1,037 research outputs found
Discovery of X-ray pulsations from IGR J16320-4751 = AX J1631.9-4752
We report a discovery of strong modulations of the X-ray flux detected from
IGR J16320-4751 = AX J1631.9-4752 with a period of P~1300 sec. We reanalyzed
the data of an XMM-Newton ToO performed soon after the discovery of the source
by INTEGRAL and found the modulation at a period of P=1309+/-40 sec with a high
significance. Modulations of the source flux with two possible periods of ~1300
and ~1500 sec were identified in the ASCA archival data. It is very likely that
the modulation can be interpreted as X-ray pulsations, favouring a pulsar as
the compact object in IGR/AX J16320-4752. Thus for the moment this source
became the fourth source from a new class of highly absorbed binary systems for
which the pulsations are observed.Comment: accepted for the publication in A&A Letters, 4 pages, 4 figure
Precision Timing of Two Anomalous X-Ray Pulsars
We report on long-term X-ray timing of two anomalous X-ray pulsars, 1RXS
J170849.0-400910 and 1E 2259+586, using the Rossi X-ray Timing Explorer. In
monthly observations made over 1.4 yr and 2.6 yr for the two pulsars,
respectively, we have obtained phase-coherent timing solutions which imply that
these objects have been rotating with great stability throughout the course of
our observations. For 1RXS J170849.0-400910, we find a rotation frequency of
0.0909169331(5) Hz and frequency derivative -15.687(4) x 10^(-14) Hz/s, for
epoch MJD 51215.931. For 1E 2259+586, we find a rotation frequency of
0.1432880613(2)Hz, and frequency derivative -1.0026(7) x 10^(-14) Hz/s, for
epoch MJD 51195.583. RMS phase residuals from these simple models are only
about 0.01 cycles for both sources. We show that the frequency derivative for
1E 2259+586 is inconsistent with that inferred from incoherent frequency
observations made over the last 20 yr. Our observations are consistent with the
magnetar hypothesis and make binary accretion scenarios appear unlikely.Comment: 12 pages including 3 figures. To appear in ApJ Letter
Further evidence that 1RXS J170849.0-400910 is an Anomalous X-ray pulsar
We report the results of two ROSAT HRI observations of the recently
discovered 11s X-ray pulsar 1RXS J170849.0-400910. A refined position with a
smaller error radius (10" uncertainty) and a new spin period measurement were
obtained. These results allowed to derive a period derivative of about 7 times
10^-4 s yr^-1 and to perform a photometric and spectroscopic study of the
possible optical counterparts of the source. The limits derived from the
optical to X-ray flux ratio exclude the presence of a massive OB companion.
These findings, together with the nearly constant X-ray flux, the stability of
the pulse shape and pulsed fraction across observations spanning three years,
strongly support the inclusion of this 11s pulsar in the class of Anomalous
X-ray Pulsars (AXPs).Comment: 4 pages plus 4 postscript figures. emulateapj style. Accepted for
publication in Astrophysical Journal Letter
Emission Spectra of Fallback Disks Around Young Neutron Stars
The nature of the energy source powering anomalous X-ray pulsars is
uncertain. Proposed scenarios involve either an ultramagnetized neutron star,
or accretion onto a neutron star. We consider the accretion model proposed
recently by Chatterjee, Hernquist & Narayan, in which a disk is fed by fallback
material following a supernova. We compute the optical, infrared, and
submillimeter emission expected from such a disk, including both viscous
dissipation and reradiation of X-ray flux impinging on the disk from the
pulsar. We find that it is possible with current instruments to put serious
constraints on this and on other accretion models of AXPs. Fallback disks could
also be found around isolated radio pulsars and we compute the corresponding
spectra. We show that the excess emission in the R and I bands observed for the
pulsar PSR 0656+14 is broadly consistent with emission from a disk.Comment: 12 pages, 1 table, 4 figures, submitted to Ap
The dust-enshrouded microquasar candidate AX J1639.0-4642 = IGR J16393-4643
We present a multiwavelength study of the field containing the unidentified
X-ray source AX J1639.0-4642, discovered with the ASCA observatory and recently
detected with the IBIS telescope, onboard the INTEGRAL satellite, dubbed IGR
J16393-4643. The huge hydrogen column density towards the source, the hard
spectral index in the 0.7-10 keV band and its flux variability suggest that the
source is a High Mass X-ray Binary (HMXB) enshrouded by dust. Our search
reveals the presence of a non-thermal radio counterpart within the X-ray error
box. After a study of the broadband emission from X-rays to the radio domain,
we propose that AX J1639.0-4642 is a dust-enshrouded Microquasar (MQ)
candidate. In addition, the X-ray source is well within the 95% location
contour of the unidentified gamma-ray source 3EG J1639-4702. The main
properties of AX J1639.0-4642/3EG J1639-4702 are consistent with those of two
other MQs previously proposed to display high-energy gamma-ray emission.Comment: 7 pages, 5 figures. Accepted for publication in A&A. Title and
discussion on the possible NIR counterpart have been modifie
Photon Propagation Around Compact Objects and the Inferred Properties of Thermally Emitting Neutron Stars
Anomalous X-ray pulsars, compact non-pulsing X-ray sources in supernova
remnants, and X-ray bursters are three distinct types of sources for which
there are viable models that attribute their X-ray emission to thermal emission
from the surface of a neutron star. Inferring the surface area of the emitting
regions in such systems is crucial in assessing the viability of different
models and in providing bounds on the radii of neutron stars. We show that the
inferred areas of the emitting regions may be over- or under-estimated by a
factor of <=2, because of the geometry of the system and general relativistic
light deflection, combined with the effects of phase averaging. Such effects
make the determination of neutron-star radii uncertain, especially when
compared to the ~5% level required for constraining the equation of state of
neutron-star matter. We also note that, for a given spectral shape, the
inferred source luminosities and pulse fractions are anticorrelated because
they depend on the same properties of the emitting regions, namely their sizes
and orientations, i.e., brighter sources have on average weaker pulsation
amplitudes than fainter sources. We argue that this property can be used as a
diagnostic tool in distinguishing between different spectral models. As an
example, we show that the high inferred pulse fraction and brightness of the
pulsar RXS J1708-40 are inconsistent with isotropic thermal emission from a
neutron-star surface. Finally, we discuss the implication of our results for
surveys in the soft X-rays for young, cooling neutron stars in supernova
remnants and show that the absence of detectable pulsations from the compact
source at the center of Cas A (at a level of >=30%) is not a strong argument
againts its identification with a spinning neutron star.Comment: 6 pages, 6 figures, to appear in the Astrophysical Journal; minor
change
A new search strategy for microquasar candidates using NVSS/2MASS and XMM-Newton data
Microquasars are ideal natural laboratories for understanding
accretion/ejection processes, studying the physics of relativistic jets, and
testing gravitational phenomena. Nevertheless, these objects are difficult to
find in our Galaxy. The main goal of this work is to increase the number of
known systems of this kind. We have developed an improved search strategy based
on positional cross-identification with very restrictive selection criteria to
find new MQs, taking advantage of more sensitive modern radio and X-ray data.
We find 86 sources with positional coincidence in the NVSS/XMM catalogs at
|b|<10 deg. Among them, 24 are well-known objects and the remaining 62 sources
are unidentified. For the fully coincident sources, whenever possible, we
analyzed color-color and hardness ratio diagrams and found that at least 3 of
them display high-mass X-ray binary characteristics, making them potential
microquasar candidates.Comment: 9 pages, 3 figures, accepted for publication in A&
General Relativistic Constraints on Emission Models of Anomalous X-ray Pulsars
Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray
spectra and pulsations by means of radiation processes that occur on the
surfaces of neutron stars. For any such model, general relativistic deflection
of light severely suppresses the amplitude of the observed pulsations. We
calculate the expected pulsation amplitudes of AXPs according to various models
and compare the results with observations. We show that the high (<= 70%) pulse
amplitudes observed in some AXPs can be accounted for only if the surface
emission is localized (spot radius <40 degrees) and strongly beamed
(cos^n[theta'] with n>2, where theta' is the angle to the normal). These
constraints are incompatible with those cooling and magnetar models in which
the observed X-rays originate as thermal emission from the neutron-star
surface. Accretion models, on the other hand, are compatible with observations
for a wide range of parameters. Finally, definitive conclusions cannot be
reached on magnetospheric models, since their localization and beaming
properties are not well understood.Comment: 7 pages, 9 figures, submitted to The Astrophysical Journa
A Chandra Observation of Supernova Remnant G350.1-0.3 and Its Central Compact Object
We present a new Chandra observation of supernova remnant (SNR) G350.1-0.3.
The high resolution X-ray data reveal previously unresolved filamentary
structures and allow us to perform detailed spectroscopy in the diffuse regions
of this SNR. Spectral analysis demonstrates that the region of brightest
emission is dominated by hot, metal-rich ejecta while the ambient material
along the perimeter of the ejecta region and throughout the remnant's western
half is mostly low-temperature, shocked interstellar/circumstellar medium
(ISM/CSM) with solar-type composition. The data reveal that the emission
extends far to the west of the ejecta region and imply a lower limit of 6.6 pc
on the diameter of the source (at a distance of 4.5 kpc). We show that
G350.1-0.3 is likely in the free expansion (ejecta-dominated) stage and
calculate an age of 600-1200 years. The derived relationship between the shock
velocity and the electron/proton temperature ratio is found to be entirely
consistent with that of other SNRs. We perform spectral fits on the X-ray
source XMMU J172054.5-372652, a candidate central compact object (CCO), and
find that its spectral properties fall within the typical range of other CCOs.
We also present archival 24 um data of G350.1-0.3 taken with the Spitzer Space
Telescope during the MIPSGAL galactic survey and find that the infrared and
X-ray morphologies are well-correlated. These results help to explain this
remnant's peculiar asymmetries and shed new light on its dynamics and
evolution
X-ray observations of the high magnetic field radio pulsar PSR J1814-1744
PSR J1814-1744 is a 4 s radio pulsar with surface dipole magnetic field
strength 5.5*10^13 G, inferred assuming simple magnetic dipole braking. This
pulsar's spin parameters are very similar to those of anomalous X-ray pulsars
(AXPs), suggesting that this may be a transition object between the radio
pulsar and AXP population, if AXPs are isolated, high magnetic field neutron
stars as has recently been hypothesized. We present archival X-ray observations
of PSR J1814-1744 made with ROSAT and ASCA. X-ray emission is not detected from
the position of the radio pulsar. The derived upper flux limit implies an X-ray
luminosity significantly smaller than those of all known AXPs. This conclusion
is insensitive to the possibility that X-ray emission from PSR J1814-1744 is
beamed or that it undergoes modest variability. When interpreted in the context
of the magnetar mechanism, these results argue that X-ray emission from AXPs
must depend on more than merely the inferred surface magnetic field strength.
This suggests distinct evolutionary paths for radio pulsars and AXP, despite
their proximity in period--period derivative phase space.Comment: 11 pages, including 2 embedded figures. Accepted by Ap
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