1,477 research outputs found
Spectral and timing properties of the accreting X-ray millisecond pulsar IGR J17498-2921
We analyze the spectral and timing properties of IGR J17498-2921 and the
characteristics of X-ray bursts to constrain the physical processes responsible
for the X-ray production in this class of sources. The broad-band average
spectrum is well-described by thermal Comptonization with an electron
temperature of kT_e ~ 50 keV, soft seed photons of kT_bb ~ 1 keV, and Thomson
optical depth \taut ~ 1 in a slab geometry. The slab area corresponds to a
black body radius of R_bb ~9 km. During the outburst, the spectrum stays
remarkably stable with plasma and soft seed photon temperatures and scattering
optical depth that are constant within the errors. This behavior has been
interpreted as indicating that the X-ray emission originates above the neutron
star (NS) surface in a hot slab (either the heated NS surface or the accretion
shock). The INTEGRAL, RXTE, and Swift data reveal the X-ray pulsation at a
period of 2.5 milliseconds up to ~65 keV. The pulsed fraction is consistent
with being constant, i.e. energy independent and has a typical value of 6-7%.
The nearly sinusoidal pulses show soft lags that seem to saturate near 10 keV
at a rather small value of ~ -60\mu s with those observed in other accreting
pulsars. The short burst profiles indicate that there is a hydrogen-poor
material at ignition, which suggests either that the accreted material is
hydrogen-deficient, or that the CNO metallicity is up to a factor of about two
times solar. However, the variation in the burst recurrence time as a function
of \dot{m} (inferred from the X-ray flux) is much smaller than predicted by
helium-ignition models.Comment: 9 pages, 8 figures, accepted for publication in A&A. arXiv admin
note: text overlap with arXiv:1012.022
Swift monitoring of the new accreting millisecond X-ray pulsar IGRJ17511-3057 in outburst
A new accreting millisecond X-ray pulsar, IGR J17511-3057, was discovered in
outburst on 2009 September 12 during the INTEGRAL Galactic bulge monitoring
programme. To study the evolution of the source X-ray flux and spectral
properties during the outburst, we requested a Swift monitoring of
IGRJ17511-3057. In this paper we report on the results of the first two weeks
of monitoring the source. The persistent emission of IGR J17511-3057 during the
outburst is modeled well with an absorbed blackbody (kT~0.9 keV) and a
power-law component (photon index~1-2), similar to what has been observed from
other previously known millisecond pulsars. Swift also detected three type-I
Xray bursts from this source. By assuming that the peak luminosity of these
bursts is equal to the Eddington value for a pure helium type-I X-ray burst, we
derived an upper limit to the source distance of ~10 kpc. The theoretical,
expected recurrence time of the bursts according to the helium burst hypothesis
is 0.2-0.9 days, in agreement with the observations.Comment: Accepted for publication in A&A Letters. V2: corrected some typos and
added one referenc
Swift J1734.5-3027: a new long type-I X-ray bursting source
Swift J1734.5-3027 is a hard X-ray transient discovered by Swift while
undergoing an outburst in September 2013. Archival observations showed that
this source underwent a previous episode of enhanced X-ray activity in May-June
2013. In this paper we report on the analysis of all X-ray data collected
during the outburst in September 2013, the first that could be intensively
followed-up by several X-ray facilities. Our data-set includes INTEGRAL, Swift,
and XMM-Newton observations. From the timing and spectral analysis of these
observations, we show that a long type-I X-ray burst took place during the
source outburst, making Swift J1734.5-3027 a new member of the class of
bursting neutron star low-mass X-ray binaries. The burst lasted for about 1.9
ks and reached a peak flux of (6.01.8)10 erg cm
s in the 0.5-100 keV energy range. The estimated burst fluence in the
same energy range is (1.100.10)10 erg cm. By
assuming that a photospheric radius expansion took place during the first
200 s of the burst and that the accreted material was predominantly
composed by He, we derived a distance to the source of 7.21.5 kpc.Comment: Accepted for publication on A&
The ephemeris, orbital decay, and masses of 10 eclipsing HMXBs
We take advantage of more than 10 years of monitoring of the eclipsing HMXB
systems LMC X-4, Cen X-3, 4U 1700-377, 4U 1538-522, SMC X-1, IGR J18027-2016,
Vela X-1, IGR J17252-3616, XTE J1855-026, and OAO 1657-415 with the ASM
on-board RXTE and ISGRI on-board INTEGRAL to update their ephemeris. These
results are used to refine previous measurements of the orbital period decay of
all sources (where available) and provide the first accurate values of the
apsidal advance in Vela X-1 and 4U 1538-522. Updated values for the masses of
the neutron stars hosted in the ten HMXBs are also provided, as well as the
long-term lightcurves folded on the sources best determined orbital parameters.
These lightcurves reveal complex eclipse ingresses and egresses, that are
understood mostly as being due to the presence of accretion wakes. The results
reported in this paper constitute a database to be used for population and
evolutionary studies of HMXBs, as well as theoretical modelling of long-term
accretion in wind-fed X-ray binaries.Comment: Accepted for publication on A&
The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
We present for the first time a study of the 3-200 keV broad band spectra of
the bursting atoll source 4U 1728-34 (GX 354-0) along its hardness intensity
diagram. The analysis was done using the INTEGRAL public and Galactic Center
deep exposure data ranging from February 2003 to October 2004. The spectra are
well described by a thermal Comptonization model with an electron temperature
from 35 keV to 3 keV and Thomson optical depth, tau_T, from 0.5 to 5 in a slab
geometry. The source undergoes a transition from an intermediate/hard to a soft
state where the source luminosity increases from 2 to 12% of Eddington. We have
also detected 36 type I X-ray bursts two of which show photospheric radius
expansion. The energetic bursts with photospheric radius expansion occurred at
an inferred low mass accretion rate per unit area of \dot m ~ 1.7x10E3 g/cm2/s,
while the others at a higher one between 2.4x10E3 - 9.4x10E3 g/cm2/s. For
4U1728-34 the bursts' total fluence, and the bursts' peak flux are
anti-correlated with the mass accretion rate. The type I X-ray bursts involve
pure helium burning either during the hard state, or during the soft state of
the source.Comment: 11 pages, 7 figures, and 2 tables. Accepted for publication in A&
Eradication of Candida albicans persister cell biofilm by the membranotropic peptide gH625
Biofilm formation poses an important clinical trouble due to resistance to antimicrobial agents; therefore, there is an urgent demand for new antibiofilm strategies that focus on the use of alternative compounds also in combination with conventional drugs. Drug-tolerant persisters are present in Candida albicans biofilms and are detected following treatment with high doses of amphotericin B. In this study, persisters were found in biofilms treated with amphotericin B of two clinical isolate strains, and were capable to form a new biofilm in situ. We investigated the possibility of eradicating persister-derived biofilms from these two Candida albicans strains, using the peptide gH625 analogue (gH625-M). Confocal microscopy studies allowed us to characterize the persister-derived biofilm and understand the mechanism of interaction of gH625-M with the biofilm. These findings confirm that persisters may be responsible for Candida biofilm survival, and prove that gH625-M was very effective in eradicating persister-derived biofilms both alone and in combination with conventional antifungals, mainly strengthening the antibiofilm activity of fluconazole and 5-flucytosine. Our strategy advances our insights into the development of effective antibiofilm therapeutic approaches
Characterization of Strombolian events by using independent component analysis
We apply Independent Component Analysis (ICA) to seismic signals recorded at Stromboli volcano. Firstly, we show how ICA works considering synthetic signals, which are generated by dynamical systems. We prove that Strombolian signals, both tremor and explosions, in the high frequency band (>0.5 Hz), are similar in time domain. This seems to give some insights to the organ pipe model generation for the source of these events. Moreover, we are able to recognize in the tremor signals a low frequency component (<0.5 Hz), with a well defined peak corresponding to 30s
- …