527 research outputs found
Infrared observations of the candidate double neutron star system PSR J1811-1736
PSR J1811-1736 (P=104 ms) is an old (~1.89 Gyrs) binary pulsar (P_orb=18.8 d)
in a highly eccentric orbit (e=0.828) with an unidentified companion.
Interestingly enough, the pulsar timing solution yields an estimated companion
mass 0.93 M_{\odot}<M_C<1.5 M_{\odot}, compatible with that of a neutron star.
As such, it is possible that PSR J1811-1736 is a double neutron star (DNS)
system, one of the very few discovered so far. This scenario can be
investigated through deep optical/infrared (IR) observations. We used J, H,
K-band images, obtained as part of the UK Infrared Telescope (UKIRT) Infrared
Deep Sky Survey (UKIDSS), and available in the recent Data Release 9 Plus, to
search for its undetected companion of the PSR J1811-1736 binary pulsar. We
detected a possible companion star to PSR J1811-1736 within the 3 sigma radio
position uncertainty (1.32 arcsec), with magnitudes J=18.61+/-0.07,
H=16.65+/-0.03, and K=15.46+/-0.02. The star colours are consistent with either
a main sequence (MS) star close to the turn-off or a lower red giant branch
(RGB) star, at a pulsar distance of ~5.5 kpc and with a reddening of
E(B-V)~4.9. The star mass and radius would be compatible with the constraints
on the masses and orbital inclination of the binary system inferred from the
mass function and the lack of radio eclipses near superior conjunction. Thus,
it is possible that it is the companion to PSR J1811-1736. However, based on
the star density in the field, we estimated a quite large chance coincidence
probability of ~0.27 between the pulsar and the star, which makes the
association unlikely. No other star is detected within the 3 sigma pulsar radio
position down to J~20.5, H~19.4$ and K~18.6, which would allow us to rule out a
MS companion star earlier than a mid-to-late M spectral type.Comment: 10 pages, 6 figures, accepted for publication on Monthly Notices of
the Royal Astronomical Societ
Gemini optical observations of binary millisecond-pulsars
Milli-second pulsars (MSPs) are rapidly spinning neutron stars, with spin
periods P_s <= 10 ms, which have been most likely spun up after a phase of
matter accretion from a companion star. In this work we present the results of
the search for the companion stars of four binary milli-second pulsars, carried
out with archival data from the Gemini South telescope. Based upon a very good
positional coincidence with the pulsar radio coordinates, we likely identified
the companion stars to three MSPs, namely PSRJ0614-3329 (g=21.95 +- 0.05),
J1231-1411 (g=25.40 +-0.23), and J2017+0603 (g=24.72 +- 0.28). For the last
pulsar (PSRJ0613-0200) the identification was hampered by the presence of a
bright star (g=16 +- 0.03) at \sim 2" from the pulsar radio coordinates and we
could only set 3-sigma upper limits of g=25.0, r= 24.3, and i= 24.2 on the
magnitudes of its companion star. The candidate companion stars to
PSRJ0614-3329, J1231-1411, and J2017+0603 can be tentatively identified as He
white dwarfs (WDs) on the basis of their optical colours and brightness and the
comparison with stellar model tracks. From the comparison of our multi-band
photometry with stellar model tracks we also obtained possible ranges on the
mass, temperature, and gravity of the candidate WD companions to these three
MSPs. Optical spectroscopy observations are needed to confirm their possible
classification as He WDs and accurately measure their stellar parameters.Comment: 17 pages, 7 figures, 6 tables, accepted for publication in MNRA
VLT/FORS2 observations of the optical counterpart of the isolated neutron star RBS 1774
X-ray observations performed with ROSAT led to the discovery of a group
(seven to date) of X-ray dim and radio-silent middle-aged isolated neutron
stars (a.k.a. XDINSs), which are characterised by pure blackbody spectra
(kT~40-100 eV), long X-ray pulsations (P=3-12 s), and appear to be endowed with
relatively high magnetic fields, (B~10d13-14 G). RBS 1774 is one of the few
XDINSs with a candidate optical counterpart, which we discovered with the VLT.
We performed deep observations of RBS 1774 in the R band with the VLT to
disentangle a non-thermal power-law spectrum from a Rayleigh-Jeans, whose
contributions are expected to be very much different in the red part of the
spectrum. We did not detect the RBS 1774 candidate counterpart down to a 3
sigma limiting magnitude of R~27. The constraint on its colour, (B-R)<0.6,
rules out that it is a background object, positionally coincident with the
X-ray source. Our R-band upper limit is consistent with the extrapolation of
the B-band flux (assuming a 3 sigma uncertainty) for a set of power-laws F_nu
~nu^alpha with spectral indeces alpha<0.07. If the optical spectrum of RBS 1774
were non-thermal, its power-law slope would be very much unlike those of all
isolated neutron stars with non-thermal optical emission, suggesting that it is
most likely thermal. For instance, a Rayleigh-Jeans with temperature T_O = 11
eV, for an optically emitting radius r_O=15 km and a source distance d=150 pc,
would be consistent with the optical measurements. The implied low distance is
compatible with the 0.04 X-ray pulsed fraction if either the star spin axis is
nearly aligned with the magnetic axis or with the line of sight, or it is
slightly misaligned with respect to both the magnetic axis and the line of
sight by 5-10 degreesComment: 8 pages, 8 postscript figures, accepted for publication in Astronomy
& Astrophysic
Studies of Neutron Stars at Optical/IR Wavelengths
In the last years, optical studies of Isolated Neutron Stars (INSs) have expanded from the more classical rotation-powered ones to other categories, like the Anomalous X-ray Pulsars (AXPs) and the Soft Gamma-ray Repeaters (SGRs), which make up the class of the magnetars, the radio-quiet INSs with X-ray thermal emission and, more recently, the enigmatic Compact Central Objects (CCOs) in supernova remnants. Apart from 10 rotation-powered pulsars, so far optical/IR counterparts have been found for 5 magnetars and for 4 INSs. In this work we present some of the latest observational results obtained from optical/IR observations of different types of INSs
Optical spectroscopy of the radio pulsar PSR B0656+14
We have obtained the spectrum of a middle-aged PSR B0656+14 in the 4300-9000
AA range with the ESO/VLT/FORS2. Preliminary results show that at 4600-7000 AA
the spectrum is almost featureless and flat with a spectral index $\alpha_nu ~
-0.2 that undergoes a change to a positive value at longer wavelengths.
Combining with available multiwavelength data suggests two wide, red and blue,
flux depressions whose frequency ratio is about 2 and which could be the 1st
and 2nd harmonics of electron/positron cyclotron absorption formed at magnetic
fields ~10^8G in upper magnetosphere of the pulsar.Comment: 4 pages, 4 figures, To appear in Astrophysics and Space Science,
Proceedings of "Isolated Neutron Stars: from the Interior to the Surface",
eds. D. Page, R. Turolla and S. Zan
Multi-wavelength observations of 3FGL J2039.6-5618: a candidate redback millisecond pulsar
We present multi-wavelength observations of the unassociated gamma-ray source
3FGL J2039.6-5618 detected by the Fermi Large Area Telescope. The source
gamma-ray properties suggest that it is a pulsar, most likely a millisecond
pulsar, for which neither radio nor -ray pulsations have been detected
yet. We observed 3FGL J2039.6-5618 with XMM-Newton and discovered several
candidate X-ray counterparts within/close to the gamma-ray error box. The
brightest of these X-ray sources is variable with a period of 0.22450.0081
d. Its X-ray spectrum can be described by a power law with photon index
, and hydrogen column density cm, which gives an unabsorbed 0.3--10 keV X-ray flux of erg cm s. Observations with the Gamma-Ray Burst
Optical/Near-Infrared Detector (GROND) discovered an optical counterpart to
this X-ray source, with a time-average magnitude . The counterpart
features a flux modulation with a period of 0.227480.00043 d that
coincides, within the errors, with that of the X-ray source, confirming the
association based on the positional coincidence. We interpret the observed
X-ray/optical periodicity as the orbital period of a close binary system where
one of the two members is a neutron star. The light curve profile of the
companion star, with two asymmetric peaks, suggests that the optical emission
comes from two regions at different temperatures on its tidally-distorted
surface. Based upon its X-ray and optical properties, we consider this source
as the most likely X-ray counterpart to 3FGL J2039.6-5618, which we propose to
be a new redback system.Comment: 35 pages, 8 figures, accepted for publication on Astrophysical
Journa
Observations of Isolated Neutron Stars with the ESO Multi-Conjugate Adaptive Optics Demonstrator
High-energy observations have unveiled peculiar classes of isolated neutron
stars which, at variance with radio pulsars, are mostly radio silent and not
powered by the star rotation. Among these objects are the magnetars,
hyper-magnetized neutron stars characterized by transient X-ray/gamma-ray
emission, and neutron stars with purely thermal, and in most cases stationary,
X-ray emission (a.k.a., X-ray dim isolated neutron stars or XDINSs). While
apparently dissimilar in their high-energy behavior and age, both magnetars and
XDINSs have similar periods and unusually high magnetic fields. This suggests a
tantalizing scenario where the former evolve into the latter.Discovering so far
uninvestigated similarities between the multi-wavelength properties of these
two classes would be a further step forward to establish an evolutionary
scenario. A most promising channels is the near infrared (NIR) one, where
magnetars are characterized by a distinctive spectral flattening with respect
to the extrapolation of the soft X-ray spectrum.We observed the two XDINSs RX
J0420.0-5022 and RX J1856.5-3754 with the Multi-Conjugate Adaptive Optics
Demonstrator (MAD) at the Very Large Telescope (VLT) as part of the instrument
guaranteed time observations program, to search for their NIR counterparts.
Both RX J1856.5-3754 and RX J0420.0-5022 were not detected down to K_s ~20 and
Ks ~21.5, respectively. In order to constrain the relation between XDINSs and
magnetars it would be of importance to perform deeper NIR observations. A good
candidate is 1RXS J214303.7+065419 which is the XDINS with the highest inferred
magnetic field.Comment: Accepted for publication in Astronomy and Astrophysic
Search for the Optical Counterpart of the Vela Pulsar X-ray Nebula
Observations of the Vela pulsar region with the Chandra X-ray observatory
have revealed the fine structure of its synchrotron pulsar-wind nebula (PWN),
which showed an overall similarity with the Crab PWN. However, contrary to the
Crab, no firm detection of the Vela PWN in optical has been reported yet. To
search for the optical counterpart of the X-ray PWN, we analyzed deep optical
observations performed with different telescopes. We compared the optical
images with those obtained with the Chandra ACIS to search for extended
emission patterns which could be identified as counterparts of the X-ray nebula
elements. Although some features are seen in the optical images, we find no
correlation with the X-ray structure. Thus, we conclude that the diffuse
optical emission is more likely associated with filaments in the host Vela SNR.
The derived upper limits on the optical flux from the PWN are compatibile,
within the uncertainties, with the values expected on the basis of the
extrapolations of the X-ray data.Comment: 19 pages, 6 figures. Accepted for publication in Ap
Signature of relic heavy stable neutrinos in underground experiments
Considering heavy stable neutrinos of 4th generation we calculate the relic
density of such neutrinos in the Universe. Taking into account the condensation
of heavy neutrinos in the Galaxy and applying the results of calculations to
experimental data from underground experiments on search for WIMPs in elastic
neutral current scattering on nuclei we found an exclusion region of neutrino
mass 60 GeV < m < 290 GeV. The bounds obtained from present underground
experiments while confirming the previous bounds derived from analysis of
cosmic ray spectra are more relible ones. We discuss also the first indication
of elastic scattering induced by WIMP in DAMA experiment finding a very narrow
window of neutrino mass 45 GeV < m < 50 GeV compatible with the possible signal
rate in the detector.Comment: 12 pages, 3 figure
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