124 research outputs found
SecureMEMS: Selective Deposition of Energetic Materials
There exists a pressing operational need to secure and control access to high-valued electromechanical systems, and in some cases render them inoperable. Developing a reliable method for depositing energetic materials will allow for the near-seamless integration of electromechanical systems and energetic material, and, in turn, provide the pathway for security and selective destruction that is needed. In this work, piezoelectric inkjet printing was used to selectively deposit energetic materials. Nanothermites, comprising of nanoscale aluminum and nanoscale copper oxide suspended in dimethyl-formamide (DMF), were printed onto silicon wafers, which enabled both thermal and thrust measurements of the decomposing energetic material. Various solids loadings were studied in order to optimize printing characteristics. Going forward, further studies will focus on the plausibility of inkjet printing other energetic materials for the purposes of the degradation of electromechanical systems
High Speed X-ray Phase Contrast Imaging of Energetic Composites under Dynamic Compression
Fracture of crystals and frictional heating are associated with the formation of âhot spotsâ (localized heating) in energetic composites such as polymer bonded explosives (PBXs). Traditional high speed optical imaging methods cannot be used to study the dynamic sub-surface deformation and the fracture behavior of such materials due to their opaque nature. In this study, high speed synchrotron X-ray experiments are conducted to visualize the in situ deformation and the fracture mechanisms in PBXs composed of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and hydroxyl-terminated polybutadiene binder doped with iron (III) oxide. A modified Kolsky bar apparatus was used to apply controlled dynamic compression on the PBX specimens, and a high speed synchrotron X-ray phase contrast imaging (PCI) setup was used to record the in situ deformation and failure in the specimens. The experiments show that synchrotron X-ray PCI provides a sufficient contrast between the HMX crystals and the doped binder, even at ultrafast recording rates. Under dynamic compression, most of the cracking in the crystals was observed to be due to the tensile stress generated by the diametral compression applied from the contacts between the crystals. Tensile stress driven cracking was also observed for some of the crystals due to the transverse deformation of the binder and superior bonding between the crystal and the binder. The obtained results are vital to develop improved understanding and to validate the macroscopic and mesoscopic numerical models for energetic composites so that eventually hot spot formation can be predicted
Turning Points in the Evolution of Isolated Neutron Stars' Magnetic Fields
During the life of isolated neutron stars (NSs) their magnetic field passes
through a variety of evolutionary phases. Depending on its strength and
structure and on the physical state of the NS (e.g. cooling, rotation), the
field looks qualitatively and quantitatively different after each of these
phases. Three of them, the phase of MHD instabilities immediately after NS's
birth, the phase of fallback which may take place hours to months after NS's
birth, and the phase when strong temperature gradients may drive thermoelectric
instabilities, are concentrated in a period lasting from the end of the
proto--NS phase until 100, perhaps 1000 years, when the NS has become almost
isothermal. The further evolution of the magnetic field proceeds in general
inconspicuous since the star is in isolation. However, as soon as the product
of Larmor frequency and electron relaxation time, the so-called magnetization
parameter, locally and/or temporally considerably exceeds unity, phases, also
unstable ones, of dramatic changes of the field structure and magnitude can
appear. An overview is given about that field evolution phases, the outcome of
which makes a qualitative decision regarding the further evolution of the
magnetic field and its host NS.Comment: References updated, typos correcte
X-ray emission line gas in the LINER galaxy M81
We present the soft X-ray spectrum of the LINER galaxy M81 derived from a
long observation with the XMM RGS. The spectrum is dominated by continuum
emission from the active nucleus, but also contains emission lines from Fe L,
and H-like and He-like N, O, and Ne. The emission lines are significantly
broader than the RGS point-source spectral resolution; in the cross dispersion
direction the emission lines are detected adjacent to, as well as coincident
with, the active nucleus. This implies that they originate in a region of a few
arc-minutes spatial extent (1 arc-minute ~ 1 kpc in M81). The flux ratios of
the OVII triplet suggest that collisional processes are responsible for the
line emission. A good fit to the whole RGS spectrum is obtained using a model
consisting of an absorbed power law from the active nucleus and a 3 temperature
optically thin thermal plasma. Two of the thermal plasma components have
temperatures of 0.18+-0.04 keV and 0.64+-0.04 keV, characteristic of the hot
interstellar medium produced by supernovae; the combined luminosity of the
plasma at these two temperatures accounts for all the unresolved bulge X-ray
emission seen in the Chandra observation by Tennant et al. (2001). The third
component has a higher temperature (~1.7 keV), and we argue that this, along
with some of the 0.64 keV emission, comes from X-ray binaries in the bulge of
M81.Comment: 7 pages, 4 figures, accepted for publication in A&
Complex variations of X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431
We report on \ixpe observations of the Be-transient X-ray pulsar LS V +44
17/RX J0440.9+4431 at two luminosity levels during the giant outburst in
January--February 2023. Considering the observed spectral variability and
changes in the pulse profiles, the source was likely caught in super- and
sub-critical states with significantly different emission region geometry,
associated with the presence of accretion columns and hot spots, respectively.
We focus here on the pulse-phase resolved polarimetric analysis and find that
the observed dependencies of the polarization degree and polarization angle
(PA) on pulse phase are indeed drastically different for the two observations.
The observed differences, if interpreted within the framework of the rotating
vector model (RVM), imply dramatic variations of the spin axis inclination and
the position angle and the magnetic colatitude by tens of degrees within just a
few days separating the observations. We suggest that the apparent changes in
the observed PA phase dependence are predominantly related to the presence of a
polarized unpulsed component in addition to the polarized radiation associated
with the pulsar itself. We show that the observed PA phase dependence in both
observations can then be explained with a single set of RVM parameters defining
the pulsar's geometry. We also suggest that the additional polarized component
is likely produced by scattering of the pulsar radiation off the equatorial
disk wind.Comment: 9 pages, 5 figures, submitted to A&
Discovery of strongly variable X-ray polarization in the neutron star low-mass X-ray binary transient XTE J1701-462
CONTEXT:
After about 16 years since its first outburst, the transient neutron star low-mass X-ray binary XTE J1701â462 turned on again in September 2022, allowing for the first study of its X-ray polarimetric characteristics by a dedicated observing program with the Imaging X-ray Polarimeter Explorer (IXPE).
AIMS:
Polarimetric studies of XTE J1701â462 have been expected to improve our understanding of accreting weakly magnetized neutron stars, in particular, the physics and the geometry of the hot inner regions close to the compact object.
METHOD:
The IXPE data of two triggered observations were analyzed using time-resolved spectroscopic and polarimetric techniques, following the source along its Z-track of the colorâcolor diagram.
RESULTS:
During the first pointing on 2022 September 29, an average 2â8 keV polarization degree of (4.6â
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0.4)% was measured, the highest value found up to now for this class of sources. Conversely, only a âŒ0.6% average degree was obtained during the second pointing ten days later.
CONCLUSIONS:
The polarimetric signal appears to be strictly related to the higher energy blackbody component associated with the boundary layer (BL) emission and its reflection from the inner accretion disk, and it is as strong as 6.1% and 1.2% (> 95% significant) above 3â4 keV for the two measurements, respectively. The variable polarimetric signal is apparently related to the spectral characteristics of XTE J1701â462, which is the strongest when the source was in the horizontal branch of its Z-track and the weakest in the normal branch. These IXPE results provide new important observational constraints on the physical models and geometry of the Z-sources. Here, we discuss the possible reasons for the presence of strong and variable polarization among these sources
Uncovering the geometry of the hot X-ray corona in the Seyfert galaxy NGC 4151 with IXPE
We present an X-ray spectropolarimetric analysis of the bright Seyfert galaxy NGC 4151. The source has been observed with the Imaging X-ray Polarimetry Explorer (IXPE) for 700 ks, complemented with simultaneous XMMâNewton (50 ks) and NuSTAR (100 ks) pointings. A polarization degree Î Â = 4.9 ± 1.1 per cent and angle Κ = 86° ± 7° east of north (68 perâcent confidence level) are measured in the 2â8 keV energy range. The spectropolarimetric analysis shows that the polarization could be entirely due to reflection. Given the low reflection flux in the IXPE band, this requires, however, a reflection with a very large (>38 per cent) polarization degree. Assuming more reasonable values, a polarization degree of the hot corona ranging from âŒ4 to âŒ8 per cent is found. The observed polarization degree excludes a âsphericalâ lamppost geometry for the corona, suggesting instead a slab-like geometry, possibly a wedge, as determined via Monte Carlo simulations. This is further confirmed by the X-ray polarization angle, which coincides with the direction of the extended radio emission in this source, supposed to match the disc axis. NGC 4151 is the first active galactic nucleus with an X-ray polarization measure for the corona, illustrating the capabilities of X-ray polarimetry and IXPE in unveiling its geometry
The X-ray polarization of the Seyfert 1 galaxy IC 4329A
We present an X-ray spectro-polarimetric analysis of the bright Seyfert galaxy IC 4329A. The Imaging X-ray Polarimetry Explorer (IXPE) observed the source for âŒ500 ks, supported by XMMâNewton (âŒ60 ks) and NuSTAR (âŒ80 ks) exposures. We detect polarization in the 2â8 keV band with 2.97Ï confidence. We report a polarization degree of 3.3 ± 1.1 perâcent and a polarization angle of 78° ± 10° (errors are 1Ï confidence). The X-ray polarization is consistent with being aligned with the radio jet, albeit partially due to large uncertainties on the radio position angle. We jointly fit the spectra from the three observatories to constrain the presence of a relativistic reflection component. From this, we obtain constraints on the inclination angle to the inner disc (<39° at 99 perâcent confidence) and the disc inner radius (<11 gravitational radii at 99 perâcent confidence), although we note that modelling systematics in practice add to the quoted statistical error. Our spectropolarimetric modelling indicates that the 2â8 keV polarization is consistent with being dominated by emission directly observed from the X-ray corona, but the polarization of the reflection component is completely unconstrained. Our constraints on viewer inclination and polarization degree tentatively favour more asymmetric, possibly out-flowing, coronal geometries that produce more highly polarized emission, but the coronal geometry is unconstrained at the 3Ï level
The geometry of the hot corona in MCG-05-23-16 constrained by X-ray polarimetry
We report on the second observation of the radio-quiet active galactic nucleus MCG-05-23-16 performed with the Imaging X-ray Polarimetry Explorer (IXPE). The observation started on 2022 November 6 for a net observing time of 640 ks, and was partly simultaneous with NuSTAR (86 ks). After combining these data with those obtained in the first IXPE pointing on 2022 May (simultaneous with XMMâNewton and NuSTAR) we find a 2â8 keV polarization degree Î Â = 1.6 ± 0.7 (at 68 perâcent confidence level), which corresponds to an upper limit Î Â = 3.2 perâcent (at 99 perâcent confidence level). We then compare the polarization results with Monte Carlo simulations obtained with the monk code, with which different coronal geometries have been explored (spherical lamppost, conical, slab, and wedge). Furthermore, the allowed range of inclination angles is found for each geometry. If the best-fitting inclination value from a spectroscopic analysis is considered, a cone-shaped corona along the disc axis is disfavoured
X-ray polarimetry and spectroscopy of the neutron star low-mass X-ray binary GX 9+9: An in-depth study with IXPE and NuSTAR
We report on a comprehensive analysis of simultaneous X-ray polarimetric and spectral data of the bright atoll source GX 9+9 with the Imaging X-ray Polarimetry Explorer (IXPE) and NuSTAR. The source is significantly polarized in the 4â8 keV band, with a degree of 2.2%ââ
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â0.5% (uncertainty at the 68% confidence level). The NuSTAR broad-band spectrum clearly shows an iron line, and is well described by a model including thermal disc emission, a Comptonized component, and reflection. From a spectro-polarimetric fit, we obtain an upper limit to the polarization degree of the disc of 4% (at the 99% confidence level), while the contribution of Comptonized and reflected radiation cannot be conclusively separated. However, the polarization is consistent with resulting from a combination of Comptonization in a boundary or spreading layer, plus reflection off the disc, which significantly contributes in any realistic scenario
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