791 research outputs found
A Comment on "A note on polarized light from Magnetars: QED effects and axion-like particles" by L.M. Capparelli, L. Maiani and A.D. Polosa
The recent detection of a large polarization degree in the optical emission
of an isolated neutron star led to the suggestion that this has been the first
evidence of vacuum polarization in a strong magnetic field, an effect predicted
by quantum electrodynamics but never observed before. This claim was challanged
in a paper by Capparelli, Maiani & Polosa (2017), according to whom a much
higher polarization degree would be necessary to positively identify vacuum
polarization. Here we show that their conclusions are biased by several
inadequate assumptions and have no impact on the original claim.Comment: 10 pages, 2 figure
Evidence of vacuum birefringence from the polarisation of the optical emission from an Isolated Neutron Star
Isolated Neutron Stars are some of the most exciting stellar objects known to
astronomers: they have the most extreme magnetic fields, with values up to
G, and, with the exception of stellar-mass black holes, they are the
most dense stars, with densities of g cm. As such,
they are perfect laboratories to test theories of electromagnetism and nuclear
physics under conditions of magnetic field and density unattainable on Earth.
In particular, the interaction of radiation with strong magnetic fields is the
cause of the {\em vacuum birefringence}, an effect predicted by quantum
electrodynamics in 1936 but that lacked an observational evidence until now.
Here, we show how the study of the polarisation of the optical radiation from
the surface of an isolated neutron star yielded such an observational evidence,
opening exciting perspectives for similar studies at other wavelengths.Comment: 5 pages, 1 figure, Contributed to the 13th Patras Workshop on Axions,
WIMPs and WISPs, Thessaloniki, May 15 to 19, 201
Observational Properties of Accreting Neutron Stars with an Optically Thick Envelope
Pulsating Ultra Luminous X-ray sources (PULXs) are thought to be X-ray bright, accreting, magnetized neutron stars. Their measured soft (<20 keV) X-ray luminosity can exceed the Eddington luminosity for a neutron star (NS) by a few orders of magnitude. Although several scenarios have been proposed to explain the different components observed in the X-ray spectrum and the characteristics of the X-ray pulse profile of these system, detailed quantitative calculations are still missing. In particular, the observed soft X-ray pulse profiles are almost sinusoidal and show an increase in the pulsed fraction (from 8 % up to even 30 %) with increasing energy. In this work, we discuss how emission originating from an optically thick envelope, expected to be formed during super-Eddington accretion, can match the observed spectra and pulse profiles
PULXs as Accreting Magnetars: Observational Manifestations
Pulsating Ultra Luminous X-ray sources (PULXs) are thought to be X-ray bright, accreting, magnetized neutron stars, and could be the first and only evidence for the existence of magnetars in binary systems. Their apparent soft (< 20 keV) X-ray luminosity can exceed the Eddington luminosity for a neutron star (NS) by a few orders of magnitude. Although several scenarios have been proposed to explain the different components observed in the X-ray spectra and the characteristics of the X-ray lightcurve of these system, detailed quantitative calculations are still missing. In particular, the observed soft X-ray lightcurves are almost sinuosidal and show an increase in the pulsed fraction (from 8% up to even 30%) with increasing energy. Here, we present how emission originating from an optically thick envelope, expected to be formed during super-Eddington accretion, can result in pulsed fractions similar to observations
A strong X-ray polarization signal from the magnetar 1RXS J170849.0-400910
Magnetars are the most strongly magnetized neutron stars, and one of the most promising targets for X-ray polarimetric measurements. We present here the first Imaging X-ray Polarimetry Explorer observation of the magnetar 1RXS J170849.0-400910, jointly analyzed with a new Swift observation and archival NICER data. The total (energy- and phase-integrated) emission in the 2–8 keV energy range is linerarly polarized, at a ∼35% level. The phase-averaged polarization signal shows a marked increase with energy, ranging from ∼20% at 2–3 keV up to ∼80% at 6–8 keV, while the polarization angle remains constant. This indicates that radiation is mostly polarized in a single direction. The spectrum is well reproduced by a combination of either two thermal (blackbody) components or a blackbody and a power law. Both the polarization degree and angle also show a variation with the spin phase, and the former is almost anticorrelated with the source counts in the 2–8 and 2–4 keV bands. We discuss the possible implications and interpretations, based on a joint analysis of the spectral, polarization, and pulsation properties of the source. A scenario in which the surface temperature is not homogeneous, with a hotter cap covered by a gaseous atmosphere and a warmer region in a condensed state, provides a satisfactory description of both the phase- and energy-dependent spectro-polarimetric data. The (comparatively) small size of the two emitting regions, required to explain the observed pulsations, does not allow to reach a robust conclusion about the presence of vacuum birefringence effects
Vacuum birefringence and X-ray polarimetry in transient magnetars
Recent optical polarimetry observations of an X-ray dim isolated neutron star, RX J1856.5-3754, showed a first evidence for QED vacuum birefringence induced by a strong magnetic field. This important result can be confirmed by performing systematically polarimetry observations in the X-ray band for other strongly magnetized neutron stars, such as transient or persistent magnetars. We computed the phase averaged polarization fraction (PF) and polarization angle (PA) expected in the thermal emission from transient magnetars in the soft X-ray energy band. We found that the detection of a PF higher than 60% is a strong evidence for vacuum birefringence. We also found that a steady change in the PA measured from transient magnetars during their outburst decay (up to 23 degrees for a magnetospheric untwisting of 0.5 rad) is a strong signature of vacuum birefringence. This latter detection would also provide an independent check of the magnetospheric untwisting model for these sources. Simulations show that these measurements are achievable by future polarimetric missions such as XIPE and IXPE with 20-380 ks of observational time, and with eXTP with 3-60 ks
Evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star RX J1856.5−3754
The ‘Magnificent Seven’ (M7) are a group of radio-quiet isolated neutron stars discovered in the soft X-rays through their purely thermal surface emission. Owing to the large inferred magnetic fields (B ≈ 1013 G), radiation from these sources is expected to be substantially polarized, independently of the mechanism actually responsible for the thermal emission. A large observed polarization degree (PD) is, however, expected only if quantum-electrodynamic (QED) polarization effects are present in the magnetized vacuum around the star. The detection of a strong linearly polarized signal would therefore provide the first observational evidence of QED effects in the strong-field regime. While polarization measurements in the soft X-rays are not feasible yet, optical polarization measurements are within reach also for quite faint targets, like the M7 which have optical counterparts with magnitudes ≈26–28. Here, we report on the measurement of optical linear polarization for the prototype, and brightest member, of the class, RX J1856.5−3754 (V ∼ 25.5), the first ever for one of the M7, obtained with the Very Large Telescope. We measured a PD = 16.43 ± 5.26 per cent and a polarization position angle PA = 145°.39 ± 9°.44, computed east of the North Celestial Meridian. The PD that we derive is large enough to support the presence of vacuum birefringence, as predicted by QED
Evidence of vacuum birefringence from the polarisation of the optical emission from an Isolated Neutron Star
Isolated Neutron Stars are some of the most exciting stellar objects known to astronomers:
they have the most extreme magnetic fields, with values up to 1015 G, and, with the exception
of stellar-mass black holes, they are the most dense stars, with densities of ≈ 1014
g cm−3
. As such, they are perfect laboratories to test theories of electromagnetism and
nuclear physics under conditions of magnetic field and density unattainable on Earth. In
particular, the interaction of radiation with strong magnetic fields is the cause of the vacuum
birefringence, an effect predicted by quantum electrodynamics in 1936 but that lacked
an observational evidence until now. Here, we show how the study of the polarisation of the
optical radiation from the surface of an isolated neutron star yielded such an observational
evidence, opening exciting perspectives for similar studies at other wavelengths
Finite size effects on thermal denaturation of globular proteins
Finite size effects on the cooperative thermal denaturation of proteins are
considered. A dimensionless measure of cooperativity, Omega, scales as N^zeta,
where N is the number of amino acids. Surprisingly, we find that zeta is
universal with zeta = 1 + gamma, where the exponent gamma characterizes the
divergence of the susceptibility for a self-avoiding walk. Our lattice model
simulations and experimental data are consistent with the theory. Our finding
rationalizes the marginal stability of proteins and substantiates the earlier
predictions that the efficient folding of two-state proteins requires the
folding transition temperature to be close to the collapse temperature.Comment: 3 figures. Physical Review Letters (in press
Evidence for vacuum birefringence from the first optical-polarimetry measurement of the isolated neutron star RX J1856.5-3754
The "Magnificent Seven" (M7) are a group of radio-quiet Isolated Neutron
Stars (INSs) discovered in the soft X-rays through their purely thermal surface
emission. Owing to the large inferred magnetic fields ( G),
radiation from these sources is expected to be substantially polarised,
independently on the mechanism actually responsible for the thermal emission. A
large observed polarisation degree is, however, expected only if
quantum-electrodynamics (QED) polarisation effects are present in the
magnetised vacuum around the star. The detection of a strongly linearly
polarised signal would therefore provide the first observational evidence of
QED effects in the strong-field regime. While polarisation measurements in the
soft X-rays are not feasible yet, optical polarisation measurements are within
reach also for quite faint targets, like the M7 which have optical counterparts
with magnitudes --. Here, we report on the measurement of
optical linear polarisation for the prototype, and brightest member, of the
class, RX\, J1856.53754 (), the first ever for one of the M7,
obtained with the Very Large Telescope. We measured a polarisation degree
and a polarisation position angle
\mathrm{P.A.}=145\fdg39\pm9\fdg44, computed east of the North Celestial
Meridian. The that we derive is large enough to support the
presence of vacuum birefringence, as predicted by QED.Comment: 9 pages, 7 figures, accepted for publication on MNRA
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