We present the discovery of a strongly phase-variable absorption feature in the X-ray spectrum of the nearby, thermally-emitting, isolated neutron star RX J0720.4-3125. The absorption line was detected performing detailed phase-resolved spectroscopy
in 20 XMM-Newton observations, covering the period May 2000 - September 2012. The feature has an energy of ~750eV, an equivalent width of ~30eV, and it is significantly detected for only ~20% of the pulsar rotation. The absorption feature appears to be stable over the timespan covered by the observations. Given its strong dependence on the pulsar rotational phase and its narrow width, a plausible interpretation is in terms of resonant proton cyclotron absorption/scattering in a confined magnetic structure very close to the neutron star surface. The inferred field in such a magnetic loop is B_loop ~ 2 x 10^{14} G, a factor of ~7 higher than the surface dipolar magnetic field.
The discovery of millisecond pulsars switching between states powered either by the rotation of their magnetic field or by the accretion of matter, has recently proved the tight link shared by millisecond radio pulsars and neutron stars in low-mass X
-ray binaries. Transitional millisecond pulsars also show an enigmatic intermediate state in which the neutron star is surrounded by an accretion disk, it emits coherent X-ray pulsations, but is sub-luminous in X-rays with respect to accreting neutron stars, and is brighter in gamma-rays than millisecond pulsars in the rotation-powered state. Here, we model the X-ray and gamma-ray emission observed from PSR J1023+0038 in such a state based on the assumption that most of the disk in-flow is propelled away by the rapidly rotating neutron star magnetosphere, and that electrons can be accelerated to energies of a few GeV at the turbulent disk-magnetosphere boundary. We show that the synchrotron and self-synchrotron Compton emission coming from such a region, together with the hard disk emission typical of low states of accreting compact objects, is able to explain the radiation observed in the X-ray and gamma-ray band. The average emission observed from PSR J1023+0038 is modelled by a disk in-flow with a rate of $(1-3)times10^{-11} M_{odot}/yr$, truncated at a radius ranging between 30 and 45 km, compatible with the hypothesis of a propelling magnetosphere. We compare the results we obtained with models that rather assume that a rotation-powered pulsar is turned on, showing how the spin down power released in similar scenarios is hardly able to account for the magnitude of the observed emission.
We study the effects of energy transport in the Sun by asymmetric dark matter with momentum and velocity-dependent interactions, with an eye to solving the decade-old Solar Abundance Problem. We study effective theories where the dark matter-nucleon
scattering cross-section goes as $v_{rm rel}^{2n}$ and $q^{2n}$ with $n = -1, 0, 1 $ or $2$, where $v_{rm rel}$ is the dark matter-nucleon relative velocity and $q$ is the momentum exchanged in the collision. Such cross-sections can arise generically as leading terms from the most basic nonstandard DM-quark operators. We employ a high-precision solar simulation code to study the impact on solar neutrino rates, the sound speed profile, convective zone depth, surface helium abundance and small frequency separations. We find that the majority of models that improve agreement with the observed sound speed profile and depth of the convection zone also reduce neutrino fluxes beyond the level that can be reasonably accommodated by measurement and theory errors. However, a few specific points in parameter space yield a significant overall improvement. A 3-5 GeV DM particle with $sigma_{SI} propto q^2$ is particularly appealing, yielding more than a $6sigma$ improvement with respect to standard solar models, while being allowed by direct detection and collider limits. We provide full analytical capture expressions for $q$- and $v_{rm rel}$-dependent scattering, as well as complete likelihood tables for all models.
To clarify the nature of correlations in Hund metals and its relationship with Mott physics we analyze the electronic correlations in multiorbital systems as a function of intraorbital interaction U, Hunds coupling JH and electronic filling n. We sho
w that the main process behind the enhancement of correlations in Hund metals is the suppression of the double-occupancy of a given orbital, as it also happens in the Mott-insulator at half-filling. However, contrary to what happens in Mott correlated states the reduction of the quasiparticle weight Z with JH can happen on spite of increasing charge fluctuations. Therefore, in Hund metals the quasiparticle weight and the mass enhancement are not good measurements of the charge localization. Using simple energetic arguments we explain why the spin polarization induced by Hunds coupling produces orbital decoupling. We also discuss how the behavior at moderate interactions, with correlations controlled by the atomic spin polarization, changes at large $U$ and $J_H$ due to the proximity to a Mott insulating state.
We study the outburst of the newly discovered X-ray transient 3XMM J185246.6+003317, re-analysing all available XMM-Newton, observations of the source to perform a phase-coherent timing analysis, and derive updated values of the period and period der
ivative. We find the source rotating at P=11.55871346(6) s (90% confidence level; at epoch MJD 54728.7) but no evidence for a period derivative in the 7 months of outburst decay spanned by the observations. This translates in a 3sigma upper limit for the period derivative of Pdot<1.4x10^{-13} s/s, which, assuming the classical magneto-dipolar braking model, gives a limit on the dipolar magnetic field of B_dip<4.1x10^{13} G . The X-ray outburst and spectral characteristics of 3XMM J185246.6+003317 confirms the identification as a magnetar, but the magnetic field upper limit we derive defines it as the third low-B magnetar discovered in the past three years, after SGR 0418+5729 and Swift J1822.3-1606. We have also obtained an upper limit to the quiescent luminosity (< 4x10^{33} erg/s), in line with the expectations for an old magnetar. The discovery of this new low field magnetar reaffirms the prediction of about one outburst per year from the hidden population of aged magnetars.
The evolution of white dwarfs is a simple gravothermal process. This process can be tested in two ways, through the luminosity function of these stars and through the secular variation of the period of pulsation of those stars that are variable. Here
we show how the mass of the axion can be constrained using the white dwarf luminosity function.
Higgs bundles over a closed orientable surface can be defined for any real reductive Lie group G. In this paper we examine the case G=SO*(2n). We describe a rigidity phenomenon encountered in the case of maximal Toledo invariant. Using this and Morse
theory in the moduli space of Higgs bundles, we show that the moduli space is connected in this maximal Toledo case. The Morse theory also allows us to show connectedness when the Toledo invariant is zero. The correspondence between Higgs bundles and surface group representations thus allows us to count the connected components with zero and maximal Toledo invariant in the moduli space of representations of the fundamental group of the surface in SO*(2n).
Aims. Excitation of far-infrared and submillimetric molecular lines may originate from nonreactive collisions, chemical formation, or far infrared, near-infrared, and optical fluorescences. As a template, we investigate the impact of each of these pr
ocesses on the excitation of the methylidyne cation CH+ and on the intensities of its rotational transitions recently detected in emission in dense photodissociation regions (PDRs) and in planetary nebulae. Methods. We have developed a nonlocal thermodynamic equilibrium (non-LTE) excitation model that includes the entire energy structure of CH+, i.e. taking into account the pumping of its vibrational and bound and unbound electronic states by near-infrared and optical photons. The model includes the theoretical cross-sections of nonreactive collisions with H, H2, He, and e-, and a Boltzmann distribution is used to describe the probability of populating the excited levels of CH+ during its chemical formation by hydrogenation of C+. To confirm our results we also performed an extensive analytical study, which we use to predict the main excitation process of several diatomic molecules, namely HF, HCl, SiO, CS, and CO. Results. At densities nH = 10^4 cm-3, the excitation of the rotational levels of CH+ is dominated by the radiative pumping of its electronic, vibrational, and rotational states if the intensities of the radiation field at sim 0.4, sim 4, and sim 300 mum are stronger than 10^5, 10^8, and 10^4 times those of the local interstellar radiation field (ISRF). Below these values, the chemical pumping is the dominant source of excitation of the J > 1 levels, even at high kinetic temperatures (sim 1000 K). The far-infrared emission lines of CH+ observed in the Orion Bar and the NGC 7027 PDRs are consistent with the predictions of our excitation model assuming an incident far-ultraviolet (FUV) radiation field of sim 3 times 10^4 (in Draines unit) and densities of sim 5 times 10^4 and sim 2 times 10^5 cm-3. In the case of NGC 7027, the estimate of the density is 10 to 100 times lower than those deduced by traditional excitation codes. Applying our model to other X1Sigma+ ground state diatomic molecules, we find that HF, and SiO and HCl are the species the most sensitive to the radiative pumping of their vibrational and bound electronic states. In both cases, the minimal near-infrared and optical/UV radiation field intensities required to modify their rotational level populations are sim 10^3 times those of the local ISRF at densities nH = 10^4 cm-3. All these results point towards interstellar and circumstellar media with densities lower than previously established and cast doubts on the clumpiness of well-studied molecular clouds.
Among the many different classes of stellar objects, neutron stars provide a unique environment where we can test (at the same time) our understanding of matter with extreme density, temperature, and magnetic field. In particular, the properties of m
atter under the influence of magnetic fields and the role of electromagnetism in physical processes are key areas of research in physics. However, despite decades of research, our limited knowledge on the physics of strong magnetic fields is clear: we only need to note that the strongest steady magnetic field achieved in terrestrial labs is some millions of Gauss, only thousands of times stronger than a common refrigerator magnet. In this general context, I will review here the state of the art of our research on the most magnetic objects in the Universe, a small sample of neutron stars called magnetars. The study of the large high-energy emission, and the flares from these strongly magnetized (~10^{15} Gauss) neutron stars is providing crucial information about the physics involved at these extremes conditions, and favoring us with many unexpected surprises.
Aims. Young, massive stars have been found at projected distances R < 0.5 pc from supermassive black hole, Sgr A* at the center of our Galay. In recent years, increasing evidence has been found for the presence of young, massive stars also at R > 0.5
pc. Our goal in this work is a systematic search for young, massive star candidates throughout the entire region within R ~ 2.5 pc of the black hole. Methods. The main criterion for the photometric identification of young, massive early-type stars is the lack of CO-absorption in the spectra. We used narrow-band imaging with VLT/ISAAC to search for young, massive stars within ~2.5 pc of Sgr A*. Results. We have found 63 early-type star candidates at R < 2.5 pc, with an estimated erroneous identification rate of only about 20%. Considering their K-band magnitudes and interstellar extinction, they are candidates for Wolf-Rayet stars, supergiants, or early O-type stars. Of these, 31 stars are so far unknown young, massive star candidates, all of which lie at R>0.5pc. The surface number density profile of the young, massive star candidates can be well fit by a single power-law, with Gamma = 1.6 +- 0.17 at R < 2.5 pc, which is significantly steeper than that of the late-type giants that make up the bulk of the observable stars in the NSC. Intriguingly, this power-law is consistent with the power-law that describes the surface density of young, massive stars in the same brightness range at R < 0.5 pc. Conclusions. The finding of a significant number of newly identified early-type star candidates at the Galactic center suggests that young, massive stars can be found throughout the entire cluster which may require us to modify existing theories for star formation at the Galactic center. Follow-up studies are needed to improve the existing data and lay the foundations for a unified theory of star formation in the Milky Ways NSC.
