60 research outputs found
New Limits on Radio Emission from X-ray Dim Isolated Neutron Stars
We have carried out a search for radio emission at 820 MHz from six X-ray dim
isolated neutron stars with the Robert C. Byrd Green Bank Radio Telescope. No
transient or pulsed emission was found using fast folding, fast Fourier
transform, and single-pulse searches. The corresponding flux limits are about
0.01 mJy for pulsed emission, depending on the integration time for the
particular source and assuming a duty cycle of 2%, and 20 mJy for single
dispersed pulses. These are the most sensitive limits to date on radio emission
from X-ray dim isolated neutron stars. There is no evidence for isolated radio
pulses, as seen in a class of neutron stars known as rotating radio transients.
Our results imply that either the radio luminosities of these objects are lower
than those of any known radio pulsars, or they could simply be long-period
nearby radio pulsars with high magnetic fields beaming away from the Earth. To
test the latter possibility, we would need around 40 similar sources to provide
a 1 sigma probability of at least one of them beaming toward us. We also give a
detailed description of our implementation of the Fast Folding Algorithm.Comment: 16 pages, 8 figures, 3 tables, accepted to Ap
The continued spectral and temporal evolution of RX J0720.4-3125
RX J0720.4-3125 is the most peculiar object among a group of seven isolated
X-ray pulsars (the so-called "Magnificent Seven"), since it shows long-term
variations of its spectral and temporal properties on time scales of years.
This behaviour was explained by different authors either by free precession
(with a seven or fourteen years period) or possibly a glitch that occurred
around .
We analysed our most recent XMM-Newton and Chandra observations in order to
further monitor the behaviour of this neutron star. With the new data sets, the
timing behaviour of RX J0720.4-3125 suggests a single (sudden) event (e.g. a
glitch) rather than a cyclic pattern as expected by free precession. The
spectral parameters changed significantly around the proposed glitch time, but
more gradual variations occurred already before the (putative) event. Since
the spectra indicate a very slow cooling by
2 eV over 7 years.Comment: seven pages, three figures, three tables; accepted by MNRA
The Magnificent Seven: Magnetic fields and surface temperature distributions
Presently seven nearby radio-quiet isolated neutron stars discovered in ROSAT
data and characterized by thermal X-ray spectra are known. They exhibit very
similar properties and despite intensive searches their number remained
constant since 2001 which led to their name ``The Magnificent Seven''. Five of
the stars exhibit pulsations in their X-ray flux with periods in the range of
3.4 s to 11.4 s. XMM-Newton observations revealed broad absorption lines in the
X-ray spectra which are interpreted as cyclotron resonance absorption lines by
protons or heavy ions and / or atomic transitions shifted to X-ray energies by
strong magnetic fields of the order of 10^13 G. New XMM-Newton observations
indicate more complex X-ray spectra with multiple absorption lines. Pulse-phase
spectroscopy of the best studied pulsars RX J0720.4-3125 and RBS 1223 reveals
variations in derived emission temperature and absorption line depth with pulse
phase. Moreover, RX J0720.4-3125 shows long-term spectral changes which are
interpreted as due to free precession of the neutron star. Modeling of the
pulse profiles of RX J0720.4-3125 and RBS 1223 provides information about the
surface temperature distribution of the neutron stars indicating hot polar caps
which have different temperatures, different sizes and are probably not located
in antipodal positions.Comment: 10 pages, 8 figures, to appear in Astrophysics and Space Science, in
the proceedings of "Isolated Neutron Stars: from the Interior to the
Surface", edited by D. Page, R. Turolla and S. Zan
Accurate X-ray position and multiwavelength observations of the isolated neutron star RBS 1774
We report on X-ray, optical, infrared and radio observations of the X-ray dim
isolated neutron star (XDINS) 1RXS J214303.7+065419 (also known as RBS 1774).
The X-ray observation was performed with the High Resolution Camera on board of
the Chandra X-ray Observatory, allowing us to derive the most accurate position
for this source (alpha = 21h43m3.38s, delta= +6deg54'17".53; 90% uncertainty of
0."6). Furthermore, we confirmed with a higher spatial accuracy the point-like
nature of this X-ray source. Optical and infrared observations were taken in B,
V, r', i', J, H and Ks filters using the Keck, VLT, Blanco and Magellan
telescopes, while radio observations were obtained from the ATNF Parkes single
dish at 2.9GHz and 708MHz. No plausible optical and/or infrared counterpart for
RBS 1774 was detected within the refined sub-arsecond Chandra X-ray error
circle. Present upper limits to the optical and infrared magnitudes are r'>25.7
and J>22.6 (5 sigma confidence level). Radio observations did not show evidence
for radio pulsations down to a luminosity at 1.4 GHz of L < 0.02 mJy kpc^2, the
deepest limit up to date for any XDINS, and lower than what expected for the
majority of radio pulsars. We can hence conclude that, if RBS 1774 is active as
radio pulsar, its non detection is more probably due to a geometrical bias
rather than to a luminosity bias. Furthermore, no convincing evidence for
RRAT-like radio bursts have been found. Our results on RBS 1774 are discussed
and compared with the known properties of other thermally emitting neutron
stars and of the radio pulsar population.Comment: 8 pages, 9 figures, accepted for publication on MNRA
The isolated neutron star X-ray pulsars RX J0420.0â5022 and RX J0806.4â4123 : new X-ray and optical observations
We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs). RX J0806.4â4123 was observed with XMM-Newton in April 2003, 2.5 years after
the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and N H = 4 Ă 10 19 cm â2 without significant changes between the two observations. Four XMM-Newton observations of RX
J0420.0â5022 between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT = 45 eV, the lowest value found from the small group of XDINs and N H = 1.0 Ă 10 20 cm â2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from RBS1223, RX J1605.3+3249 and RX J0720.4â3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0â5022 with B = 26.6 ± 0.3 mag from VLT imaging
Influences of neutron star parameters on evolutions of different types of pulsar; evolutions of anomalous X-ray pulsars, soft gamma repeaters and dim isolated thermal neutron stars on the P-\.{P} diagram
Influences of the mass, moment of inertia, rotation, absence of stability in
the atmosphere and some other parameters of neutron stars on the evolution of
pulsars are examined. It is shown that the locations and evolutions of soft
gamma repeaters, anomalous X-ray pulsars and other types of pulsar on the
period versus period derivative diagram can be explained adopting values of
B G for these objects. This approach gives the possibility to explain
many properties of different types of pulsar.Comment: 18 pages, 1 figur
X-ray emission from isolated neutron stars
X-ray emission is a common feature of all varieties of isolated neutron stars
(INS) and, thanks to the advent of sensitive instruments with good
spectroscopic, timing, and imaging capabilities, X-ray observations have become
an essential tool in the study of these objects. Non-thermal X-rays from young,
energetic radio pulsars have been detected since the beginning of X-ray
astronomy, and the long-sought thermal emission from cooling neutron star's
surfaces can now be studied in detail in many pulsars spanning different ages,
magnetic fields, and, possibly, surface compositions. In addition, other
different manifestations of INS have been discovered with X-ray observations.
These new classes of high-energy sources, comprising the nearby X-ray Dim
Isolated Neutron Stars, the Central Compact Objects in supernova remnants, the
Anomalous X-ray Pulsars, and the Soft Gamma-ray Repeaters, now add up to
several tens of confirmed members, plus many candidates, and allow us to study
a variety of phenomena unobservable in "standard'' radio pulsars.Comment: Chapter to be published in the book of proceedings of the 1st Sant
Cugat Forum on Astrophysics, "ICREA Workshop on the high-energy emission from
pulsars and their systems", held in April, 201
Isolated neutron stars and studies of their interiors
In these lectures presented at Baikal summer school on physics of elementary
particles and astrophysics 2011, I present a wide view of neutron star
astrophysics with special attention paid to young isolated compact objects and
studies of the properties of neutron star interiors using astronomical methods.Comment: 28 pages, lecture notes for the Baikal-2011 summer school on physics
of elementary particles and astrophysic
Is there a compact companion orbiting the late O-type binary star HD 164816?
We present a multi-wavelength (X-ray, -ray, optical and radio) study
of HD 194816, a late O-type X-ray detected spectroscopic binary. X-ray spectra
are analyzed and the X-ray photon arrival times are checked for pulsation. In
addition, newly obtained optical spectroscopic monitoring data on HD 164816 are
presented. They are complemented by available radio data from several large
scale surveys as well as the \emph{FERMI} -ray data from its
\emph{Large Area Telescope}. We report the detection of a low energy excess in
the X-ray spectrum that can be described by a simple absorbed blackbody model
with a temperature of 50 eV as well as a 9.78 s pulsation of the X-ray
source. The soft X-ray excess, the X-ray pulsation, and the kinematical age
would all be consistent with a compact object like a neutron star as companion
to HD 164816. The size of the soft X-ray excess emitting area is consistent
with a circular region with a radius of about 7 km, typical for neutron stars,
while the emission measure of the remaining harder emission is typical for late
O-type single or binary stars. If HD 164816 includes a neutron star born in a
supernova, this supernova should have been very recent and should have given
the system a kick, which is consistent with the observation that the star HD
164816 has a significantly different radial velocity than the cluster mean. In
addition we confirm the binarity of HD 164816 itself by obtaining an orbital
period of 3.82 d, projected masses = 2.355(69) M,
= 2.103(62) M apparently seen at low inclination
angle, determined from high-resolution optical spectra.Comment: Accepted for publication by MNRAS, 11 pages, 6 figures, 4 table
Exotic phases in compact stars
We discuss how the co-existence of hyperons, antikaon condensate and color
superconducting quark matter in neutron star interior influences the gross
properties of compact stars such as, the equation of state and mass-radius
relationship. We compare our results with the recent observations. We also
discuss about superdense stars in the third family branch which may contain a
pure color-flavor-locked (CFL) core.Comment: 6 pages, presented in "Strange Quarks in Matter" (SQM2003)
conference, Atlantic Beach, NC, USA, March 12-17, 2003 and to be published in
J. Phys.
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