster center (similar to6') suggested that this system has recently (less than or similar to1 Gyr) been ejected from the cluster core as the result of a strong dynamical interaction. The data presented here allow us to constrain the cooling age of the companion within a fairly narrow range (similar to1.2 - 2.8 Gyr), therefore suggesting that such a dynamical encounter must have acted on an already recycled millisecond pulsar
Blue stragglers, young white dwarfs, and UV-excess stars in the core of 47 Tucanae
No full textWe 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
The puzzling properties of the helium white dwarf orbiting the millisecond pulsar PSR J1911-5958A in NGC 6752
No full textWe have used phase-resolved high-resolution images and low-resolution spectra taken at the ESO Very Large Telescope to study the properties of the low-mass helium white dwarf companion to the millisecond pulsar PSR J1911-5958A (COM J1911-5958A), in the halo of the Galactic globular cluster NGC 6752. The radial velocity curve confirms that COM J1911-5958A is orbiting the pulsar and allows us to derive a systemic velocity of the binary system nicely in agreement with that of NGC 6752. This strongly indicates that the system is a member of the cluster, despite its very offset position (similar to 74 core radii) with respect to the core. Constraints on the orbital inclination (greater than or similar to 70 degrees) and pulsar mass (1.2-1.5 M-circle dot) are derived from the mass ratio M-PSR/M-COM = 7.49 +/- 0.64 and photometric properties of COM J1911-5958A. The light curve in the B band shows two phases of unequal brightening (Delta mag similar to 0.3 and 0.2, respectively) located close to quadratures and superimposed on an almost steady baseline emission: this feature is quite surprising and needs to be further investigated
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The puzzling dynamical status of the core of the globular cluster NGC 6752
No full textWe have used high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and ground-based wide-field images to determine the center of gravity and construct an extended radial density and brightness pro. le of the cluster NGC 6752 including, for the first time, detailed star counts in the very inner region. The barycenter of the nine innermost X-ray sources detected by Chandra is located only 1."9 off the new center of gravity. Both the density and the brightness profile of the central region are best fitted by a double King model, suggesting that NGC 6752 is experiencing a post-core-collapse bounce. Taking advantage of our new optical data, we discuss the puzzling nature of the accelerations displayed by the innermost millisecond pulsars detected in this cluster. We discuss two possible origins to the accelerations: ( 1) the overall cluster gravitational potential, which would require a central projected mass-to-light ratio of the order of 6 - 7 and the existence of a few thousand solar masses of low-luminosity matter within the inner 0.08 pc of NGC 6752, and (2) the existence of a local perturber(s) of the pulsar dynamics, such as a recently proposed binary black hole of intermediate (100 - 200 M(.)) mass
The complex H alpha line profile of the bright companion to PSR J1740-5340 in NGC 6397
No full textWe present a detailed study of the Ha and He I spectral features of COM J1740-5340 ( the companion to PSR J1740-5340 in the Galactic globular cluster NGC 6397), exploiting a series of high-resolution spectra obtained at different orbital phases. The Ha absorption line shows a complex two-component structure, revealing that optically thin hydrogen gas resides outside the Roche lobe of COM J1740-5340. The line morphology precludes the existence of any residual disk around the millisecond pulsar and suggests the presence of a stream of material going from the companion toward the neutron star. This material never reaches the neutron star surface, being driven back by the pulsar radiation far beyond COM J1740-5340. By analyzing the He I absorption lines as a function of orbital phase, we infer the presence of an overheated longitudinal strip (about 150 times narrower than it is long) on the COM J1740-5340 surface facing the radio pulsar
A puzzling millisecond pulsar companion in NGC 6266
No full textWe report on the optical identification of the companion to the eclipsing millisecond pulsar PSR J1701-3006B in the globular cluster NGC 6266. A relatively bright star with an anomalous red color and an optical variability (similar to 0.2 mag) that nicely correlates with the orbital period of the pulsar (similar to 0.144 days) has been found nearly coincident with the pulsar nominal position. This star is also found to lie within the error box position of an Xray source detected by Chandra observations, thus supporting the hypothesis that some interaction is occurring between the pulsar wind and the gas streaming off the companion. Although the shape of the optical light curve is suggestive of a tidally deformed star which has nearly completely filled its Roche lobe, the luminosity (similar to 1.9 L(circle dot)) and the surface temperature (similar to 6000 K) of the star, deduced from the observed magnitude and colors, would imply a stellar radius significantly larger than the Roche lobe radius. Possible explanations for this apparent inconsistency are discussed
Accurate mass ratio and heating effects in the dual-line millisecond binary pulsar in NGC 6397
No full textBy means of high-resolution spectra, we have measured radial velocities of the companion (hereafter COM J1740-5340) to the eclipsing millisecond pulsar PSR J1740-5340 in the galactic globular cluster NGC 6397. The radial velocity curve fully confirms that COM J1740-5340 is orbiting the pulsar, and this enables us to derive the most accurate mass ratio (M-PSR/M-COM = 5.85 +/- 0.13) for any nonrelativistic binary system containing a neutron star. Assuming a pulsar mass in the range 1.3-1.9 M-., the mass of COM J1740-5340 spans the interval 0.22-0.32 M-., the inclination of the system is constrained within, and the Roche lobe 56degrees greater than or similar to i greater than or similar to 47degrees and the Roche lobe radius is r(RL) similar to1.5-1.7 R-.. A preliminary chemical abundance analysis confirms that COM J1740-5340 has a metallicity compatible with that measured for other stars in this metal-poor globular, but the unexpected detection of strong He I absorption lines implies the existence of regions at K, which are significantly warmer T > 10,000 than the rest of the star. The intensity of this line correlates with the orbital phase, suggesting the presence of a region on the companion surface, heated by the millisecond pulsar flux
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