27 research outputs found
A new X-ray look into four old pulsars
We report on the X-ray properties of four rotation-powered pulsars with
characteristic ages in the range 0.3-5 Myr, derived from the analysis of
XMM-Newton archival observations. We found convincing evidence of thermal
emission only in the phase-averaged spectrum of PSR B0114+58, that is well
fitted by a blackbody with temperature kT = keV and emitting
radius R = m, consistent with the size of its polar cap. The
other three considered pulsars, PSR B0628-28, PSR B0919+06 and PSR B1133+16,
have phase-averaged spectra well described by single power-laws with photon
index {\Gamma} ~ 3. The 3{\sigma} upper limits on the bolometric luminosity of
a possible thermal component with temperature in the range ~ 0.05-2 keV are
L_bol erg/s and L_bol
erg/s, for PSR B0628-28 and PSR B0919+06, respectively. On the other hand, we
found possible evidence that the pulsed emission of PSR B0628-28 is thermal.
Two absorption lines at ~0.22 keV and ~0.44 keV are detected in the spectrum of
PSR B1133+16. They are best interpreted as proton cyclotron features, implying
the presence of multipolar components with a field of a few G at the
neutron star polar caps. We discuss our results in the context of high-energy
emission models of old rotation-powered pulsars.Comment: Accepted by A&A, 15 pages, 11 figures, 7 table
The radio and X-ray mode-switching pulsar PSR B0943+10
Observations obtained in the last years challenged the widespread notion that
rotation-powered neutron stars are steady X-ray emitters. Besides a few
allegedly rotation-powered neutron stars that showed "magnetar-like"
variability, a particularly interesting case is that of PSR B0943+10. Recent
observations have shown that this pulsar, well studied in the radio band where
it alternates between a bright and a quiescent mode, displays significant X-ray
variations, anticorrelated in flux with the radio emission. The study of such
synchronous radio/X-ray mode switching opens a new window to investigate the
processes responsible for the pulsar radio and high-energy emission. Here we
review the main X-ray properties of PSR B0943+10 derived from recent
coordinated X-ray and radio observations.Comment: Published in Journal of Astrophysics and Astronomy special issue on
'Physics of Neutron Stars and Related Objects', celebrating the 75th
birth-year of G. Srinivasa
Thermal emission and magnetic beaming in the radio and X-ray mode-switching PSR B0943+10
PSR B0943+10 is a mode-switching radio pulsar characterized by two emission
modes with different radio and X-ray properties. Previous studies, based on
simple combinations of blackbody and power law models, showed that its X-ray
flux can be decomposed in a pulsed thermal plus an unpulsed non-thermal
components. However, if PSR B0943+10 is a nearly aligned rotator seen pole-on,
as suggested by the radio data, it is difficult to reproduce the high observed
pulsed fraction unless magnetic beaming is included. In this work we reanalyze
all the available X-ray observations of PSR B0943+10 with simultaneous radio
coverage, modeling its thermal emission with polar caps covered by a magnetized
hydrogen atmosphere or with a condensed iron surface. The condensed surface
model provides good fits to the spectra of both pulsar modes, but, similarly to
the blackbody, it can not reproduce the observed pulse profiles, unless an
additional power law with an ad hoc modulation is added. Instead, the pulse
profiles and phase-resolved spectra are well described using the hydrogen
atmosphere model to describe the polar cap emission, plus an unpulsed power
law. For the X-ray brighter state (Q-mode) we obtain a best fit with a
temperature kT~0.09 keV, an emitting radius R~260 m, a magnetic field
consistent with the value of the dipole field of 4x10^12 G inferred from the
timing parameters, and a small angle between the magnetic and spin axis,
=5. The corresponding parameters for the X-ray fainter state (B-mode) are
kT~0.08 keV and R~170 m.Comment: 16 pages, 10 figures, accepted for publication in Ap
Timing the X-ray pulsating companion of the hot-subdwarf HD 49798 with NICER
HD 49798 is a hot subdwarf of O spectral type in a 1.55 day orbit with the
X-ray source RX J0648.0-4418, a compact object with spin period of 13.2 s. We
use recent data from the NICER instrument, joined with archival data from
XMM-Newton and ROSAT, to obtain a phase-connected timing solution spanning ~30
years. Contrary to previous works, that relied on parameters determined through
optical observations, the new timing solution could be derived using only X-ray
data. We confirm that the compact object is steadily spinning up with Pdot =
-2.28(2)x10^-15 s/s and obtain a refined measure of the projected semi-major
axis of the compact object aX sini = 9.60(5) lightsec. This allows us to
determine the inclination and masses of the system as i = 84.5(7) deg, MX =
1.220(8) Msun and Mopt = 1.41(2) Msun. We also study possible long term (~year)
and orbital variations of the soft X-ray pulsed flux, without finding evidence
for variability. In the light of the new findings, we discuss the nature of the
compact object, concluding that the possibility of a neutron star in the
subsonic propeller regime is unlikely, while accretion of the subdwarf wind
onto a massive white dwarf can explain the observed luminosity and spin-up rate
for a wind velocity of ~800 km/s.Comment: Accepted for publication in MNRAS, 7 pages, 4 figures, 2 table
XMM-Newton and INTEGRAL observations of the bright GRB 230307A : vanishing of the local absorption and limits on the dust in the Magellanic Bridge
230307A is the second brightest gamma ray burst detected in more than 50
years of observations and is located in the direction of the Magellanic Bridge.
Despite its long duration, it is most likely the result of the compact merger
of a binary ejected from a galaxy in the local universe (redshift z=0.065). Our
XMM-Newton observation of its afterglow at 4.5 days shows a power-law spectrum
with photon index , unabsorbed flux erg cm s and no absorption in
excess of that produced in our Galaxy and in the Magellanic Bridge. We derive a
limit of cm on the absorption
at the GRB redshift, which is a factor 5 below the value measured
during the prompt phase. We searched for the presence of dust scattering rings
with negative results and set an upper limit of the order of on the
absorption from dust in the Magellanic Bridge.Comment: Version accepted for publication on The Astrophysical Journal (a few
changes and more figures
Candidate isolated neutron stars in the 4XMM-DR10 catalog of X-ray sources
Most isolated neutron stars have been discovered thanks to the detection of
their pulsed non-thermal emission, at wavelengths spanning from radio to
gamma-rays. However, if the beamed non-thermal radiation does not intercept our
line of sight or it is too faint or absent, isolated neutron stars can also be
detected through their thermal emission, which peaks in the soft X-ray band and
is emitted nearly isotropically. In the past thirty years, several
thermally-emitting isolated neutron stars have been discovered thanks to X-ray
all-sky surveys, observations targeted at the center of supernova remnants, or
as serendipitous X-ray sources. Distinctive properties of these relatively rare
X-ray sources are very soft spectra and high ratios of X-ray to optical flux.
The recently released 4XMM-DR10 catalog contains more than half a million X-ray
sources detected with the XMM-Newton telescope in the 0.2-10 keV range in
observations carried out from 2000 to 2019. Based on a study of the spectral
properties of these sources and on cross-correlations with catalogs of possible
counterparts, we have carried out a search of isolated neutron stars, finding
four potential candidates. The spectral and long-term variability analysis of
these candidates, using also Chandra and Swift-XRT data, allowed us to point
out the most interesting sources deserving further multiwavelength
investigations
Strongly pulsed thermal X-rays from a single extended hot spot on PSR J2021+4026
The radio-quiet pulsar PSR J2021+4026 is mostly known because it is the only
rotation-powered pulsar that shows variability in its {\gamma}-ray emission.
Using XMM-Newton archival data, we first confirmed that its flux is steady in
the X-ray band, and then we showed that both the spectral and timing X-ray
properties, i.e. the narrow pulse profile, the high pulsed fraction of 80-90%
and its dependence on the energy, can be better reproduced using a magnetized
atmosphere model instead of a simply blackbody. With a maximum likelihood
analysis in the energy-phase space, we inferred that the pulsar has, in
correspondence of one magnetic pole, a hot spot of temperature T~1 MK and
colatitude extension {\theta}~20{\deg}. For the pulsar distance of 1.5 kpc,
this corresponds to a cap of R~5-6 km, greater than the standard dimension of
the dipolar polar caps. The large pulsed fraction further argues against
emission from the entire star surface, as it would be expected in the case of
secular cooling. An unpulsed (<40% pulsed fraction), non-thermal component,
probably originating in a wind nebula, is also detected. The pulsar geometry
derived with our spectral fits in the X-ray is relatively well constrained
({\chi}=90{\deg} and {\xi}=20-25{\deg}) and consistent with that deduced from
{\gamma}-ray observations, provided that only one of the two hemispheres is
active. The evidence for an extended hot spot in PSR J2021+4026, found also in
other pulsars of similar age but not in older objects, suggests a possible age
dependence of the emitting size of thermal X-rays.Comment: Accepted for publication in A&A, 10 pages, 3 figures, 3 table
Thermal and non-thermal X-ray emission from the rotation-powered radio/γ-ray pulsar PSR J1740+1000
We report the results of new XMM-Newton observations of the middle-aged (τc = 1.1 × 105 yr) radio pulsar PSR J1740+1000 carried out in 2017–2018. These long pointings (∼530 ks) show that the non-thermal emission, well described by a power-law spectrum with photon index Γ = 1.80 ± 0.17, is pulsed with a ∼30 per cent pulsed fraction above 2 keV. The thermal emission can be well-fit with the sum of two blackbodies of temperatures kT1 = 70 ± 4 eV and kT2 = 137 ± 7 eV, and emitting radii R1=5.4+1.3−0.9 km and R2=0.70+0.15−0.13 km (for a distance of 1.2 kpc). We found no evidence for absorption lines as those observed in the shorter XMM-Newton observations (∼67 ks) of this pulsar carried out in 2006. The X-ray thermal and non-thermal components peak in antiphase and none of them is seen to coincide in phase with the radio pulse. This, coupled with the small difference in the emission radii of the two thermal components, disfavours an interpretation in which the dipolar polar cap is heated by magnetospheric backward-accelerated particles. Comparison with the other thermally emitting isolated neutron stars with spectra well described by the sum of two components at different temperatures shows that the ratios T2/T1 and R2/R1 are similar for objects of different classes. The observed values cannot be reproduced with simple temperature distributions, such as those caused by a dipolar field, indicating the presence of more complicated thermal maps
Two decades of X-ray observations of the isolated neutron star RX J1856.5-3754: detection of thermal and non-thermal hard X-rays and refined spin-down measurement
The soft X-ray pulsar RX J1856.5-3754 is the brightest member of a small
class of thermally-emitting, radio-silent, isolated neutron stars. Its X-ray
spectrum is almost indistinguishable from a blackbody with eV, but evidence of harder emission above keV has been recently
found. We report on a spectral and timing analysis of RX J1856.5-3754 based on
the large amount of data collected by XMM-Newton in 2002--2022, complemented by
a dense monitoring campaign carried out by NICER in 2019. Through a
phase-coherent timing analysis we obtained an improved value of the spin-down
rate Hz s, reducing by more than one
order magnitude the uncertainty of the previous measurement, and yielding a
characteristic spin-down field of G. We also detect two
spectral components above keV: a blackbody-like one with
eV and emitting radius m, and a power law
with photon index . The power-law 2--8\,keV flux,
erg cm s, corresponds to an
efficiency of , in line with that seen in other pulsars. We also
reveal a small difference between the -- keV and -- keV
pulse profiles, as well as some evidence for a modulation above keV.
These results show that, notwithstanding its simple spectrum, \eighteen still
has a non-trivial thermal surface distribution and features non-thermal
emission as seen in other pulsars with higher spin-down power.Comment: 10 pages, 7 figures, 5 tables, accepted for publication in MNRA
EXTraS discovery of a peculiar flaring X-ray source in the Galactic globular cluster NGC 6540
We report the discovery of a flaring X-ray source in the globular cluster NGC
6540, obtained during the EXTraS project devoted to a systematic search for
variability in archival data of the XMM-Newton satellite. The source had a
quiescent X-ray luminosity of the order of ~10^32 erg/s in the 0.5-10 keV range
(for a distance of NGC 6540 of 4 kpc) and showed a flare lasting about 300 s.
During the flare, the X-ray luminosity increased by more than a factor 40, with
a total emitted energy of ~10^36 erg. These properties, as well as Hubble Space
Telescope photometry of the possible optical counterparts, suggest the
identification with a chromospherically active binary. However, the flare
luminosity is significantly higher than what commonly observed in stellar
flares of such a short duration, leaving open the possibility of other
interpretations.Comment: To appear in Astronomy and Astrophysic