Mean-field phase diagram of interacting e_g electrons

We investigate the magnetic phase diagram of the two-dimensional model for e_g electrons which describes layered nickelates. One finds a generic tendency towards magnetic order accompanied by orbital polarization. For two equivalent orbitals with diagonal hopping such orbitally polarized phases are induced by finite crystal field.Comment: 2 pages, 2 figure

Pulsar spin-down: the glitch-dominated rotation of PSR J0537-6910

The young, fast-spinning, X-ray pulsar J0537-6910 displays an extreme glitch activity, with large spin-ups interrupting its decelerating rotation every ~100 days. We present nearly 13 years of timing data from this pulsar, obtained with the {\it Rossi X-ray Timing Explorer}. We discovered 22 new glitches and performed a consistent analysis of all 45 glitches detected in the complete data span. Our results corroborate the previously reported strong correlation between glitch spin-up size and the time to the next glitch, a relation that has not been observed so far in any other pulsar. The spin evolution is dominated by the glitches, which occur at a rate ~3.5 per year, and the post-glitch recoveries, which prevail the entire inter-glitch intervals. This distinctive behaviour provides invaluable insights into the physics of glitches. The observations can be explained with a multi-component model which accounts for the dynamics of the neutron superfluid present in the crust and core of neutron stars. We place limits on the moment of inertia of the component responsible for the spin-up and, ignoring differential rotation, the velocity difference it can sustain with the crust. Contrary to its rapid decrease between glitches, the spin-down rate increased over the 13 years, and we find the long-term braking index $n_{\rm l}=-1.22(4)$, the only negative braking index seen in a young pulsar. We briefly discuss the plausible interpretations of this result, which is in stark contrast to the predictions of standard models of pulsar spin-down.Comment: Minor changes to match the MNRAS accepted versio

The 32-GHz performance of the DSS-14 70-meter antenna: 1989 configuration

The results of preliminary 32 GHz calibrations of the 70 meter antenna at Goldstone are presented. Measurements were done between March and July 1989 using Virgo A and Venus as the primary efficiency calibrators. The flux densites of theses radio sources at 32 GHz are not known with high accuracy, but were extrapolated from calibrated data at lower frequencies. The measured value of efficiency (0.35) agreed closely with the predicted value (0.32), and the results are very repeatable. Flux densities of secondary sources used in the observations were subsequently derived. These measurements were performed using a beamswitching radiometer that employed an uncooled high-electron mobility transistor (HEMT) low-noise amplifier. This system was installed primarily to determine the performance of the antenna in its 1989 configuration, but the experience will also aid in successful future calibration of the Deep Space Network (DSN) at this frequency

183 GHz water line variation: An energetic outburst in orion KL

Observations of the 3(13)-2(20) transition of water vapor in the direction of Ori MC1 in 1980 February show a 50% flux increase and an apparent additional red shift of approximately 2 km/s relative to the line observed in 1977 December. From a detailed examination of the amplitude and frequency calibration, it appears unlikely that the effect is due to systematic error. The increase is attributed to the appearance of a new component at a velocity of 12 km/s with respect to the local standard of rest. The new component also has broad wings. Increased emission from a region in the high-velocity core of Ori MC1 can be due either to additional far-IR radiation to pump the 1983 GHz transition or to a change in the physical conditions in the gas. Statistical equilibrium calculations using the large-velocity-gradient formalism were carried out to develop a model for the emission. The calculations support a model in which the gas in the region of enhanced emission is hotter than the dust. The temporal coincidence between the 183 GHZ increase and the 22 GH1 water maser outburst suggests a common, impulsive cause, which has heated the gas in a part of the HV source, enhancing the emission in both transitions

The 2-10 keV emission properties of PSR B1937+21

We present the results of a BeppoSAX observation of the fastest pulsar known: PSR B1937+21. The ~ 200 ks observation (78.5 (34) ks MECS (LECS) exposure times) allowed us to investigate with high statistical significance both the spectral properties and the pulse profile shape. The absorbed power law spectral model gave a photon index of ~ 1.7 and N_H ~ 2.3 x 10^22 cm^-2. These values explain both a) the ROSAT non-detection and b) the deviant estimate of a photon index of ~ 0.8 obtained by ASCA. The pulse profile appears, for the first time, clearly double peaked with the main component much stronger than the other. The statistical significance is 10 sigma (main peak) and 5 sigma (secondary peak). The 1.6-10 keV pulsed fraction is consistent with 100%; only in the 1.6-4 keV band there is a ~ 2 sigma indication for a DC component. The secondary peak is detected significantly only for energies above 3 / 4 keV. The unabsorbed (2-10 keV) flux is F_2-10 = 3.7 x 10^-13 erg cm^-2 s^-1, implying a luminosity of L_X = 4.6 x 10^31 Theta (d/3.6 kpc)^2 erg s^-1 and an X-ray efficiency of eta = 4 x 10^-5 Theta, where Theta is the solid angle spanned by the emission beam. These results are in agreement with those obtained by ASCA.Comment: 4 pages, 4 figures, 2 tables. To appear in the Proceedings of the 270. WE-Heraeus Seminar on Neutron Stars, Pulsars and Supernova Remnants, Jan. 21-25, 2002, Physikzentrum Bad Honnef, eds W. Becker, H. Lesch & J. Truemper. Proceedings are available as MPE-Report 27

BeppoSAX observation of PSR B1937+21

We present the results of a BeppoSAX observation of the fastest rotating pulsar known: PSR B1937+21. The ~200 ks observation (78.5 ks MECS/34 ks LECS on-source time) allowed us to investigate with high statistical significance both the spectral properties and the pulse profile shape. The pulse profile is clearly double peaked at energies > ~4 keV. Peak widths are compatible with the instrumental time resolution and the second pulse lags the main pulse 0.52 in phase, like is the case in the radio. In the 1.3-4 keV band we detect a ~45% DC component; conversely the 4-10 keV pulsed fraction is consistent with 100%. The on-pulse spectrum is fitted with an absorbed power-law of spectral index ~1.2, harder than that of the total flux which is ~1.9. The total unabsorbed (2-10 keV) flux is F_{2-10} = 4.1 10^-13 cgs, implying a luminosity of L_X = 5.0 10^31 \Theta (d/3.6 kpc)^2 erg s^-1 and a X-ray efficiency of \eta = 4.5 10^-5 \Theta, where \Theta is the solid angle spanned by the emission beam. These results are in agreement with those obtained by ASCA and a more recent Rossi-XTE observation. The hydrogen column density N_H ~2 10^22 cm^-2 is ~10 times higher than expected from the radio dispersion measure and average Galactic density of e-. Though it is compatible (within 2\sigma) with the Galactic (HI derived) value of ~1 10^22 cm^-2, inspection of dust extinction maps reveal that the pulsar falls in a highly absorbed region. In addition, 1.4 GHz radio map shows that the nearby (likely unrelated) HII source 4C21.53W is part of a circular emission region ~4' across.Comment: 8 pages, 5 figures; accepted for publication in A&

On the observability of bow shocks of Galactic runaway OB stars

Massive stars that have been ejected from their parent cluster and supersonically sailing away through the interstellar medium (ISM) are classified as exiled. They generate circumstellar bow shock nebulae that can be observed. We present two-dimensional, axisymmetric hydrodynamical simulations of a representative sample of stellar wind bow shocks from Galactic OB stars in an ambient medium of densities ranging from n_ISM=0.01 up to 10.0/cm3. Independently of their location in the Galaxy, we confirm that the infrared is the most appropriated waveband to search for bow shocks from massive stars. Their spectral energy distribution is the convenient tool to analyze them since their emission does not depend on the temporary effects which could affect unstable, thin-shelled bow shocks. Our numerical models of Galactic bow shocks generated by high-mass (~40 Mo) runaway stars yield H$\alpha$ fluxes which could be observed by facilities such as the SuperCOSMOS H-Alpha Survey. The brightest bow shock nebulae are produced in the denser regions of the ISM. We predict that bow shocks in the field observed at Ha by means of Rayleigh-sensitive facilities are formed around stars of initial mass larger than about 20 Mo. Our models of bow shocks from OB stars have the emission maximum in the wavelength range 3 <= lambda <= 50 micrometer which can be up to several orders of magnitude brighter than the runaway stars themselves, particularly for stars of initial mass larger than 20 Mo.Comment: 13 pages, 12 figures. Accepted to MNRAS (2016