124 research outputs found
The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440
We used a combination of deep optical and Halpha images of the Galactic
globular cluster NGC 6440, acquired with the Hubble Space Telescope, to
identify the optical counterpart to the accreting millisecond X-ray pulsar SAX
J1748.9-2021during quiescence. A strong Halpha emission has been detected from
a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15"
from the nominal position of the X-ray source. The position of the star also
agrees with the optical counterpart found by Verbunt et al. (2000) during an
outburst. We propose this star as the most likely optical counterpart to the
binary system. By direct comparison with isochrones, we estimated that COM-SAX
J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun
and a superficial temperature of 5250pm80 K. These parameters combined with the
orbital characteristics of the binary suggest that the system is observed at a
very low inclination angle (~8 deg -14 deg) and that the star is filling or
even overflowing its Roche Lobe. This, together with the equivalent width of
the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The
possibile presence of such a on-going mass transfer during a quiescence state
also suggests that the radio pulsar is not active yet and thus this system,
despite its similarity with the class of redback millisecond pulsars, is not a
transitional millisecond pulsar.Comment: 8 pages, 6 figures. Accepted for publication in Ap
Optical Identification of He White Dwarfs Orbiting Four Millisecond Pulsars in the Globular Cluster 47 Tucanae
We used ultra-deep UV observations obtained with the Hubble Space Telescope
to search for optical companions to binary millisecond pulsars (MSPs) in the
globular cluster 47 Tucanae. We identified four new counterparts (to MSPs
47TucQ, 47TucS, 47TucT and 47TucY) and confirmed those already known (to MSPs
47TucU and 47TucW). In the color magnitude diagram, the detected companions are
located in a region between the main sequence and the CO white dwarf cooling
sequences, consistent with the cooling tracks of He white dwarfs of mass
between 0.15 Msun and 0.20 Msun. For each identified companion, mass, cooling
age, temperature and pulsar mass (as a function of the inclination angle) have
been derived and discussed. For 47TucU we also found that the past accretion
history likely proceeded in a sub-Eddington rate. The companion to the redback
47TucW is confirmed to be a non degenerate star, with properties particularly
similar to those observed for black widow systems. Two stars have been
identified within the 2-sigma astrometric uncertainty from the radio positions
of 47TucH and 47TucI, but the available data prevent us from firmly assessing
whether they are the true companions of these two MSPs.Comment: 27 pages, 7 figures, Accepted for publication by Ap
Probing the MSP prenatal stage: the optical identification of the X-ray burster EXO 1745-248 in Terzan 5
We report on the optical identification of the neutron star burster EXO
1745-248 in Terzan 5. The identification was performed by exploiting HST/ACS
images acquired in Director's Discretionary Time shortly after (approximately 1
month) the Swift detection of the X-ray burst. The comparison between these
images and previous archival data revealed the presence of a star that
currently brightened by ~3 magnitudes, consistent with expectations during an
X-ray outburst. The centroid of this object well agrees with the position, in
the archival images, of a star located in the Turn-Off/Sub Giant Branch region
of Terzan 5. This supports the scenario that the companion should has recently
filled its Roche Lobe. Such a system represents the pre-natal stage of a
millisecond pulsar, an evolutionary phase during which heavy mass accretion on
the compact object occurs, thus producing X-ray outbursts and re-accelerating
the neutron star.Comment: ApJ Letter, in pres
Radio Timing and Optical Photometry of the Black Widow System PSR J1953+1846A in the Globular Cluster M71
We report on the determination of the astrometric, spin and orbital
parameters for PSR J1953+1846A, a "black widow" binary millisecond pulsar in
the globular cluster M71. By using the accurate position and orbital parameters
obtained from radio timing, we identified the optical companion in ACS/Hubble
Space Telescope images. It turns out to be a faint (m_F606W>=24, m_F814W>=23)
and variable star located at only ~0.06" from the pulsar timing position. The
light curve shows a maximum at the pulsar inferior conjunction and a minimum at
the pulsar superior conjunction, thus confirming the association with the
system. The shape of the optical modulation suggests that the companion star is
heated, likely by the pulsar wind. The comparison with the X-ray light curve
possibly suggests the presence of an intra-binary shock due to the interaction
between the pulsar wind and the material released by the companion. This is the
second identification (after COM-M5C) of an optical companion to a black widow
pulsar in a globular cluster. Interestingly, the two companions show a similar
light curve and share the same position in the color magnitude diagram.Comment: Accepted for publication by ApJ; 33 Pages, 10 Figures, 3 Table
Discovery of three new millisecond pulsars in Terzan 5
We report on the discovery of three new millisecond pulsars (namely
J1748-2446aj, J1748-2446ak and J1748-2446al) in the inner regions of the dense
stellar system Terzan 5. These pulsars have been discovered thanks to a method,
alternative to the classical search routines, that exploited the large set of
archival observations of Terzan 5 acquired with the Green Bank Telescope over 5
years (from 2010 to 2015). This technique allowed the analysis of stacked power
spectra obtained by combining ~206 hours of observation. J1748-2446aj has a
spin period of ~2.96 ms, J1748-2446ak of ~1.89 ms (thus it is the fourth
fastest pulsar in the cluster) and J1748-2446al of ~5.95 ms. All the three
millisecond pulsars are isolated and currently we have timing solutions only
for J1748-2446aj and J1748-2446ak. For these two systems, we evaluated the
contribution to the measured spin-down rate of the acceleration due to the
cluster potential field, thus estimating the intrinsic spin-down rates, which
are in agreement with those typically measured for millisecond pulsars in
globular clusters. Our results increase to 37 the number of pulsars known in
Terzan 5, which now hosts 25% of the entire pulsar population identified, so
far, in globular clusters.Comment: 10 pages, 6 figures. ApJ (accepted
Interplay between bending and stretching in carbon nanoribbons
We investigate the bending properties of carbon nanoribbons by combining
continuum elasticity theory and tight-binding atomistic simulations. First, we
develop a complete analysis of a given bended configuration through continuum
mechanics. Then, we provide by tight-binding calculations the value of the
bending rigidity in good agreement with recent literature. We discuss the
emergence of a stretching field induced by the full atomic-scale relaxation of
the nanoribbon architecture. We further prove that such an in-plane strain
field can be decomposed into a first contribution due to the actual bending of
the sheet and a second one due to edge effects.Comment: 5 pages, 6 figure
Correlated electron-hole plasma in organometal perovskites
Organic-inorganic perovskites are a class of solution-processed semiconductors holding promise for the realization of low-cost efficient solar cells and on-chip lasers. Despite the recent attention they have attracted, fundamental aspects of the photophysics underlying device operation still remain elusive. Here we use photoluminescence and transmission spectroscopy to show that photoexcitations give rise to a conducting plasma of unbound but Coulomb-correlated electron-hole pairs at all excitations of interest for light-energy conversion and stimulated optical amplification. The conductive nature of the photoexcited plasma has crucial consequences for perovskite-based devices: in solar cells, it ensures efficient charge separation and ambipolar transport while, concerning lasing, it provides a low threshold for light amplification and justifies a favourable outlook for the demonstration of an electrically driven laser. We find a significant trap density, whose cross-section for carrier capture is however low, yielding a minor impact on device performance
The double blue-straggler sequence in NGC 2173: an artifact of field contamination
Here we discuss the case of the double blue straggler star (BSS) sequence recently detected in the young stellar cluster NGC2173 in the Large Magellanic Cloud (LMC) by Li et al. (2018, ApJ, 856, 25). In order to investigate this feature we made use of two Hubble Space Telescope (HST) sets of observations, one (the same one used by Li et al.) probing the cluster central regions, and the other sampling the surrounding field. We demonstrate that when field star decontamination is applied, _40% of the BSS population selected by Li et al. turns out to be composed of field stars interlopers. This contamination mainly affects one of the two sequences, which therefore disappears in the decontaminated color-magnitude diagram. We analyze the result of tens different decontamination realizations: we find no evidence of a double BSS sequence in any of them. We therefore conclude that NGC2173 harbors a normal single (poorly populated) BSS sequence and that particular care needs to be devoted to the field decontamination process in any study aimed at probing stellar population features or star counts in the LMC clusters
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