1,201 research outputs found
Vibrational analysis of the structure of gramicidin A. II. Vibrational spectra
Raman and infrared spectra have been obtained of gramicidin A (GA) in the crystalline state both in the native form and complexed with CsSCN and KSCN, in solution in dioxane, and incorporated into lipid vesicles. Based on predictions from normal mode calculations of a number of relevant single- and double-stranded beta-helix conformations (Naik and Krimm, 1986), it has been possible to assign the structures of GA that are present under the above conditions. In the crystalline state, native GA has a double-stranded increases decreases beta 5.6 structure, whereas complexes with CsSCN or KSCN adopt a increases decreases beta 7.2 structure. In dioxane solution, the increases decreases beta 5.6 structure predominates. In lipid vesicles, the single-stranded beta 6.3-helix is found, which converts to a double-stranded helix on drying the sample. These results support our previous studies in showing that normal mode analysis can be a powerful technique in obtaining three-dimensional structural information from vibrational spectra
Evidence of two unique variability classes from IGR J17091-3624
IGR J17091-3624 is the second black hole X-ray binary after GRS 1915+105,
which showed large and distinct variabilities. The study of these variability
classes can be useful to understand the accretion-ejection mechanisms of
accreting black holes, and hence to probe the strong gravity regime. We report
the discovery of two new variability classes (C1 and C2) from IGR J17091-3624
from the 2011 outburst Rossi X-ray Timing Explorer data. These unique classes
will be useful to have complete details about the source, and to learn new
aspects about variabilities. For examples, the C1 class shows that the
intensity and period of oscillations, energy spectrum and power spectrum can
clearly evolve in tens of seconds. Moreover, in such a small time scale,
soft-lag becomes hard-lag. The C2 class shows that the variability and the
nonvariability can occur at similar energy spectrum, and a soft state is not
required for variability to happen.Comment: 5 pages, 6 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society Letter
Swift/XRT orbital monitoring of the candidate supergiant fast X-ray transient IGR J17354-3255
We report on the Swift/X-ray Telescope (XRT) monitoring of the field of view
around the candidate supergiant fast X-ray transient (SFXT) IGR J17354-3255,
which is positionally associated with the AGILE/GRID gamma-ray transient AGL
J1734-3310. Our observations, which cover 11 days for a total on-source
exposure of about 24 ks, span 1.2 orbital periods (P_orb=8.4474 d) and are the
first sensitive monitoring of this source in the soft X-rays. These new data
allow us to exploit the timing variability properties of the sources in the
field to unambiguously identify the soft X-ray counterpart of IGR J17354-3255.
The soft X-ray light curve shows a moderate orbital modulation and a dip. We
investigated the nature of the dip by comparing the X-ray light curve with the
prediction of the Bondi-Hoyle-Lyttleton accretion theory, assuming both
spherical and nonspherical symmetry of the outflow from the donor star. We
found that the dip cannot be explained with the X-ray orbital modulation. We
propose that an eclipse or the onset of a gated mechanism is the most likely
explanation for the observed light curve.Comment: Accepted for publication in Astronomy and Astrophysics. 9 page
Evidence of Exponential Decay Emission in the Swift Gamma-ray Bursts
We present a systematic study of the steep decay emission from gamma-ray
bursts (GRBs) observed by the Swift X-Ray Telescope (XRT). In contrast to the
analysis described in recent literature, we produce composite Burst Alert
Telescope (BAT) and XRT light curves by extrapolating the XRT data (2-10 keV)
into the BAT energy range (15-25 keV) rather than extrapolating the BAT data
into the XRT energy band (0.3-10 keV). Based on the fits to the composite light
curves, we have confirmed the existence of an exponential decay component which
smoothly connects the BAT prompt data to the XRT steep decay for several GRBs.
We also find that the XRT steep decay for some of the bursts can be well fit by
a combination of a power-law with an exponential decay model. We discuss this
exponential component within the frame work of both the internal and the
external shock model.Comment: 33 pages, 34 figures; accepted for publication in Ap
Long-term variability of AGN at hard X-rays
Variability at all observed wavelengths is a distinctive property of AGN.
Hard X-rays provide us with a view of the innermost regions of AGN, mostly
unbiased by absorption along the line of sight. Swift/BAT offers the unique
opportunity to follow, on time scales of days to years and with a regular
sampling, the 14-195 keV emission of the largest AGN sample available up to
date for this kind of investigation. We study the amplitude of the variations,
and their dependence on sub-class and on energy, for a sample of 110 radio
quiet and radio loud AGN selected from the BAT 58-month survey. About 80% of
the AGN in the sample are found to exhibit significant variability on months to
years time scales, radio loud sources being the most variable. The amplitude of
the variations and their energy dependence are incompatible with variability
being driven at hard X-rays by changes of the absorption column density. In
general, the variations in the 14-24 and 35-100 keV bands are well correlated,
suggesting a common origin of the variability across the BAT energy band.
However, radio quiet AGN display on average 10% larger variations at 14-24 keV
than at 35-100 keV and a softer-when-brighter behavior for most of the Seyfert
galaxies with detectable spectral variability on month time scale. In addition,
sources with harder spectra are found to be more variable than softer ones.
These properties are generally consistent with a variable power law continuum,
in flux and shape, pivoting at energies >~ 50 keV, to which a constant
reflection component is superposed. When the same time scales are considered,
the timing properties of AGN at hard X-rays are comparable to those at lower
energies, with at least some of the differences possibly ascribable to
components contributing differently in the two energy domains (e.g.,
reflection, absorption).Comment: 17 pages, 11 figures, accepted for publication in A&
Surface Grafting of Poly(L-glutamates). 2. Helix Orientation
In this paper the average helix orientation of surface-grafted poly(γ-benzyl L-glutamate) (PBLG), poly(γ-methyl L-glutamate) (PMLG), and poly(γ-methyl L-glutamate)-co-(γ-n-stearyl L-glutamate) (PMLGSLG 70/30) was investigated by means of FT-IR transmission spectroscopy. The theoretical relation between the average tilt angle (θ) and the absorption peak areas of three different backbone amide bands could be calculated because their transition dipole moment directions with respect to the helix axis were known. From the normalized absorptions, the average tilt angles of grafted helices of PBLG, PMLG, and PMLGSLG 70/30 were determined. The somewhat larger average angle of PMLG helices of 35 ± 5° with respect to the substrate compared to the value of 32 ± 5° of PBLG was due to the higher grafting density of PMLG. Because of the smaller helix diameter as a result of the smaller size of the methyl side group, more PMLG helices grew on the same surface area. Sterical hindrance and unfavorable polar interactions between unidirectional aligned helices forced the PMLG helices in a more upright arrangement. The even more perpendicular orientation of PMLGSLG 70/30 (48 ± 6°) could be the result of incorporation of mainly γ-methyl L-glutamate N-carboxyanhydride (MLG-NCA) monomers during the initiation step. Incorporation of the much larger γ-n-stearyl L-glutamate N-carboxyanhydride (SLG-NCA) monomers afterward lead to enlarged angles with respect to the substrate. Due to swelling, a pronounced change in helix orientation of grafted PMLGSLG 70/30 in n-hexadecane was observed, resulting in an almost perpendicular helix orientation.
The 2008 October Swift detection of X-ray bursts/outburst from the transient SGR-like AXP 1E 1547.0-5408
We report on the detailed study of the 2008 October outburst from the
anomalous X-ray pulsar (AXP) 1E 1547.0-5408 discovered through the Swift/Burst
Alert Telescope (BAT) detection of SGR-like short X-ray bursts on 2008 October
3. The Swift/X-ray Telescope (XRT) started observing the source after less than
100 s since the BAT trigger, when the flux (about 6E-11 erg/cm^2/s in the 2-10
keV range) was >50 times higher than its quiescent level. Swift monitored the
outbursting activity of 1E 1547.0-5408 on a daily basis for approximately three
weeks. This strategy allowed us to find a phase-coherent solution for the
source pulsations after the burst, which, besides period and period derivative,
requires a positive Period second derivative term (spin-down increase). The
time evolution of the pulse shape is complex and variable, with the pulsed
fraction increasing from 20% to 50% within the Swift observational window. The
XRT spectra can be fitted well by means of a single component, either a
power-law (PL) or a blackbody (BB). During the very initial phases of the
outburst the spectrum is hard, with a PL photon index about 2 (or kT about 1.4
keV) which steepens to about 4 (or kT about 0.8 keV) within one day from the
BAT trigger, though the two components are likely present simultaneously during
the first day spectra. An INTEGRAL observation carried out five days after the
trigger provided an upper limit of about 2E-11 erg/cm^2/s to the emission of 1E
1547.0-5408 in the 18-60 keV band.Comment: 10 pages, 2 tables, 6 figures; accepted for publication in MNRA
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
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