1,715 research outputs found
A new model for the X-ray continuum of the magnetized accreting pulsars
Accreting highly magnetized pulsars in binary systems are among the brightest
X-ray emitters in our Galaxy. Although a number of high statistical quality
broad-band (0.1-100 keV) X-ray observations are available, the spectral energy
distribution of these sources is usually investigated by adopting pure
phenomenological models, rather than models linked to the physics of accretion.
In this paper, a detailed spectral study of the X-ray emission recorded from
the high-mass X-ray binary pulsars Cen X-3, 4U 0115+63, and Her X-1 is carried
out by using BeppoSAX and joined Suzaku+NuStar data, together with an advanced
version of the compmag model. The latter provides a physical description of the
high energy emission from accreting pulsars, including the thermal and bulk
Comptonization of cyclotron and bremsstrahlung seed photons along the neutron
star accretion column. The compmag model is based on an iterative method for
solving second-order partial differential equations, whose convergence
algorithm has been improved and consolidated during the preparation of this
paper. Our analysis shows that the broad-band X-ray continuum of all considered
sources can be self-consistently described by the compmag model. The cyclotron
absorption features, not included in the model, can be accounted for by using
Gaussian components. From the fits of the compmag model to the data we inferred
the physical properties of the accretion columns in all sources, finding values
reasonably close to those theoretically expected according to our current
understanding of accretion in highly magnetized neutron stars. The updated
version of the compmag model has been tailored to the physical processes that
are known to occur in the columns of highly magnetized accreting neutron stars
and it can thus provide a better understanding of the high energy radiation
from these sources.Comment: 19 pages, 10 figures, accepted for publication in A&
Swift monitoring of the new accreting millisecond X-ray pulsar IGRJ17511-3057 in outburst
A new accreting millisecond X-ray pulsar, IGR J17511-3057, was discovered in
outburst on 2009 September 12 during the INTEGRAL Galactic bulge monitoring
programme. To study the evolution of the source X-ray flux and spectral
properties during the outburst, we requested a Swift monitoring of
IGRJ17511-3057. In this paper we report on the results of the first two weeks
of monitoring the source. The persistent emission of IGR J17511-3057 during the
outburst is modeled well with an absorbed blackbody (kT~0.9 keV) and a
power-law component (photon index~1-2), similar to what has been observed from
other previously known millisecond pulsars. Swift also detected three type-I
Xray bursts from this source. By assuming that the peak luminosity of these
bursts is equal to the Eddington value for a pure helium type-I X-ray burst, we
derived an upper limit to the source distance of ~10 kpc. The theoretical,
expected recurrence time of the bursts according to the helium burst hypothesis
is 0.2-0.9 days, in agreement with the observations.Comment: Accepted for publication in A&A Letters. V2: corrected some typos and
added one referenc
Swift J1734.5-3027: a new long type-I X-ray bursting source
Swift J1734.5-3027 is a hard X-ray transient discovered by Swift while
undergoing an outburst in September 2013. Archival observations showed that
this source underwent a previous episode of enhanced X-ray activity in May-June
2013. In this paper we report on the analysis of all X-ray data collected
during the outburst in September 2013, the first that could be intensively
followed-up by several X-ray facilities. Our data-set includes INTEGRAL, Swift,
and XMM-Newton observations. From the timing and spectral analysis of these
observations, we show that a long type-I X-ray burst took place during the
source outburst, making Swift J1734.5-3027 a new member of the class of
bursting neutron star low-mass X-ray binaries. The burst lasted for about 1.9
ks and reached a peak flux of (6.01.8)10 erg cm
s in the 0.5-100 keV energy range. The estimated burst fluence in the
same energy range is (1.100.10)10 erg cm. By
assuming that a photospheric radius expansion took place during the first
200 s of the burst and that the accreted material was predominantly
composed by He, we derived a distance to the source of 7.21.5 kpc.Comment: Accepted for publication on A&
Glancing through the accretion column of EXO 2030+375
We took advantage of the large collecting area and good timing capabilities
of the EPIC cameras on-board XMM-Newton to investigate the accretion geometry
onto the magnetized neutron star hosted in the high mass X-ray binary EXO
2030+375 during the rise of a source Type-I outburst in 2014. We carried out a
timing and spectral analysis of the XMM-Newton observation as function of the
neutron star spin phase. We used a phenomenological spectral continuum model
comprising the required fluorescence emission lines. Two neutral absorption
components are present: one covering fully the source and one only partially.
The same analysis was also carried out on two Suzaku observations of the source
performed during outbursts in 2007 and 2012, to search for possible spectral
variations at different luminosities. The XMM-Newton data caught the source at
an X-ray luminosity of erg s and revealed the presence
of a narrow dip-like feature in its pulse profile that was never reported
before. The width of this feature corresponds to about one hundredth of the
neutron star spin period. From the results of the phase-resolved spectral
analysis we suggest that this feature can be ascribed to the self-obscuration
of the accretion stream passing in front of the observer line of sight. We
inferred from the Suzaku observation carried out in 2007 that the
self-obscuration of the accretion stream might produce a significantly wider
feature in the neutron star pulsed profile at higher luminosities
( erg s).Comment: Accepted for publication on A&
Properties and observability of glitches and anti-glitches in accreting pulsars
Several glitches have been observed in young, isolated radio pulsars, while a
clear detection in accretion-powered X-ray pulsars is still lacking. We use the
Pizzochero snowplow model for pulsar glitches as well as starquake models to
determine for the first time the expected properties of glitches in accreting
pulsars and their observability. Since some accreting pulsars show
accretion-induced long-term spin-up, we also investigate the possibility that
anti-glitches occur in these stars. We find that glitches caused by quakes in a
slow accreting neutron star are very rare and their detection extremely
unlikely. On the contrary, glitches and anti-glitches caused by a transfer of
angular momentum between the superfluid neutron vortices and the non-superfluid
component may take place in accreting pulsars more often. We calculate the
maximum jump in angular velocity of an anti-glitch and we find that it is
expected to be about 1E-5 - 1E-4 rad/s. We also note that since accreting
pulsars usually have rotational angular velocities lower than those of isolated
glitching pulsars, both glitches and anti-glitches are expected to have long
rise and recovery timescales compared to isolated glitching pulsars, with
glitches and anti-glitches appearing as a simple step in angular velocity.
Among accreting pulsars, we find that GX 1+4 is the best candidate for the
detection of glitches with currently operating X-ray instruments and future
missions such as the proposed Large Observatory for X-ray Timing (LOFT).Comment: Accepted for publication in Astronomy & Astrophysics. 6 pages. Minor
changes to match the final A&A versio
Discovery of a new accreting millisecond X-ray pulsar in the globular cluster NGC 2808
We report on the discovery of coherent pulsations at a period of 2.9 ms from
the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We
observed X-ray pulsations at a frequency of Hz in three different
observations of the source performed with XMM-Newton and NuSTAR during the
source outburst. This newly discovered accreting millisecond pulsar is part of
an ultra-compact binary system characterised by an orbital period of
minutes and a projected semi-major axis of lt-ms. Based on the mass
function we estimate a minimum companion mass of 0.024 M, which
assumes a neutron star mass of 1.4 M and a maximum inclination angle
of (derived from the lack of eclipses and dips in the light-curve
of the source). We find that the companion star's Roche-Lobe could either be
filled by a hot ( K) pure helium white dwarf with a 0.028
M mass (implying ) or an old (>5 Gyr) brown dwarf
with metallicity abundances between solar/sub-solar and mass ranging in the
interval 0.0650.085 M (16 < < 21). During the outburst the
broad-band energy spectra are well described by a superposition of a weak
black-body component (kT 0.5 keV) and a hard cutoff power-law with photon
index 1.7 and cut-off at a temperature kT 130 keV. Up to
the latest Swift-XRT observation performed on 2016 July 19 the source has been
observed in outburst for almost 150 days, which makes MAXI J0911-655 the second
accreting millisecond X-ray pulsar with outburst duration longer than 100 days.Comment: 7 pages, 5 figures, accepted for publication in A&
Discovery of a soft X-ray 8 mHz QPO from the accreting millisecond pulsar IGR J00291+5934
In this paper, we report on the analysis of the peculiar X-ray variability
displayed by the accreting millisecond X-ray pulsar IGR J00291+5934 in a 80
ks-long joint NuSTAR and XMM-Newton observation performed during the source
outburst in 2015. The light curve of the source was characterized by a
flaring-like behavior, with typical rise and decay time scales of ~120 s. The
flares are accompanied by a remarkable spectral variability, with the X-ray
emission being generally softer at the peak of the flares. A strong quasi
periodic oscillation (QPO) is detected at ~8 mHz in the power spectrum of the
source and clearly associated with the flaring-like behavior. This feature has
the strongest power at soft X-rays (<3 keV). We carried out a dedicated
hardness-ratio resolved spectral analysis and a QPO phase-resolved spectral
analysis, together with an in-depth study of the source timing properties, to
investigate the origin of this behavior. We suggest that the unusual
variability of IGR J00291+5934 observed by XMM-Newton and NuSTAR could be
produced by an heartbeat-like mechanism, similar to that operating in
black-hole X-ray binaries. The possibility that this variability, and the
associated QPO, are triggered by phases of quasi-stable nuclear burning, as
suggested in the literature for a number of other neutron star binaries
displaying a similar behavior, cannot be solidly tested in the case of IGR
J00291+5934 due to the paucity of type-I X-ray bursts observed from this
source.Comment: Submitted to MNRAS on 23 Sept 2016. Modified according to the
referee's suggestions. Comments are welcomed. One reference updated in this
versio
Spectral and timing properties of IGR J00291+5934 during its 2015 outburst
We report on the spectral and timing properties of the accreting millisecond
X-ray pulsar IGR J00291+5934 observed by XMM-Newton and NuSTAR during its 2015
outburst. The source is in a hard state dominated at high energies by a
comptonization of soft photons ( keV) by an electron population with
kT keV, and at lower energies by a blackbody component with
kT keV. A moderately broad, neutral Fe emission line and four narrow
absorption lines are also found. By investigating the pulse phase evolution, we
derived the best-fitting orbital solution for the 2015 outburst. Comparing the
updated ephemeris with those of the previous outbursts, we set a
confidence level interval s/s s/s on the orbital period derivative. Moreover, we
investigated the pulse profile dependence on energy finding a peculiar
behaviour of the pulse fractional amplitude and lags as a function of energy.
We performed a phase-resolved spectroscopy showing that the blackbody component
tracks remarkably well the pulse-profile, indicating that this component
resides at the neutron star surface (hot-spot).Comment: 9 pages, 7 figures. Accepted for publication in MNRA
The supergiant fast X-ray transients XTE J1739-302 and IGR J08408-4503 in quiescence with XMM-Newton
Context. Supergiant fast X-ray transients are a subclass of high mass X-ray
binaries that host a neutron star accreting mass from the wind of its OB
supergiant companion. They are characterized by an extremely pronounced and
rapid variability in X-rays, which still lacks an unambiguous interpretation. A
number of deep pointed observations with XMM-Newton have been carried out to
study the quiescent emission of these sources and gain insight into the
mechanism that causes their X-ray variability. Aims. We continued this study by
using three XMM-Newton observations of the two supergiant fast X-ray transient
prototypes XTEJ1739-302 and IGR J08408-4503 in quiescence. Methods. An in-depth
timing and spectral analysis of these data have been carried out. Results. We
found that the quiescent emission of these sources is characterized by both
complex timing and spectral variability, with multiple small flares occurring
sporadically after periods of lower X-ray emission. Some evidence is found in
the XMM-Newton spectra of a soft component below ~2 keV, similar to that
observed in the two supergiant fast X-ray transients AXJ1845.0-0433 and
IGRJ16207-5129 and in many other high mass X-ray binaries. Conclusions.We
suggest some possible interpretations of the timing and spectral properties of
the quiescent emission of XTEJ1739- 302 and IGR J08408-4503 in the context of
the different theoretical models proposed to interpret the behavior of the
supergiant fast X-ray transients.Comment: 13 pages, 14 figures. Accepted for publication in A&A. V2: Corrected
few typo
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