20 research outputs found
A Study of the 20 Day Superorbital Modulation in the High-Mass X-ray Binary IGR J16493-4348
We report on Nuclear Spectroscopic Telescope Array (NuSTAR), Neil Gehrels
Swift Observatory (Swift) X-ray Telescope (XRT) and Swift Burst Alert Telescope
(BAT) observations of IGR J16493-4348, a wind-fed Supergiant X-ray Binary
(SGXB) showing significant superorbital variability. From a discrete Fourier
transform of the BAT light curve, we refine its superorbital period to be
20.058 0.007 days. The BAT dynamic power spectrum and a fractional root
mean square analysis both show strong variations in the amplitude of the
superorbital modulation, but no observed changes in the period were found. The
superorbital modulation is significantly weaker between MJD 55,700 and MJD
56,300. The joint NuSTAR and XRT observations, which were performed near the
minimum and maximum of one cycle of the 20 day superorbital modulation, show
that the flux increases by more than a factor of two between superorbital
minimum and maximum. We find no significant changes in the 3-50 keV pulse
profiles between superorbital minimum and maximum, which suggests a similar
accretion regime. Modeling the pulse-phase averaged spectra we find a possible
Fe K emission line at 6.4 keV at superorbital maximum. The feature is
not significant at superorbital minimum. While we do not observe any
significant differences between the pulse-phase averaged spectral continua
apart from the overall flux change, we find that the hardness ratio near the
broad main peak of the pulse profile increases from superorbital minimum to
maximum. This suggests the spectral shape hardens with increasing luminosity.
We discuss different mechanisms that might drive the observed superorbital
modulation.Comment: 17 pages, 14 figures, 3 tables, accepted for publication in The
Astrophysical Journal on 2019 May 1
Investigating the superorbital modulations in 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 with Swift XRT, BAT and NuSTAR observations
A puzzling variety of superorbital modulations have been discovered in
several supergiant High-Mass X-ray binaries (sgHMXBs). To investigate the
mechanisms driving these superorbital modulations, we have analyzed long-term
Neil Gehrels Swift Observatory (Swift) Burst Alert Telescope (BAT) observations
of three sgHMXBs: 4U 1909+07, IGR J16418-4532 and IGR J16479-4514 and
constructed their dynamic power spectra and superorbital intensity profiles.
These Swift BAT observations are complemented by pointed Swift X-ray Telescope
(XRT) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations performed
near the predicted maximum and minimum phase of a single superorbital cycle for
each of these sources. The BAT dynamic power spectra show changes in the
strength of the superorbital modulation on timescales of years, with either the
peak at the fundamental frequency and/or the second harmonic present at
different times for all three sources. The pointed Swift XRT and NuSTAR
observations show no significant differences between the pulse profiles and
spectral parameters at the superorbital maximum and minimum phase. This is
likely due to the fact the superorbital modulation had weakened significantly
during the times when the NuSTAR observations were carried out for all three
sources. The results from the Swift XRT, BAT and NuSTAR analysis indicate the
possible presence of multiple co-rotating interaction regions (CIRs) in the
stellar winds of the supergiant stars, although a structured stellar wind from
the supergiant star due to tidal oscillations cannot be ruled out.Comment: Accepted for publication in the Astrophysical Journa
Plane waves in periodic, quadratically nonlinear slab waveguides: stability and exact Fourier structure
We consider the propagation of broad optical beams through slab waveguides
with a purely quadratic nonlinearity and containing linear and nonlinear
long-period quasi-phase-matching gratings. An exact Floquet analysis on the
periodic, plane-wave solution shows that the periodicity can drastically alter
the growth rate of the modulational instability but that it never completely
removes the instability. The results are confirmed by direct numerical
simulation, as well as through a simpler, approximate theory for the averaged
fields that accurately predicts the low-frequency part of the spectrum.Comment: 10 Pages, 13 figures (some in two parts) new version has some typos
removed and extra references and explanation adde
Plasma Cleaning of LCLS-II-HE verification cryomodule cavities
Plasma cleaning is a technique that can be applied in superconducting
radio-frequency (SRF) cavities in situ in cryomodules in order to decrease
their level of field emission. We developed the technique for the Linac
Coherent Light Source II (LCLS-II) cavities and we present in this paper the
full development and application of plasma processing to the LCLS-II High
Energy (HE) verification cryomodule (vCM). We validated our plasma processing
procedure on the vCM, fully processing four out of eight cavities of this CM,
demonstrating that cavities performance were preserved in terms of both
accelerating field and quality factor. Applying plasma processing to this
clean, record breaking cryomodule also showed that no contaminants were
introduced in the string, maintaining the vCM field emission-free up to the
maximum field reached by each cavity. We also found that plasma processing
eliminates multipacting (MP) induced quenches that are typically observed
frequently within the MP band field range. This suggests that plasma processing
could be employed in situ in CMs to mitigate both field emission and
multipacting, significantly decreasing the testing time of cryomodules, the
linac commissioning time and cost and increasing the accelerator reliability.Comment: 11 pages, 10 figure
Accreting on the edge: a luminosity-dependent cyclotron line in the Be/X-ray Binary 2S 1553-542 accompanied by accretion regimes transition
Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which
the luminosity-dependent spectral and timing features of the neutron star's
emission can be analyzed in detail, thus shedding light on the accretion regime
at work. We took advantage of a monitoring campaign performed with NuSTAR,
Swift/XRT, AstroSat and NICER, to follow the Be/X-ray Binary 2S 1553-542 along
one of its rare outbursts and trace its spectral and timing evolution. We
report the discovery of a luminosity-dependent cyclotron line energy for the
first time in this source. The pulse profiles and pulsed fraction also show
variability along the outburst, consistently with the interpretation that the
source transitions from the sub-critical to the super-critical accretion
regime, separated by a critical luminosity of L
erg/s.Comment: Accepted on ApJ. 11 pages, 7 figures, 3 table
Rab protein evolution and the history of the eukaryotic endomembrane system
Spectacular increases in the quantity of sequence data genome have facilitated major advances in eukaryotic comparative genomics. By exploiting homology with classical model organisms, this makes possible predictions of pathways and cellular functions currently impossible to address in intractable organisms. Echoing realization that core metabolic processes were established very early following evolution of life on earth, it is now emerging that many eukaryotic cellular features, including the endomembrane system, are ancient and organized around near-universal principles. Rab proteins are key mediators of vesicle transport and specificity, and via the presence of multiple paralogues, alterations in interaction specificity and modification of pathways, contribute greatly to the evolution of complexity of membrane transport. Understanding system-level contributions of Rab proteins to evolutionary history provides insight into the multiple processes sculpting cellular transport pathways and the exciting challenges that we face in delving further into the origins of membrane trafficking specificity