894 research outputs found
Properties and observability of glitches and anti-glitches in accreting pulsars
Several glitches have been observed in young, isolated radio pulsars, while a
clear detection in accretion-powered X-ray pulsars is still lacking. We use the
Pizzochero snowplow model for pulsar glitches as well as starquake models to
determine for the first time the expected properties of glitches in accreting
pulsars and their observability. Since some accreting pulsars show
accretion-induced long-term spin-up, we also investigate the possibility that
anti-glitches occur in these stars. We find that glitches caused by quakes in a
slow accreting neutron star are very rare and their detection extremely
unlikely. On the contrary, glitches and anti-glitches caused by a transfer of
angular momentum between the superfluid neutron vortices and the non-superfluid
component may take place in accreting pulsars more often. We calculate the
maximum jump in angular velocity of an anti-glitch and we find that it is
expected to be about 1E-5 - 1E-4 rad/s. We also note that since accreting
pulsars usually have rotational angular velocities lower than those of isolated
glitching pulsars, both glitches and anti-glitches are expected to have long
rise and recovery timescales compared to isolated glitching pulsars, with
glitches and anti-glitches appearing as a simple step in angular velocity.
Among accreting pulsars, we find that GX 1+4 is the best candidate for the
detection of glitches with currently operating X-ray instruments and future
missions such as the proposed Large Observatory for X-ray Timing (LOFT).Comment: Accepted for publication in Astronomy & Astrophysics. 6 pages. Minor
changes to match the final A&A versio
Glancing through the accretion column of EXO 2030+375
We took advantage of the large collecting area and good timing capabilities
of the EPIC cameras on-board XMM-Newton to investigate the accretion geometry
onto the magnetized neutron star hosted in the high mass X-ray binary EXO
2030+375 during the rise of a source Type-I outburst in 2014. We carried out a
timing and spectral analysis of the XMM-Newton observation as function of the
neutron star spin phase. We used a phenomenological spectral continuum model
comprising the required fluorescence emission lines. Two neutral absorption
components are present: one covering fully the source and one only partially.
The same analysis was also carried out on two Suzaku observations of the source
performed during outbursts in 2007 and 2012, to search for possible spectral
variations at different luminosities. The XMM-Newton data caught the source at
an X-ray luminosity of erg s and revealed the presence
of a narrow dip-like feature in its pulse profile that was never reported
before. The width of this feature corresponds to about one hundredth of the
neutron star spin period. From the results of the phase-resolved spectral
analysis we suggest that this feature can be ascribed to the self-obscuration
of the accretion stream passing in front of the observer line of sight. We
inferred from the Suzaku observation carried out in 2007 that the
self-obscuration of the accretion stream might produce a significantly wider
feature in the neutron star pulsed profile at higher luminosities
( erg s).Comment: Accepted for publication on A&
Spatial distribution of unidentified infrared bands and extended red emission in the compact galactic HII region Sh 152
We present visible and near IR images of the compact HII region Sh 152. Some
of these images reveal the presence of Extended Red Emission (ERE) around 698
nm and emission from Unidentified Infra Red Bands (UIRBs) at 3.3 and 6.2
micron. Other images show the near infrared (7-12 micron) continuous emission
of the nebula. The ERE emission is found to coincide with the ionized region
and significantly differ from the UIRBs location. Also some evidence is found
in favor of grains as carriers for ERE.Comment: 3 pages, 4 figures, to be published in the proceedings of the
colloquium "The universe as seen by ISO" help in Paris, October 20-23, 1998 ;
available in html format at http://www.obs-hp.fr/preprints.htm
Swift, NuSTAR, and INTEGRAL observations of the symbiotic X-ray binary IGR J16194-2810
We report on a simultaneous observational campaign with both Swift/XRT and
NuSTAR targeting the symbiotic X-ray binary IGR J16194-2810. The main goal of
the campaign was to investigate the possible presence of cyclotron scattering
absorption features in the broad-band spectrum of the source, and help advance
our understanding of the process of neutron star formation via the
accretion-induced collapse of a white dwarf. The 1-30 keV spectrum of the
source, as measured during our campaign, did not reveal the presence of any
statistically significant absorption feature. The spectrum could be well
described using a model comprising a thermal black-body hot component, most
likely emerging from the surface of the accreting neutron star, and a power-law
with no measurable cut-off energy (and affected by a modest absorption column
density). Compared to previous analyses in the literature, we could rule out
the presence of a colder thermal component emerging from an accretion disk,
compatible with the idea that IGR J16194-2810 is a wind-fed binary (as most of
the symbiotic X-ray binaries). Our results were strengthened by exploiting the
archival XRT and INTEGRAL data, extending the validity of the spectral model
used up to 0.3-40 keV and demonstrating that IGR J16194-2810 is unlikely to
undergo significant spectral variability over time in the X-ray domain.Comment: Accepted for publication on MNRA
Giant outburst from the supergiant fast X-ray transient IGR J17544-2619: accretion from a transient disc?
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries
associated with OB supergiant companions and characterised by an X-ray flaring
behaviour whose dynamical range reaches 5 orders of magnitude on timescales of
a few hundred to thousands of seconds. Current investigations concentrate on
finding possible mechanisms to inhibit accretion in SFXTs and explain their
unusually low average X-ray luminosity. We present the Swift observations of an
exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014
October 10 when the source achieved a peak luminosity of erg
s. This extends the total source dynamic range to 10, the
largest (by a factor of 10) recorded so far from an SFXT. Tentative evidence
for pulsations at a period of 11.6 s is also reported. We show that these
observations challenge, for the first time, the maximum theoretical luminosity
achievable by an SFXT and propose that this giant outburst was due to the
formation of a transient accretion disc around the compact object.Comment: Accepted for publication in Astronomy and Astrophysics Letters. 5
pages, 5 figures, 2 table
Biogenic Amines Content of Four Types of "Pecorino" Cheese Manufactured in Tuscany
Biogenic amines content of four types of Tuscan ewes' milk "pecorino" cheese was evaluated using HPLC-UV analysis. All cheeses were manufactured in the same dairy factory with different combinations of milk (raw or pasteurized) and type of ripening. Total biogenic amines and tyramine levels of a raw milk "pecorino" ripened five months, partly in a traditional cave, were significantly higher than those of a pasteurized milk "pecorino" with a similar ripening; and of a two months raw milk "pecorino" ripened in the dairy plant. No statistical significant difference was found when comparing total biogenic amines and tyramine contents of the same five month ripened raw milk "pecorino" with a pasteurized milk "pecorino" ripened six months, partly in a traditional "fossa." In raw milk cave-ripened and "fossa"-ripened cheeses, total biogenic amines exceeded 1000 mg/kg. In cheeses manufactured with raw milk and/or in particular ripening environments, specific hygienic cares are needed to limit biogenic amines form..
Constraining duty cycles through a Bayesian technique
The duty cycle (DC) of astrophysical sources is generally defined as the
fraction of time during which the sources are active. However, DCs are
generally not provided with statistical uncertainties, since the standard
approach is to perform Monte Carlo bootstrap simulations to evaluate them,
which can be quite time consuming for a large sample of sources. As an
alternative, considerably less time-consuming approach, we derived the
theoretical expectation value for the DC and its error for sources whose state
is one of two possible, mutually exclusive states, inactive (off) or flaring
(on), as based on a finite set of independent observational data points.
Following a Bayesian approach, we derived the analytical expression for the
posterior, the conjugated distribution adopted as prior, and the expectation
value and variance. We applied our method to the specific case of the
inactivity duty cycle (IDC) for supergiant fast X-ray transients. We also
studied IDC as a function of the number of observations in the sample. Finally,
we compare the results with the theoretical expectations. We found excellent
agreement with our findings based on the standard bootstrap method. Our
Bayesian treatment can be applied to all sets of independent observations of
two-state sources, such as active galactic nuclei, X-ray binaries, etc. In
addition to being far less time consuming than bootstrap methods, the
additional strength of this approach becomes obvious when considering a
well-populated class of sources () for which the prior can
be fully characterized by fitting the distribution of the observed DCs for all
sources in the class, so that, through the prior, one can further constrain the
DC of a new source by exploiting the information acquired on the DC
distribution derived from the other sources. [Abridged]Comment: Accepted for publication in Astronomy and Astrophysics. 4 pages, 2
figures, 1 table. Supporting material at
http://www.ifc.inaf.it/~romano/Sfxts/IDCSims/index.htm
Cardiovascular magnetic resonance myocardial perfusion: methods and applications in patients with coronary artery disease
Perfusion CMR is an attractive imaging modality that is becoming comparable with other clinically diagnostic tests. SPECT and PET are well clinically validated and have good accuracy for detection of significant CAD. However, these techniques have a rather low spatial resolution and are not suitable for the detection of subendocardial perfusion defect. In addition, the radiation burden, the potential for attenuation artefacts (SPECT) and the limited availability (PET) are limitations of these imaging techniques. An integrated assessment of myocardial perfusion, function and viability is feasible with CMR. In addition, compared to other clinically available imaging techniques, CMR perfusion has excellent spatial resolution and no ionising radiation exposure. However, it is not widely available and there is a need of protocol and pulse sequence standardization. Most perfusion analysis remains observer-dependent (“eyeball” analysis) or dependent on bolus dispersion (semi-quantitative analysis). Fully quantitative analysis using CMR perfusion is currently time-consuming for clinical application. Perfusion CMR is an evolving field with numerous future directions
In-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AX J0049.4-7323
AX J0049.4-7323 is a Be/X-ray binary in the Small Magellanic Cloud hosting a
~750 s pulsar which has been observed over the last ~17 years by several X-ray
telescopes. Despite numerous observations, little is known about its X-ray
behaviour. Therefore, we coherently analysed archival Swift, Chandra,
XMM-Newton, RXTE, and INTEGRAL data, and we compared them with already
published ASCA data, to study its X-ray long-term spectral and flux
variability. AX J0049.4-7323 shows a high X-ray variability, spanning more than
three orders of magnitudes, from L ~ 1.6E37 erg/s (0.3-8 keV, d=62 kpc) down to
L ~ 8E33 erg/s. RXTE, Chandra, Swift, and ASCA observed, in addition to the
expected enhancement of X-ray luminosity at periastron, flux variations by a
factor of ~ 270 with peak luminosities of ~2.1E36 erg/s far from periastron.
These properties are difficult to reconcile with the typical long-term
variability of Be/XRBs, traditionally interpreted in terms of type I and type
II outbursts. The study of AX J0049.4-7323 is complemented with a spectral
analysis of Swift, Chandra, and XMM-Newton data which showed a softening trend
when the emission becomes fainter, and an analysis of optical/UV data collected
by the UVOT telescope on board Swift. In addition, we measured a secular
spin-up rate of s day, which
suggests that the pulsar has not yet achieved its equilibrium period. Assuming
spherical accretion, we estimated an upper limit for the magnetic field
strength of the pulsar of ~3E12 G.Comment: Accepted for publication in Astronomy & Astrophysic
- …