2,826 research outputs found
Breaking the AMSP mould: the increasingly strange case of HETE J1900.1-2455
We present ongoing Rossi X-ray Timing Explorer (RXTE) monitoring observations
of the 377.3 Hz accretion-powered pulsar, HETE J1900.1-2455 Activity continues
in this system more than 3 years after discovery, at a mean luminosity of
4.4e36 erg/s (for d=5 kpc), although pulsations were present only within the
first 70 days. X-ray variability has increased each year, notably with a brief
interval of nondetection in 2007, during which the luminosity dropped to below
1e-3 of the mean level. A deep search of data from the intervals of
nondetection in 2005 revealed evidence for extremely weak pulsations at an
amplitude of 0.29% rms, a factor of ten less than the largest amplitude seen
early in the outburst.
X-ray burst activity continued through 2008, with bursts typically featuring
strong radius expansion. Spectral analysis of the most intense burst detected
by RXTE early in the outburst revealed unusual variations in the inferred
photospheric radius, as well as significant deviations from a blackbody. We
obtained much better fits instead with a comptonisation model.Comment: 8 pages, 3 figures, one table; to appear in the proceedings of the
workshop "A Decade of Accreting Millisecond X-ray Pulsars", Amsterdam, April
2008, eds. R. Wijnands et al. (AIP Conf. Proc.
Simultaneous Multi-band Radio & X-ray Observations of the Galactic Center Magnetar SGR 17452900
We report on multi-frequency, wideband radio observations of the Galactic
Center magnetar (SGR 17452900) with the Green Bank Telescope for 100
days immediately following its initial X-ray outburst in April 2013. We made
multiple simultaneous observations at 1.5, 2.0, and 8.9 GHz, allowing us to
examine the magnetar's flux evolution, radio spectrum, and interstellar medium
parameters (such as the dispersion measure (DM), the scattering timescale and
its index). During two epochs, we have simultaneous observations from the
Chandra X-ray Observatory, which permitted the absolute alignment of the radio
and X-ray profiles. As with the two other radio magnetars with published
alignments, the radio profile lies within the broad peak of the X-ray profile,
preceding the X-ray profile maximum by 0.2 rotations. We also find that
the radio spectral index is significantly negative between 2 and
9 GHz; during the final 30 days of our observations ,
which is typical of canonical pulsars. The radio flux has not decreased during
this outburst, whereas the long-term trends in the other radio magnetars show
concomitant fading of the radio and X-ray fluxes. Finally, our wideband
measurements of the DMs taken in adjacent frequency bands in tandem are
stochastically inconsistent with one another. Based on recent theoretical
predictions, we consider the possibility that the dispersion measure is
frequency-dependent. Despite having several properties in common with the other
radio magnetars, such as , an
increase in the radio flux during the X-ray flux decay has not been observed
thus far in other systems.Comment: 15 pages, 9 figures, 3 tables; accepted to Ap
A new low magnetic field magnetar: the 2011 outburst of Swift J1822.3-1606
We report on the long term X-ray monitoring with Swift, RXTE, Suzaku, Chandra
and XMM-Newton of the outburst of the newly discovered magnetar Swift
J1822.3-1606 (SGR 1822-1606), from the first observations soon after the
detection of the short X-ray bursts which led to its discovery, through the
first stages of its outburst decay (covering the time-span from July 2011,
until end of April 2012). We also report on archival ROSAT observations which
witnessed the source during its likely quiescent state, and on upper limits on
Swift J1822.3-1606's radio-pulsed and optical emission during outburst, with
the Green Bank Telescope (GBT) and the Gran Telescopio Canarias (GTC),
respectively. Our X-ray timing analysis finds the source rotating with a period
of P=8.43772016(2) s and a period derivative \dot{P}=8.3(2)x10^{-14} s s^{-1} ,
which entails an inferred dipolar surface magnetic field of B~2.7x10^{13} G at
the equator. This measurement makes Swift J1822.3-1606 the second lowest
magnetic field magnetar (after SGR 0418+5729; Rea et al. 2010). Following the
flux and spectral evolution from the beginning of the outburst, we find that
the flux decreased by about an order of magnitude, with a subtle softening of
the spectrum, both typical of the outburst decay of magnetars. By modeling the
secular thermal evolution of Swift J1822.3-1606, we find that the observed
timing properties of the source, as well as its quiescent X-ray luminosity, can
be reproduced if it was born with a poloidal and crustal toroidal fields of
B_{p}~1.5x10^{14} G and B_{tor}~7x10^{14} G, respectively, and if its current
age is ~550 kyr.Comment: 14 pages, 9 figures; new observations added; ApJ in pres
Fifteen years of XMM-Newton and Chandra monitoring of Sgr A*: Evidence for a recent increase in the bright flaring rate
We present a study of the X-ray flaring activity of Sgr A* during all the 150
XMM-Newton and Chandra observations pointed at the Milky Way center over the
last 15 years. This includes the latest XMM-Newton and Chandra campaigns
devoted to monitoring the closest approach of the very red Br-Gamma emitting
object called G2. The entire dataset analysed extends from September 1999
through November 2014. We employed a Bayesian block analysis to investigate any
possible variations in the characteristics (frequency, energetics, peak
intensity, duration) of the flaring events that Sgr A* has exhibited since
their discovery in 2001. We observe that the total bright-or-very bright flare
luminosity of Sgr A* increased between 2013-2014 by a factor of 2-3 (~3.5 sigma
significance). We also observe an increase (~99.9% significance) from
0.27+-0.04 to 2.5+-1.0 day^-1 of the bright-or-very bright flaring rate of Sgr
A*, starting in late summer 2014, which happens to be about six months after
G2's peri-center passage. This might indicate that clustering is a general
property of bright flares and that it is associated with a stationary noise
process producing flares not uniformly distributed in time (similar to what is
observed in other quiescent black holes). If so, the variation in flaring
properties would be revealed only now because of the increased monitoring
frequency. Alternatively, this may be the first sign of an excess accretion
activity induced by the close passage of G2. More observations are necessary to
distinguish between these two hypotheses.Comment: Accepted for publication in MNRA
The X-ray outburst of the Galactic Centre magnetar SGR J1745-2900 during the first 1.5 year
In 2013 April a new magnetar, SGR 1745-2900, was discovered as it entered an
outburst, at only 2.4 arcsec angular distance from the supermassive black hole
at the Centre of the Milky Way, Sagittarius A*. SGR 1745-2900 has a surface
dipolar magnetic field of ~ 2x10^{14} G, and it is the neutron star closest to
a black hole ever observed. The new source was detected both in the radio and
X-ray bands, with a peak X-ray luminosity L_X ~ 5x10^{35} erg s^{-1}. Here we
report on the long-term Chandra (25 observations) and XMM-Newton (8
observations) X-ray monitoring campaign of SGR 1745-2900, from the onset of the
outburst in April 2013 until September 2014. This unprecedented dataset allows
us to refine the timing properties of the source, as well as to study the
outburst spectral evolution as a function of time and rotational phase. Our
timing analysis confirms the increase in the spin period derivative by a factor
of ~2 around June 2013, and reveals that a further increase occurred between
2013 Oct 30 and 2014 Feb 21. We find that the period derivative changed from
6.6x10^{-12} s s^{-1} to 3.3x10^{-11} s s^{-1} in 1.5 yr. On the other hand,
this magnetar shows a slow flux decay compared to other magnetars and a rather
inefficient surface cooling. In particular, starquake-induced crustal cooling
models alone have difficulty in explaining the high luminosity of the source
for the first ~200 days of its outburst, and additional heating of the star
surface from currents flowing in a twisted magnetic bundle is probably playing
an important role in the outburst evolution.Comment: 16 pages, 12 figures; accepted for publication on MNRA
Structure and mechanism of acetolactate decarboxylase
Acetolactate decarboxylase catalyzes the conversion of both enantiomers of acetolactate to the (R)-enantiomer of acetoin, via a mechanism that has been shown to involve a prior rearrangement of the non-natural (R)-enantiomer substrate to the natural (S)-enantiomer. In this paper, a series of crystal structures of ALDC complex with designed transition state mimics are reported. These structures, coupled with inhibition studies and site-directed mutagenesis provide an improved understanding of the molecular processes involved in the stereoselective decarboxylation/protonation events. A mechanism for the transformation of each enantiomer of acetolactate is proposed
Geochemical analysis of bulk marine sediment by Inductively Coupled Plasma–Atomic Emission Spectroscopy on board the JOIDES Resolution
Geochemical analyses on board the JOIDES Resolution have been enhanced with the addition of a Jobin-Yvon Ultrace inductively coupled plasma-atomic emission spectrometer (ICP-AES) as an upgrade from the previous X-ray fluorescence facility. During Leg 199, we sought to both challenge and utilize the capabilities of the ICP-AES in order to provide an extensive bulk-sediment geochemical database during the cruise. These near real-time analyses were then used to help characterize the recovered sedimentary sequences, calculate mass accumulation rates of the different sedimentary components, and assist with cruise and postcruise sampling requests. The general procedures, sample preparation techniques, and basic protocol for ICP-AES analyses on board ship are outlined by Murray et al. (2000) in Ocean Drilling Program Tech Note, 29. We expand on those concepts and offer suggestions for ICP-AES methodology, calibration by standard reference materials, data reduction procedures, and challenges that are specific to the analysis of bulk-sediment samples. During Leg 199, we employed an extensive bulk-sediment analytical program of ~600 samples of varying lithologies, thereby providing several opportunities for refinement of techniques. We also discuss some difficulties and challenges that were faced and suggest how to alleviate such occurrences for sedimentary chemical analyses during future legs
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