Opacities and spectra of hydrogen atmospheres of moderately magnetized neutron stars

There is observational evidence that central compact objects (CCOs) in supernova remnants have moderately strong magnetic fields $B\sim10^{11}$ G. Meanwhile, available models of partially ionized hydrogen atmospheres of neutron stars with strong magnetic fields are restricted to $B\gtrsim10^{12}$ G. We extend the equation of state and radiative opacities, presented in previous papers for 10^{12}\mbox{ G}\lesssim B \lesssim 10^{15} G, to weaker fields. An equation of state and radiative opacities for a partially ionized hydrogen plasma are obtained at magnetic fields $B$, temperatures $T$, and densities $\rho$ typical for atmospheres of CCOs and other isolated neutron stars with moderately strong magnetic fields. The first- and second-order thermodynamic functions, monochromatic radiative opacities, and Rosseland mean opacities are calculated and tabulated, taking account of partial ionization, for 3\times10^{10}\mbox{ G}\lesssim B\lesssim 10^{12} G, $10^5$ K $\lesssim T\lesssim 10^7$ K, and a wide range of densities. Atmosphere models and spectra are calculated to verify the applicability of the results and to determine the range of magnetic fields and effective temperatures where the incomplete ionization of the hydrogen plasma is important.The work of AYP on calculation of the opacities and polarizabilities (Sect. 4) has been supported by the Russian Science Foundation (grant 14-12-00316). WCGH appreciates use of computer facilities at KIPAC

Osteoporotic insufficiency fractures of the pelvis

Insufficiency fractures are a subtype of stress fractures which occur when normal or physiological stresses are placed on weakened bone. The main predisposing cause is osteoporosis. The clinical and radiological features of two elderly women with pelvic insufficiency fractures are described. Isotope bone scan is the most sensitive imaging modality for the detection of these fractures. Computed tomography is useful for confirming these fractures and excluding malignancy. As pelvic insufficiency fractures are difficult to detect clinically, application of the appropriate radiological investigation is necessary for diagnosis. With early recognition of this entity, the patient could be managed appropriately, thus avoiding any unnecessary invasive investigations.published_or_final_versio

Hunting for Orphaned Central Compact Objects Among Radio Pulsars

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Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114

We report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX J0812.4-3114 in quiescence (L-X approximate to 1.6 x 10(33) erg s(-1)). Intriguingly, we find a two-component spectrum, with a hard power-law (Gamma approximate to 1.5) and a soft blackbody-like excess below keV. The blackbody component is consistent in kT with a prior quiescent Chandra observation reported by Tsygankov et al. and has an inferred blackbody radius of;----10 km, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at approximate to and/or approximate to 1.4 keV. The hard component shows pulsations at P approximate to 31.908 s (pulsed fraction 0.84 +/- 0.10), agreeing with the pulse period seen previously in outbursts, but 110 pulsations were found in the soft excess (pulsed fraction less than or similar to 31 per cent). We conclude that the pulsed hard component suggests low-level accretion on to the NS poles, while the soft excess seems to originate from the entire NS surface. Wc speculate that, in quiescence, the source switches between a soft, thermal-dominated state (when the propeller effect is at work) and a relatively hard state with low-level accretion, and use the propeller cut-off to estimate the ma.gnetic field of the system to be less than or similar to 8.4 x 10(11)G. We compare the quiescent thermal L-X predicted by the standard deep crustal heating model to our observations and find that RX J0812.4-31.14 has a high thermal L-X, at or above the prediction for minimum cooling mechanisms. This suggests that RX J0812.4-3114 either contains a relatively low-mass NS with minimum cooling, or that the system may be young enough that the NS has not fully cooled from the supernova explosion

Multiwavelength monitoring and X-ray brightening of Be X-ray binary PSR J2032+4127/MT91 213 on its approach to periastron

The radio and gamma-ray pulsar PSR J2032+4127 was recently found to be in a decades-long orbit with the Be star MT91 213, with the pulsar moving rapidly towards periastron. This binary shares many similar characteristics with the previously unique binary system PSR B1259−63/LS 2883. Here, we describe radio, X-ray, and optical monitoring of PSR J2032+4127/MT91 213. Our extended orbital phase coverage in radio, supplemented with Fermi LAT gamma-ray data, allows us to update and refine the orbital period to 45–50 yr and time of periastron passage to 2017 November. We analyse archival and recent Chandra and Swift observations and show that PSR J2032+4127/MT91 213 is now brighter in X-rays by a factor of ∼70 since 2002 and ∼20 since 2010. While the pulsar is still far from periastron, this increase in X-rays is possibly due to collisions between pulsar and Be star winds. Optical observations of the Hα emission line of the Be star suggest that the size of its circumstellar disc may be varying by ∼2 over time-scales as short as 1–2 months. Multiwavelength monitoring of PSR J2032+4127/MT91 213 will continue through periastron passage, and the system should present an interesting test case and comparison to PSR B1259−63/LS 2883

The semicentennial binary system PSR J2032+4127 at periastron: X-ray photometry, optical spectroscopy, and SPH modelling

X-ray photometry and optical spectra are presented covering the periastron passage of the highly eccentric, ∼50 yr binary system PSR J2032+4127 in 2017 November. This system consists of a 143 ms pulsar in orbit around a massive OB star, MT 91-213. The data show dramatic changes during the encounter as the pulsar wind collided with the stellar wind. The X-ray flux rose on the approach to periastron, then underwent a major dip in the few days around periastron, and then gradually declined over the next few weeks. The optical spectroscopy revealed a steady decline in the H α line strength on the approach to periastron (from an equivalent width of −15 to −7 Å) implying a truncation of the OB star’s circumstellar disc by the approaching neutron star. Smooth particle hydrodynamic modelling is used here to model the system within the context of the observed behaviour and predict the geometrical configuration of the circumstellar disc with respect to the pulsar’s orbit

Cooling of Dark-Matter Admixed Neutron Stars with density-dependent Equation of State

We propose a dark-matter (DM) admixed density-dependent equation of state where the fermionic DM interacts with the nucleons via Higgs portal. Presence of DM can hardly influence the particle distribution inside neutron star (NS) but can significantly affect the structure as well as equation of state (EOS) of NS. Introduction of DM inside NS softens the equation of state. We explored the effect of variation of DM mass and DM Fermi momentum on the NS EOS. Moreover, DM-Higgs coupling is constrained using dark matter direct detection experiments. Then, we studied cooling of normal NSs using APR and DD2 EOSs and DM admixed NSs using dark-matter modified DD2 with varying DM mass and Fermi momentum. We have done our analysis by considering different NS masses. Also DM mass and DM Fermi momentum are varied for fixed NS mass and DM-Higgs coupling. We calculated the variations of luminosity and temperature of NS with time for all EOSs considered in our work and then compared our calculations with the observed astronomical cooling data of pulsars namely Cas A, RX J0822-43, 1E 1207-52, RX J0002+62, XMMU J17328, PSR B1706-44, Vela, PSR B2334+61, PSR B0656+14, Geminga, PSR B1055-52 and RX J0720.4-3125. It is found that APR EOS agrees well with the pulsar data for lighter and medium mass NSs but cooling is very fast for heavier NS. For DM admixed DD2 EOS, it is found that for all considered NS masses, all chosen DM masses and Fermi momenta agree well with the observational data of PSR B0656+14, Geminga, Vela, PSR B1706-44 and PSR B2334+61. Cooling becomes faster as compared to normal NSs in case of increasing DM mass and Fermi momenta. It is infered from the calculations that if low mass super cold NSs are observed in future that may support the fact that heavier WIMP can be present inside neutron stars.Comment: 24 Pages, 15 Figures and 2 Tables. Version accepted in The European Physical Journal

Strongly magnetized pulsars: explosive events and evolution

Well before the radio discovery of pulsars offered the first observational confirmation for their existence (Hewish et al., 1968), it had been suggested that neutron stars might be endowed with very strong magnetic fields of $10^{10}$-$10^{14}$G (Hoyle et al., 1964; Pacini, 1967). It is because of their magnetic fields that these otherwise small ed inert, cooling dead stars emit radio pulses and shine in various part of the electromagnetic spectrum. But the presence of a strong magnetic field has more subtle and sometimes dramatic consequences: In the last decades of observations indeed, evidence mounted that it is likely the magnetic field that makes of an isolated neutron star what it is among the different observational manifestations in which they come. The contribution of the magnetic field to the energy budget of the neutron star can be comparable or even exceed the available kinetic energy. The most magnetised neutron stars in particular, the magnetars, exhibit an amazing assortment of explosive events, underlining the importance of their magnetic field in their lives. In this chapter we review the recent observational and theoretical achievements, which not only confirmed the importance of the magnetic field in the evolution of neutron stars, but also provide a promising unification scheme for the different observational manifestations in which they appear. We focus on the role of their magnetic field as an energy source behind their persistent emission, but also its critical role in explosive events.Comment: Review commissioned for publication in the White Book of "NewCompStar" European COST Action MP1304, 43 pages, 8 figure

Gravitational Waves from Gravitational Collapse

Gravitational wave emission from the gravitational collapse of massive stars has been studied for more than three decades. Current state of the art numerical investigations of collapse include those that use progenitors with realistic angular momentum profiles, properly treat microphysics issues, account for general relativity, and examine non--axisymmetric effects in three dimensions. Such simulations predict that gravitational waves from various phenomena associated with gravitational collapse could be detectable with advanced ground--based and future space--based interferometric observatories.Comment: 68 pages including 13 figures; revised version accepted for publication in Living Reviews in Relativity (http://www.livingreviews.org