1,347 research outputs found
NuSTAR discovery of a cyclotron line in the accreting X-ray pulsar IGR J16393-4643
The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was
observed by NuSTAR in the 3-79 keV energy band for a net exposure time of 50
ks. We present the results of this observation which enabled the discovery of a
cyclotron resonant scattering feature with a centroid energy of 29.3(+1.1/-1.3)
keV. This allowed us to measure the magnetic field strength of the neutron star
for the first time: B = (2.5+/-0.1)e12 G. The known pulsation period is now
observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone a
long-term spin-up trend at a rate of P' = -2e-8 s/s (-0.6 s per year, or a
frequency derivative of nu' = 3e-14 Hz/s ). In the power density spectrum, a
break appears at the pulse frequency which separates the zero slope at low
frequency from the steeper slope at high frequency. This addition of angular
momentum to the neutron star could be due to the accretion of a quasi-spherical
wind, or it could be caused by the transient appearance of a prograde accretion
disk that is nearly in corotation with the neutron star whose magnetospheric
radius is around 2e8 cm.Comment: Accepted for publication in the Astrophysical Journal, 7 pages, 8
figures, 2 table
The NuSTAR Extragalactic Surveys: unveiling rare, buried AGNs and detecting the contributors to the peak of the Cosmic X-ray Background
We report on the results of active galactic nuclei (AGNs) detection by NuSTAR
performed in three extragalactic survey fields (COSMOS, UDS, ECDFS) in three
hard bands, namely H1 (8-16 keV), H2 (16-24 keV) and VH (35-55 keV). The
aggregated area of the surveys is deg. While a large number of
sources is detected in the H1 band (72 at the level of reliability), the
H2 band directly probing close to the peak of the Cosmic X-ray Background (CXB)
returns four significant detections, and two tentative, although not
significant, detections are found in the VH band. All the sources detected
above 16 keV are also detected at lower energies. We compute the integral
number counts for sources in such bands, which show broad consistency with
population synthesis models of the CXB. We furthermore identify two
Compton-thick AGNs, one in the COSMOS field, associated with a hard and faint
Chandra source, and one in the UDS field, never detected in the X-ray band
before. Both sources are at the same redshift , which shifts their
Compton-hump into the H1 band, and were previously missed in the usually
employed NuSTAR bands, confirming the potential of using the H1 band to
discover obscured AGNs at in deep surveys.Comment: 12 pages, 5 figures, accepted for publication in The Astrophysical
Journa
NuSTAR Discovery of a Cyclotron Line in the Be/X-ray Binary RX J0520.5-6932 During Outburst
We present spectral and timing analysis of NuSTAR observations of RX
J0520.56932 in the 3-79 keV band collected during its outburst in January
2014. The target was observed on two epochs and we report the detection of a
cyclotron resonant scattering feature with central energies of keV and keV during the two
observations, respectively, corresponding to a magnetic field of G. The 3-79 keV luminosity of the system during the two epochs
assuming a nominal distance of 50 kpc was and
. Both values are much higher
than the critical luminosity of
above which a radiation dominated shock front may be expected. This adds a new
object to the sparse set of three systems that have a cyclotron line observed
at luminosities in excess of . A broad
( keV) Fe emission line is observed in the spectrum at a
central energy of keV in both epochs. The pulse profile
of the pulsar was observed to be highly asymmetric with a sharply rising and
slowly falling profile of the primary peak. We also observed minor variations
in the cyclotron line energy and width as a function of the rotation phase.% As
in observations of other cyclotron absorption line sources, there is a small
() phase difference between the peak of the cyclotron
energy variation and the peak of the flux variation.Comment: 9 pages, 4 figures, 6 tables. Accepted to Ap
Broadband X-ray Properties of the Gamma-ray Binary 1FGL J1018.6-5856
We report on NuSTAR, XMM-Newton and Swift observations of the gamma-ray
binary 1FGL J1018.6-5856. We measure the orbital period to be 16.544+/-0.008
days using Swift data spanning 1900 days. The orbital period is different from
the 2011 gamma-ray measurement which was used in the previous X-ray study of An
et al. (2013) using ~400 days of Swift data, but is consistent with a new
gamma-ray solution reported in 2014. The light curve folded on the new period
is qualitatively similar to that reported previously, having a spike at phase 0
and broad sinusoidal modulation. The X-ray flux enhancement at phase 0 occurs
more regularly in time than was previously suggested. A spiky structure at this
phase seems to be a persistent feature, although there is some variability.
Furthermore, we find that the source flux clearly correlates with the spectral
hardness throughout all orbital phases, and that the broadband X-ray spectra
measured with NuSTAR, XMM-Newton, and Swift are well fit with an unbroken
power-law model. This spectrum suggests that the system may not be
accretion-powered.Comment: 8 pages, 4 figures. Accepted for publication in Ap
Phase-resolved NuSTAR and Swift-XRT Observations of Magnetar 4U 0142+61
We present temporal and spectral analysis of simultaneous 0.5-79 keV
Swift-XRT and NuSTAR observations of the magnetar 4U 0142+61. The pulse profile
changes significantly with photon energy between 3 and 35 keV. The pulse
fraction increases with energy, reaching a value of ~20%, similar to that
observed in 1E 1841-045 and much lower than the ~80% pulse fraction observed in
1E 2259+586. We do not detect the 55-ks phase modulation reported in previous
Suzaku-HXD observations. The phase-averaged spectrum of 4U 0142+61 above 20 keV
is dominated by a hard power law with a photon index, ~ 0.65, and the
spectrum below 20 keV can be described by two blackbodies, a blackbody plus a
soft power law, or by a Comptonized blackbody model. We study the full
phase-resolved spectra using the electron-positron outflow model of Beloborodov
(2013). Our results are consistent with the parameters of the active j-bundle
derived from INTEGRAL data by Hascoet et al. (2014). We find that a significant
degeneracy appears in the inferred parameters if the footprint of the j-bundle
is allowed to be a thin ring instead of a polar cap. The degeneracy is reduced
when the footprint is required to be the hot spot inferred from the soft X-ray
data.Comment: 14 pages, 8 figures, 4 tables. Accepted for publication in Ap
Looking at A 0535+26 at low luminosities with NuSTAR
We report on two NuSTAR observations of the HMXB A 0535+26 taken toward the
end of its normal 2015 outburst at very low keV luminosities of
erg/s and erg/s which are
complemented by 9 Swift observations. The data clearly confirm indications seen
in earlier data that the source's spectral shape softens as it becomes fainter.
The smooth, exponential rollover at high energies present in the first
observation evolves to a much more abrupt steepening of the spectrum at
keV. The continuum evolution can be well described with emission from a
magnetized accretion column, modeled using the compmag model modified by an
additional Gaussian emission component for the fainter observation. Between the
two observations, the optical depth changes from to
, the electron temperature remains constant, and there is
an indication that the column decreases in radius. Since the energy resolved
pulse profiles remain virtually unchanged in shape between the two
observations, the emission properties of the accretion column, however, reflect
the same accretion regime. This conclusion is also confirmed by our result that
the energy of the cyclotron resonant scattering feature (CRSF) at
keV is independent of the luminosity, implying that the magnetic field in the
region in which the observed radiation is produced is the same in both
observations. Finally, we also constrain the evolution of the continuum
parameters with rotational phase of the neutron star. The width of the CRSF
could only be constrained for the brighter observation. Based on Monte-Carlo
simulations of CRSF formation in single accretion columns, its pulse phase
dependence supports a simplified fan beam emission pattern. The evolution of
the CRSF width is very similar to that of the CRSF depth, which is in
disagreement with expectations.Comment: 14 pages, 11 figures, 3 tables, accepted for publication in A&
NuSTAR discovery of an unusually steady long-term spin-up of the Be binary 2RXP J130159.6-635806
We present spectral and timing analysis of NuSTAR observations of the
accreting X-ray pulsar 2RXP J130159.6-635806. The source was serendipitously
observed during a campaign focused on the gamma-ray binary PSR B1259-63 and was
later targeted for a dedicated observation. The spectrum has a typical shape
for accreting X-ray pulsars, consisting of a simple power law with an
exponential cutoff starting at ~7 keV with a folding energy of E_fold=~18 keV.
There is also an indication of the presence of a 6.4 keV iron line in the
spectrum at the ~3 sigma significance level. NuSTAR measurements of the
pulsation period reveal that the pulsar has undergone a strong and steady
spin-up for the last 20 years. The pulsed fraction is estimated to be ~80%, and
is constant with energy up to 40 keV. The power density spectrum shows a break
towards higher frequencies relative to the current spin period. This, together
with steady persistent luminosity, points to a long-term mass accretion rate
high enough to bring the pulsar out of spin equilibrium.Comment: 13 pages, 7 figures, accepted for publication in The Astrophysical
Journa
A Spatially Resolved Study of the Synchrotron Emission and Titanium in Tycho's Supernova Remnant with NuSTAR
We report results from deep observations (~750 ks) of Tycho's supernova
remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over
several energy bands to identify the regions producing the hardest X-rays and
to search for radioactive decay line emission from 44Ti. We find that the
hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where
recent Chandra observations have revealed high emissivity "stripes" associated
with particles accelerated to the knee of the cosmic-ray spectrum. We do not
find evidence of 44Ti, and we set limits on its presence and distribution
within the SNR. These limits correspond to a upper-limit 44Ti mass of M44 <
2.4x10^-4 M_sun for a distance of 2.3 kpc. We perform spatially resolved
spectroscopic analysis of sixty-six regions across Tycho. We map the best-fit
rolloff frequency of the hard X-ray spectra, and we compare these results to
measurements of the shock expansion and ambient density. We find that the
highest energy electrons are accelerated at the lowest densities and in the
fastest shocks, with a steep dependence of the roll-off frequency with shock
velocity. Such a dependence is predicted by models where the maximum energy of
accelerated electrons is limited by the age of the SNR rather than by
synchrotron losses, but this scenario requires far lower magnetic field
strengths than those derived from observations in Tycho. One way to reconcile
these discrepant findings is through shock obliquity effects, and future
observational work is necessary to explore the role of obliquity in the
particle acceleration process.Comment: 12 pages, 12 figures, ApJ in pres
Impact of pore fluid chemistry on fine‐grained sediment fabric and compressibility
Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 123 (2018): 5495-5514, doi:10.1029/2018JB015872.Fines, defined here as grains or particles, less than 75 μm in diameter, exist nearly ubiquitously in natural sediment, even those classified as coarse. Macroscopic sediment properties, such as compressibility, which relates applied effective stress to the resulting sediment deformation, depend on the fabric of fines. Unlike coarse grains, fines have sizes and masses small enough to be more strongly influenced by electrical interparticle forces than by gravity. These electrical forces acting through pore fluids are influenced by pore fluid chemistry changes. Macroscopic property dependence on pore fluid chemistry must be accounted for in sediment studies involving subsurface flow and sediment stability analyses, as well as in engineered flow situations such as groundwater pollutant remediation, hydrocarbon migration, or other energy resource extraction applications. This study demonstrates how the liquid limit‐based electrical sensitivity index can be used to predict sediment compressibility changes due to pore fluid chemistry changes. Laboratory tests of electrical sensitivity, sedimentation, and compressibility illustrate mechanisms linking microscale and macroscale processes for selected pure, end‐member fines. A specific application considered here is methane extraction via depressurization of gas hydrate‐bearing sediment, which causes a dramatic pore water salinity drop concurrent with sediment being compressed by the imposed effective stress increase.DOI U.S. Geological Survey (USGS);
U.S. Department of Energy (DOE) Grant Numbers: DE‐FE00‐28966, DE‐FE00‐261662019-01-1
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