51 research outputs found
Preparation and optical properties of novel bioactive photonic crystals obtained from core-shell poly(styrene/α-tert-butoxy-Ï-vinylbenzyl-polyglycidol) microspheres
Optical properties of polymer microspheres with polystyrene cores and polyglycidol-enriched shells poly(styrene/α-tert-butoxy-Ï-vinylbenzyl-polyglycidol) (P(S/PGL) particles with number average diameters Dn determined by scanning electron microscopy equal 237 and 271 nm), were studied before and after immobilization of ovalbumin. The particles were synthesized by emulsifier-free emulsion copolymerization of styrene and polyglycidol macromonomer (poly(styrene/α-tert-butoxy-Ï-vinylbenzyl-polyglycidol)) initiated with potassium persulfate. Molar fraction of polyglycidol units in the interfacial layer of the microspheres determined by XPS was equal 42.6 and 34.0%, for the particles with Dn equal 137 and 271 nm, respectively. Colloidal crystals from the aforementioned particles were prepared by deposition of particle suspensions on the glass slides and subsequent evaporation of water. It was found that optical properties of colloidal crystals from the P(S/PGL) microspheres strongly depend on modification of their interfacial layer by covalent immobilization of ovalbumin. The coating of particles with ovalbumin resulted in decreasing their refractive index from 1.58 to 1.52
Type-I bursts within outbursts of IGR J17473-2721
Two outbursts were observed by RXTE in the history of the atoll source IGR
J17473-2721. During the most recent outburst in 2008, the source showed a
complete series of spectral states/transitions. The neutron star system was
prolific in type-I X-ray bursts, and we investigate them in the context of
complete outbursts evolution. A total exposure of ~ 309 ks was collected by
RXTE during the two outbursts of IGR J17473-2721. We carried out a systematic
search for type-I bursts in this data set. For each burst found, we
investigated the burst profile, the peak flux, and their dependence on the
accretion rate along the evolution of the outbursts. Eighteen type-I X-ray
bursts were found from IGR J17473-2721: two from the outburst in 2005 and the
other 16 from the recent outburst in 2008. Among them, 3 bursts show
photospheric radius expansion (PRE). The distance to the source is estimated as
6.4 kpc with a 15% uncertainty based on the three bursts that show PRE. In the
recent outburst, there are 6 bursts showing up in the low/hard state prior to
the state transition to a high/soft state, 3 bursts at the end phase of the
high/soft state, and 7 in the following low/hard state. The blackbody radius of
these bursts presents a variety of interesting features. We find that at the
end of the recent outburst, the profile of the blackbody radius is
anti-correlated with the blackbody temperature and the burst flux. The
durations of the type-I burst are found to correlate with the Eddington ratio
and to have two parallel evolution groups. Along the decreasing Eddington
ratio, the burst duration decreases and ends in each group the PRE bursts
occurred. This provides new clues to the type-I bursts in the context of
outbursts for atoll XRBs.Comment: in press at A &
A superburst from 4U 1254-690
We report the detection with the BeppoSAX Wide Field Cameras of a superburst
from 4U 1254-690. The superburst is preceded by a normal type-I X-ray burst,
has a decay time that is the longest of all eight superbursts detected so far
and a peak luminosity that is the lowest. Like for the other seven superbursts,
the origin is a well-known type-I X-ray burster with a persistent luminosity
level close to one tenth of the Eddington limit. Based on WFC data of all
persistently bright X-ray bursters, the average rate of superbursts is
0.51+/-0.25 per year per persistently bright X-ray burster. Some systems may
have higher superburst rates. For all superbursters, we present evidence for a
pure helium layer which is burnt in an unstable as well as a stable manner.Comment: Accepted by A&A Letter
X-ray Spectroscopy of MXB 1728-34 with XMM-Newton
We have analysed an XMM-Newton observation of the low mass X-ray binary and
atoll source MXB 1728-34. The source was in a low luminosity state during the
XMM-Newton observation, corresponding to a bolometric X-ray luminosity of
5*10E36 d^2 erg/s, where d is the distance in units of 5.1 kpc. The 1-11 keV
X-ray spectrum of the source, obtained combining data from all the five
instruments on-board XMM-Newton, is well fitted by a Comptonized continuum.
Evident residuals are present at 6-7 keV which are ascribed to the presence of
a broad iron emission line. This feature can be equally well fitted by a
relativistically smeared line or by a self-consistent, relativistically
smeared, reflection model. Under the hypothesis that the iron line is produced
by reflection from the inner accretion disk, we can infer important information
on the physical parameters of the system, such as the inner disk radius, Rin =
25-100 km, and the inclination of the system, 44{\deg} < i < 60{\deg}.Comment: 7 pages, 3 figures, Accepted by A&A on 29.03.201
Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473-2721
We investigate the thermonuclear bursting behaviour of IGR J17473-2721, an
X-ray transient that in 2008 underwent a six month long outburst, starting
(unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts
throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range
of inferred accretion rates (between <1% and about 20% of the Eddington
accretion rate m-dot_Edd) spanned during the outburst allows us to study
changes in the nuclear burning processes and to identify up to seven different
phases. The burst rate increased gradually with the accretion rate until it
dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few
days before the outburst peak, after which bursts were not detected for a
month. As the persistent emission subsequently decreased, the bursting activity
resumed with a much lower rate than during the outburst rise. This hysteresis
may arise from the thermal effect of the accretion on the surface nuclear
burning processes, and the timescale is roughly consistent with that expected
for the neutron star crust thermal response. On the other hand, an undetected
superburst, occurring within a data gap near the outburst peak, could have
produced a similar quenching of burst activity.Comment: 18 pages, 12 figures, 1 table, accepted for publication in MNRA
The observers' view of (very) long X-ray bursts: they are super!
In many X-ray point sources on the sky, the X-ray emission arises because
hydrogen and/or helium is accreted onto a neutron star from a nearby donor
star. When this matter settles on the neutron star surface, it will undergo
nuclear fusion. For a large range of physical parameters the fusion is
unstable. The resulting thermo-nuclear explosions last from seconds to minutes.
They are observed as short flares in X-rays and are called `type I X-ray
bursts'. Recently, hours-long X-ray flares have been found in seven X-ray burst
sources with the BeppoSAX/WFC, RXTE/ASM and RXTE/PCA. They have similar
properties to the usual X-ray bursts, except they last for two or three orders
of magnitude longer (hence they are referred to as `superbursts'). This can not
be understood in the context of the standard nuclear-fusion picture mentioned
above. Instead, the superbursts are thought to be related to the unstable
burning of the leftovers from the hydrogen and/or helium fusion. I will discuss
the observational properties of these superbursts.Comment: 10 pages, 7 figures; to appear in Proc. of the 2nd BeppoSAX Meeting:
"The Restless High-Energy Universe" (Amsterdam, May 5-8, 2003), E.P.J. van
den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers (Eds), Nucl. Physics B.
Suppl. Ser., Elsevie
Discovery of type-I X-ray bursts from the low-mass X-ray binary 4U 1708-40
We report the discovery of type-I X-ray bursts from the low-mass X-ray binary
4U 1708-40 during the 100 ks observation performed by BeppoSAX on 1999 August
15-16. Six X-ray bursts have been observed. The unabsorbed 2-10 keV fluxes of
the bursts range from ~ (3-9)x10^(-10) erg cm^(-2)s^(-1). A correlation between
peak flux and fluence of the bursts is found, in agreement with the behaviour
observed in other similar sources. There is a trend of the burst flux to
increase with the time interval from the previous burst. From the value of the
persistent flux we infer a mass accretion rate Mdot~7x10^(-11) Msun/yr, that
may correspond to the mixed hydrogen/helium burning regime triggered by
thermally unstable hydrogen. We have also analysed a BeppoSAX observation
performed on 2001 August 22 and previous RXTE observations of 4U 1708-40, where
no bursts have been observed; we found persistent fluxes of more than a factor
of 7 higher than the persistent flux observed during the BeppoSAX observation
showing X-ray bursts.Comment: accepted for publication in MNRA
Photospheric radius expansion X-ray bursts as standard candles
We examined the maximum bolometric peak luminosities during type I X-ray
bursts from the persistent or transient luminous X-ray sources in globular
clusters. We show that for about two thirds of the sources the maximum peak
luminosities during photospheric radius expansion X-ray bursts extend to a
critical value of (3.79+/-0.15)x10^{38} erg/s, assuming the total X-ray burst
emission is entirely due to black-body radiation and the recorded maximum
luminosity is the actual peak luminosity. This empirical critical luminosity is
consistent with the Eddington luminosity limit for hydrogen poor material.
Since the critical luminosity is more or less always reached during
photospheric radius expansion X-ray bursts (except for one source), such bursts
may be regarded as empirical standard candles. However, because significant
deviations do occur, our standard candle is only accurate to within 15%. We
re-evaluated the distances to the twelve globular clusters in which the X-ray
bursters reside.Comment: Accepted for publication in A&A, 20 pages, 7 figure
Binary systems and their nuclear explosions
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