Search

Analysis of Chandra observations of the neutron star (NS) in the centre of the Cassiopeia A supernova remnant taken in the subarray (FAINT) mode of the Advanced CCD Imaging Spectrometer (ACIS) detector performed by Posselt and collaborators revealed, after inclusion of the most recent (2020 May) observations, a significant decrease of the source surface temperature from 2006 to 2020. The obtained cooling rate is consistent with those obtained from analysis of the 2000–2019 data taken in the GRADED mode of the ACIS detector, which is potentially more strongly affected by instrumental effects. We performed a joint spectral analysis using all ACIS data to constrain the NS parameters and cooling rate. We constrain the mass of the Cassiopeia A NS at |$M=1.55\pm 0.25\, {\rm M}_\odot$|⁠ , and its radius at R  = 13.5 ± 1.5 km. The surface temperature cooling rate is found to be 2.2 ± 0.3 per cent in 10 yr if the absorbing hydrogen column density is allowed to vary and 1.6 ± 0.2 per cent in 10 yr if it is fixed. The observed cooling can be explained by enhanced neutrino emission from the superfluid NS interior due to Cooper pair formation (CPF) process. Based on analysis of all ACIS data, we constrain the maximal critical temperature of triplet neutron pairing within the NS core at (4–9.5) × 108 K. In accordance with previous studies, the required effective strength of the CPF neutrino emission is at least a factor of 2 higher than existing microscopic calculations suggest. [ABSTRACT FROM AUTHOR]

We present analysis of multiple Chandra and XMM–Newton spectra, separated by 9–19 yr, of four of the youngest central compact objects (CCOs) with ages <2500 yr: CXOU J232327.9+584842 (Cassiopeia A), CXOU J160103.1−513353 (G330.2+1.0), 1WGA J1713.4−3949 (G347.3−0.5), and XMMU J172054.5−372652 (G350.1−0.3). By fitting these spectra with thermal models, we attempt to constrain each CCO's long-term cooling rate, composition, and magnetic field. For the CCO in Cassiopeia A, 14 measurements over 19 yr indicate a decreasing temperature at a 10-yr rate of 2.2 ± 0.2 or 2.8 ± 0.3 per cent (1σ error) for a constant or changing X-ray absorption, respectively. We obtain cooling rate upper limits of 17 per cent for CXOU J160103.1−513353 and 6 per cent for XMMU J172054.5−372652. For the oldest CCO, 1WGA J1713.4−3949, its temperature seems to have increased by 4 ± 2 per cent over a 10-yr period. Assuming each CCO's preferred distance and an emission area that is a large fraction of the total stellar surface, a non-magnetic carbon atmosphere spectrum is a good fit to spectra of all four CCOs. If distances are larger and emission areas are somewhat smaller, then equally good spectral fits are obtained using a hydrogen atmosphere with B ≤ 7 × 1010 G or B ≥ 1012 G for CXOU J160103.1−513353 and B ≤ 1010 G or B ≥ 1012 G for XMMU J172054.5−372652 and non-magnetic hydrogen atmosphere for 1WGA J1713.4−3949. In a unified picture of CCO evolution, our results suggest most CCOs, and hence a sizable fraction of young neutron stars, have a surface magnetic field that is low early in their life but builds up over several thousand years. [ABSTRACT FROM AUTHOR]

We present a detailed study of two-color two-photon excited fluorescence in indole dissolved in propylene glycol. Femtosecond excitation pulses at effective wavelengths from 268 to 293.33 nm were used to populate the two lowest indole excited states ¹La and 1Lb and polarized fluorescence was then detected. All seven molecular parameters and the two-photon polarization ratio O containing information on two-photon absorption dynamics, molecular lifetime tf, and rotation correlation time trot have been determined from experiment and analyzed as a function of the excitation wavelength. The analysis of the experimental data has shown that ¹Lb-¹La inversion occurred under the conditions of our experiment. The two-photon absorption predominantly populated the ¹La state at all excitation wavelengths but in the 287-289 nm area which contained an absorption hump of the ¹Lb state 0-0 origin. The components of the two-photon excitation tensor S were analyzed giving important information on the principal tensor axes and absorption symmetry. The results obtained are in a good agreement with the results reported by other groups. The lifetime tf and the rotation correlation time trot showed no explicit dependence on the effective excitation wavelength. Their calculated weighted average values were found to be tf = 3.83 ± 0.14 ns and trot = 0.74 ± 0.06 ns. [ABSTRACT FROM AUTHOR]

We present the full quantum mechanical expressions for the polarization differential cross sections of the photofragments produced in slow predissociation of a parent molecule via isolated rotational branches. Both radial and Coriolis nonadiabatic interactions between the molecular potential energy surfaces have been taken into account. The expressions describe the recoil angle distribution of the photofragments and the distributions of the photofragment angular momentum polarization (orientation and alignment) in terms of the anisotropy parameters of the ranks K=0,1,2. The explicit expressions for the anisotropy parameters are presented and analyzed which contain contributions from different possible photolysis mechanisms including incoherent, or coherent optical excitation of the parent molecule followed by the radial, or Coriolis nonadiabatic transitions to the dissociative states. The obtained expression for the zeroth-rank anisotropy parameter β is valid for any molecule and for an arbitrary value of the molecular total angular momentum J. The expressions for the orientation (K=1) and alignment (K=2) anisotropy parameters are given in the high-J limit in terms of the generalized dynamical functions which were analyzed for the case of photolysis of linear/diatomic molecules. As shown, the Coriolis nonadiabatic interaction results in several new photolysis mechanisms which can play an important role in the predissociation dynamics. [ABSTRACT FROM AUTHOR]

We present a theoretical framework for calculating the recoil-angle dependence of the photofragment angular momentum polarization taking into account both radial and Coriolis nonadiabatic interactions in the diatomic/linear photodissociating molecules. The parity-adapted representation of the total molecular wave function has been used throughout the paper. The obtained full quantum-mechanical expressions for the photofragment state multipoles have been simplified by using the semiclassical approximation in the high-J limit and then analyzed for the cases of direct photodissociation and slow predissociation in terms of the anisotropy parameters. In both cases, each anisotropy parameter can be presented as a linear combination of the generalized dynamical functions fK(q,q′,q,q′) of the rank K representing contribution from different dissociation mechanisms including possible radial and Coriolis nonadiabatic transitions, coherent effects, and the rotation of the recoil axis. In the absence of the Coriolis interactions, the obtained results are equivalent to the earlier published ones. The angle-recoil dependence of the photofragment state multipoles for an arbitrary photolysis reaction is derived. As shown, the polarization of the photofragments in the photolysis of a diatomic or a polyatomic molecule can be described in terms of the anisotropy parameters irrespective of the photodissociation mechanism. [ABSTRACT FROM AUTHOR]

Abstract We calculate modified Urca neutrino emission rates in the dense nuclear matter in neutron star cores. We find that these rates are strongly enhanced in the beta-stable matter in regions of the core close to the direct Urca process threshold. This enhancement can be tracked to the use of the in-medium nucleon spectrum in the virtual nucleon propagator. We describe the in-medium nucleon scattering in the non-relativistic Bruckner–Hartree–Fock framework taking into account two-body as well as the effective three-body forces, although the proposed enhancement does not rely on a particular way of the nucleon interaction treatment. Finally we suggest a simple approximate expression for the emissivity of the neutron branch of the modified Urca process that can be used in the neutron stars cooling simulations with any nucleon equation of state of dense matter. [ABSTRACT FROM AUTHOR]

According to recent results of Ho & Heinke, the Cassiopeia A supernova remnant contains a young (≈330-yr-old) neutron star (NS) which has carbon atmosphere and shows notable decline of the effective surface temperature. We report a new (2010 November) Chandra observation which confirms the previously reported decline rate. The decline is naturally explained if neutrons have recently become superfluid (in triplet state) in the NS core, producing a splash of neutrino emission due to Cooper pair formation (CPF) process that currently accelerates the cooling. This scenario puts stringent constraints on poorly known properties of NS cores: on density dependence of the temperature for the onset of neutron superfluidity [ should have a wide peak with maximum K]; on the reduction factor q of CPF process by collective effects in superfluid matter () and on the intensity of neutrino emission before the onset of neutron superfluidity (30-100 times weaker than the standard modified Urca process). This is serious evidence for nucleon superfluidity in NS cores that comes from observations of cooling NSs. [ABSTRACT FROM AUTHOR]

We explore the thermal state of the neutron star in the Cassiopeia A supernova remnant using the recent result of Ho & Heinke that the thermal radiation of this star is well described by a carbon atmosphere model and the emission comes from the entire stellar surface. Starting from neutron star cooling theory, we formulate a robust method to extract neutrino cooling rates of thermally relaxed stars at the neutrino cooling stage from observations of thermal surface radiation. We show how to compare these rates with the rates of standard candles - stars with non-superfluid nucleon cores cooling slowly via the modified Urca process. We find that the internal temperature of standard candles is a well-defined function of the stellar compactness parameter , irrespective of the equation of state of neutron star matter ( R and are circumferential and gravitational radii, respectively). We demonstrate that the data on the Cassiopeia A neutron star can be explained in terms of three parameters: , the neutrino cooling efficiency with respect to the standard candle; the compactness x; and the amount of light elements in the heat-blanketing envelope. For an ordinary (iron) heat-blanketing envelope or a low-mass () carbon envelope, we find the efficiency (standard cooling) for and (slower cooling) for a maximum compactness . A heat blanket containing the maximum mass () of light elements increases by a factor of 50. We also examine the (unlikely) possibility that the star is still thermally non-relaxed. [ABSTRACT FROM AUTHOR]

We report a general theory of the two-photon excited fluorescence (TPEF) of rotating molecules under the condition of anisotropic depolarisation. The obtained expressions for the TPEF intensity are based upon the spherical tensor approach and valid for any symmetric, or asymmetric top molecule and for any photon polarisation. The expressions are written in terms of the molecular parameters MK(R, R', t) which have clear tensor notation and contain all molecular information that can be extracted from the TPEF experiments. Two important extreme cases are discussed. If the molecular rotation period is much larger than the excited state lifetime, the obtained expressions describe the TPEF in non-rotating molecules under the condition of isotropic relaxation. The time-independent part of these expressions is in agreement with the earlier results of McClain [J. Chem. Phys., 58, 324 (1973)]. The relationship between McClain's molecular parameters [image omitted] and the parameters MK(R, R', 0) is presented and analysed. If the molecular rotation period is much smaller than the excited state lifetime, the expressions describe anisotropy of fluorescence from isolated ro-vibrational molecular states. As shown, under certain approximations the expression still can be written in terms of the M-parameters which are reduced due to molecular rotation by about a factor of (2K + 1). This result manifests that the fluorescence anisotropy is reduced, but not lost due to the molecular rotation. [ABSTRACT FROM AUTHOR]

The cooling rate of young neutron stars gives direct insight into their internal makeup. Using Chandra observations of the 330-year-old Cassiopeia A supernova remnant, we find that the temperature of the youngest-known neutron star in the Galaxy has declined by 4% over the last 10 years. The decline is explained naturally by superconductivity and superfluidity of the protons and neutrons in the stellar core. The protons became superconducting early in the life of the star and suppressed the early cooling rate; the neutron star thus remained hot before the (recent) onset of neutron superfluidity. Once the neutrons became superfluid, the Cooper pair-formation process produced a splash of neutrino emission which accelerated the cooling and resulted in the observed rapid temperature decline. This is the first time a young neutron star has been seen to cool in real time, and is the first direct evidence, from cooling observations, of superfluidity and superconductivity in the core of neutron stars. [ABSTRACT FROM PUBLISHER]

High resolution kinetic energy release spectra were obtained for C(+) and O(+) from CO multiphoton ionization followed by dissociation of CO(+). The excitation was through the CO (B (1)Sigma(+)) state via resonant two-photon excitation around 230 nm. A total of 5 and 6 photons are found to contribute to the production of carbon and oxygen cations. DC slice and Megapixel ion imaging techniques were used to acquire high quality images. Major features in both O(+) and C(+) spectra are assigned to the dissociation of some specific vibrational levels of CO(+)(X (2)Sigma(+)). The angular distributions of C(+) and O(+) are very distinct and those of various features of C(+) are also different. A dramatic change of the angular distribution of C(+) from dissociation of CO(+)(X (2)Sigma(+), nu(+) = 1) is attributed to an accidental one-photon resonance between CO(+)(X (2)Sigma(+), nu(+) = 1) and CO(+)(B (2)Sigma(+), nu(+) = 0) and explained well by a theoretical model. Both kinetic energy release and angular distributions were used to reveal the underlying dynamics.