3,174 research outputs found
The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence
We present an optical (gri) study during quiescence of the accreting
millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran
Telescopio Canarias (GTC) in August 2014. Despite the source being in
quiescence at the time of our observations, it showed a strong optical flaring
activity, more pronounced at higher frequencies (i.e. the g band). Once the
flares were subtracted, we tentatively recovered a sinusoidal modulation at the
system orbital period in all bands, even if a significant phase shift with
respect to an irradiated star, typical of accreting millisecond X-ray pulsars
is detected. We conclude that the observed flaring could be a manifestation of
the presence of an accretion disc in the system. The observed light curve
variability could be explained by the presence of a superhump, which might be
another proof of the formation of an accretion disc. In particular, the disc at
the time of our observations was probably preparing to the new outburst of the
source, that happened just a few months later, in 2015.Comment: 6 pages, 2 figures, 1 table. Accepted for publication in A&
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
The Advanced X-ray Timing Array (AXTAR)
AXTAR is an X-ray observatory mission concept, currently under study in the
U.S., that combines very large collecting area, broadband spectral coverage,
high time resolution, highly flexible scheduling, and an ability to respond
promptly to time-critical targets of opportunity. It is optimized for
submillisecond timing of bright Galactic X-ray sources in order to study
phenomena at the natural time scales of neutron star surfaces and black hole
event horizons, thus probing the physics of ultradense matter, strongly curved
spacetimes, and intense magnetic fields. AXTAR's main instrument is a
collimated, thick Si pixel detector with 2-50 keV coverage and 8 square meters
collecting area. For timing observations of accreting neutron stars and black
holes, AXTAR provides at least an order of magnitude improvement in sensitivity
over both RXTE and Constellation-X. AXTAR also carries a sensitive sky monitor
that acts as a trigger for pointed observations of X-ray transients and also
provides continuous monitoring of the X-ray sky with 20 times the sensitivity
of the RXTE ASM. AXTAR builds on detector and electronics technology previously
developed for other applications and thus combines high technical readiness and
well understood cost.Comment: 4 pages with 1 figure, to appear in the proceedings of "A Decade of
Accreting Millisecond X-ray Pulsars", Amsterdam, April 2008, eds. R. Wijnands
et al. (AIP Conf. Proc.). Footnote and references adde
Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11
Variants in NDUFB11, which encodes a structural component of complex I of the mitochondrial respiratory chain (MRC), were recently independently reported to cause histiocytoid cardiomyopathy (histiocytoid CM) and microphthalmia with linear skin defects syndrome (MLS syndrome). Here we report an additional case of histiocytoid CM, which carries a de novo nonsense variant in NDUFB11 (ENST00000276062.8: c.262C > T; p.[Arg88*]) identified using whole-exome sequencing (WES) of a family trio. An identical variant has been previously reported in association with MLS syndrome. The case we describe here lacked the diagnostic features of MLS syndrome, but a detailed clinical comparison of the two cases revealed significant phenotypic overlap. Heterozygous variants in HCCS (which encodes an important mitochondrially targeted protein) and COX7B, which, like NDUFB11, encodes a protein of the MRC, have also previously been identified in MLS syndrome including a case with features of both MLS syndrome and histiocytoid CM. However, a systematic review of WES data from previously published histiocytoid CM cases, alongside four additional cases presented here for the first time, did not identify any variants in these genes. We conclude that NDUFB11 variants play a role in the pathogenesis of both histiocytoid CM and MLS and that these disorders are allelic (genetically related)
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
A variable absorption feature in the X-ray spectrum of a magnetar
Soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are slowly
rotating, isolated neutron stars that sporadically undergo episodes of
long-term flux enhancement (outbursts) generally accompanied by the emission of
short bursts of hard X-rays. This behaviour can be understood in the magnetar
model, according to which these sources are mainly powered by their own
magnetic energy. This is supported by the fact that the magnetic fields
inferred from several observed properties of AXPs and SGRs are greater than -
or at the high end of the range of - those of radio pulsars. In the peculiar
case of SGR 0418+5729, a weak dipole magnetic moment is derived from its timing
parameters, whereas a strong field has been proposed to reside in the stellar
interior and in multipole components on the surface. Here we show that the
X-ray spectrum of SGR 0418+5729 has an absorption line, the properties of which
depend strongly on the star's rotational phase. This line is interpreted as a
proton cyclotron feature and its energy implies a magnetic field ranging from
2E14 gauss to more than 1E15 gauss.Comment: Nature, 500, 312 (including Supplementary Information
Spectral line shape of resonant four-wave mixing induced by broad-bandwidth lasers
We present a theoretical and experimental study of the line shape of resonant four-wave mixing induced by broad-bandwidth laser radiation that revises the theory of Meacher, Smith, Ewart, and Cooper (MSEC) [Phys. Rev. A 46, 2718 (1992)]. We adopt the same method as MSEC but correct for an invalid integral used to average over the distribution of atomic velocities. The revised theory predicts a Voigt line shape composed of a homogeneous, Lorentzian component, defined by the collisional rate Î, and an inhomogeneous, Doppler component, which is a squared Gaussian. The width of the inhomogeneous component is reduced by a factor of â2 compared to the simple Doppler width predicted by MSEC. In the limit of dominant Doppler broadening, the width of the homogeneous component is predicted to be 4Î, whereas in the limit of dominant homogeneous broadening, the predicted width is 2Î. An experimental measurement is reported of the line shape of the four-wave-mixing signal using a broad-bandwidth, "modeless", laser resonant with the Q1 (6) line of the A2 ÎŁ - X2 Î (0,0) system of the hydroxyl radical. The measured widths of the Voigt components were found to be consistent with the predictions of the revised theory
Intensity-hardness correlation and deep infrared observation of the anomalous X-ray pulsar 1RXS J170849-400910
We report here on X-ray and IR observations of the Anomalous X-ray Pulsar
1RXS J170849-400910. First, we report on new XMM-Newton, Swift-XRT and Chandra
observations of this AXP, which confirm the intensity-hardness correlation
observed in the long term X-ray monitoring of this source by Rea et al. (2005).
These new X-ray observations show that the AXP flux is rising again, and the
spectrum hardening. If the increase in the source intensity is indeed connected
with the glitches and a possible bursting activity, we expect this source to
enter in a bursting active phase around 2006-2007. Second, we report on deep IR
observations of 1RXS J170849-400910, taken with the VLT-NACO adaptive optics,
showing that there are many weak sources consistent with the AXP position.
Neither star A or B, previously proposed by different authors, might yet be
conclusively recognised as the IR counterpart of 1RXS J170849-400910. Third,
using Monte Carlo simulations, we re-address the calculation of the
significance of the absorption line found in a phase-resolved spectrum of this
source by Rea et al. (2003), and interpreted as a resonant scattering cyclotron
feature.Comment: 7 pages, 5 color figures; Astrophysics & Space Science, in press
("Isolated Neutron Stars"; London, UK
Long-term spectral and timing properties of the soft gamma-ray repeater SGR 1833-0832 and detection of extended X-ray emission around the radio pulsar PSR B1830-08
SGR 1833-0832 was discovered on 2010 March 19 thanks to the Swift detection
of a short hard X-ray burst and follow-up X-ray observations. Since then, it
was repeatedly observed with Swift, Rossi X-ray Timing Explorer, and
XMM-Newton. Using these data, which span about 225 days, we studied the
long-term spectral and timing characteristics of SGR 1833-0832. We found
evidence for diffuse emission surrounding SGR 1833-0832, which is most likely a
halo produced by the scattering of the point source X-ray radiation by dust
along the line of sight, and we show that the source X-ray spectrum is well
described by an absorbed blackbody, with temperature kT=1.2 keV and absorbing
column nH=(10.4+/-0.2)E22 cm^-2, while different or more complex models are
disfavoured. The source persistent X-ray emission remained fairly constant at
about 3.7E-12 erg/cm^2/s for the first 20 days after the onset of the bursting
episode, then it faded by a factor 40 in the subsequent 140 days, following a
power-law trend with index alpha=-0.5. We obtained a phase-coherent timing
solution with the longest baseline (225 days) to date for this source which,
besides period P=7.5654084(4) s and period derivative dP/dt=3.5(3)E-12 s/s,
includes higher order period derivatives. We also report on our search of the
counterpart to the SGR at radio frequencies using the Australia Telescope
Compact Array and the Parkes radio telescope. No evidence for radio emission
was found, down to flux densities of 0.9 mJy (at 1.5 GHz) and 0.09 mJy (at 1.4
GHz) for the continuum and pulsed emissions, respectively, consistently with
other observations at different epochs.Comment: 12 pages, 7 colour figures and 3 tables, accepted for publication in
MNRAS. Figure 6 in reduced quality and abstract abridged for astro-ph
submissio
Pulsating in unison at optical and X-ray energies: simultaneous high-time resolution observations of the transitional millisecond pulsar PSR J1023+0038
PSR J1023+0038 is the first millisecond pulsar discovered to pulsate in the
visible band; such a detection took place when the pulsar was surrounded by an
accretion disk and also showed X-ray pulsations. We report on the first high
time resolution observational campaign of this transitional pulsar in the disk
state, using simultaneous observations in the optical (TNG, NOT, TJO), X-ray
(XMM-Newton, NuSTAR, NICER), infrared (GTC) and UV (Swift) bands. Optical and
X-ray pulsations were detected simultaneously in the X-ray high intensity mode
in which the source spends 70% of the time, and both disappeared in the
low mode, indicating a common underlying physical mechanism. In addition,
optical and X-ray pulses were emitted within a few km, had similar pulse shape
and distribution of the pulsed flux density compatible with a power-law
relation connecting the optical and the 0.3-45 keV
X-ray band. Optical pulses were detected also during flares with a pulsed flux
reduced by one third with respect to the high mode; the lack of a simultaneous
detection of X-ray pulses is compatible with the lower photon statistics. We
show that magnetically channeled accretion of plasma onto the surface of the
neutron star cannot account for the optical pulsed luminosity (
erg/s). On the other hand, magnetospheric rotation-powered pulsar emission
would require an extremely efficient conversion of spin-down power into pulsed
optical and X-ray emission. We then propose that optical and X-ray pulses are
instead produced by synchrotron emission from the intrabinary shock that forms
where a striped pulsar wind meets the accretion disk, within a few light
cylinder radii away, 100 km, from the pulsar.Comment: 26 pages, 14 figures, first submitted to ApJ on 2019, January 1
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