- Published on Tuesday, 07 August 2018 08:00
In section 7. Stellar structure and evolution
The evolving jet spectrum of the neutron star X-ray binary Aql X-1 in transitional states during its 2016 outburst
Aql X-1 is the most prolific neutron star X-ray binary transient. Despite there being more than 30 outbursts observed so far, the simultaneous coverage in X-rays and radio band is poor. The 2016 outburst from Aql X-1 was observed seven times in the radio (ATCA) and mm (ALMA) bands, together with near-daily observations at X-ray and optical frequencies by Diaz Trigo et al. (2017). For the first time it was possible to see the rise to the outburst peak and the decay after at all frequencies. Radio and mm observations are consistent with the presence of a jet. The synchrotron emission peak shifts from ~100 GHz to ~5 GHz during the rising phase and back to ~30-100 GHz during the decay. This is the first time that these frequency shifts have been observed in a transient system containing a neutron star. A similar behaviour has been revealed in several black hole transients (even if for these sources a correlation of the radio peak with the X-ray spectrum holds, which is not detected in Aql X-1). This should indicate that physical processes at play for the jet formation depend only mildly on black hole mass or spin, or presence of a hard surface or the neutron star magnetic field, and should rely instead on the accretion disc and its corona properties.
- Published on Friday, 20 July 2018 08:00
In section 5. Galactic structure
The vertical force in the solar neighbourhood using red clump stars in TGAS and RAVE
Many estimations of the volumic mass density of the Milky Way in the solar neighborhood have already been made from a detailed census of the vertical dynamics of the stars, in order to derive the local dark matter density. Now with the first GAIA data release (TGAS), positions of stars and their proper motions have increased by almost a factor of two in precision, and most importantly, the sample of stars involved is nearly 20 times larger. It is, then, timely to revisit these estimations. The authors have studied the kinematics of red clump stars in the solar neighborhood by combining data from TGAS and RAVE. They applied the axisymmetric Jeans equations to subsets of stars representing the thin and thick disks to determine the local distribution of mass near the disk of our Galaxy. They determine two values of the z-force, Kz, for the thin and thick disks at 1.5 kpc away from the Galactic plane, assuming scale heights of 0.28 kpc and 1.12 kpc respectively. These measurements can be translated into a local dark matter density of 0.018 +/- 0.002 M_sol/pc^3, which is in very good agreement with previous estimations. However, the statistical error above is an underestimation of the actual uncertainty, which is dominated by systematic errors due to various assumptions (equilibrium or breathing of the stars, the nature of the various components, the interstellar mass, etc). A slight error of only 10% on the scale height of the thin disk leads to a 30% change in the value of the dark matter density. Future Gaia data releases will characterize the different stellar components in the Galaxy better and more precisely, and thus allow more precise determinations of the density.
- Published on Friday, 13 July 2018 08:00
In section 1. Letters to the Editor
Time-dependent molecular emission in IRC+10216
This paper presents the first results from an almost three-year long monitoring of the molecular emission from the Mira variable star IRC+10216. Several rotational lines from species such as HC3N, HC5N, CCH, C4H, C5H, and CN present evidence of periodical variability, with periods consistent with the stellar IR variability. The different phase lags in the light curves suggest that the variability of the molecular lines results from radiative transfer and pumping effects, and not from chemical changes in the envelope composition.
- Published on Thursday, 28 June 2018 08:00
In section 14. Catalogs and data
The stellar content of the XMM-Newton slew survey
Wide-field optical telescopes survey the sky every night. This is especially difficult in the X-ray band, for which we rely only on the ROSAT all-sky survey performed in 1990-1991. In recent years the time spent by the XMM-Newton satellite to slew from one target to the other has been scientifically exploited. This has given us the XMM-Newton Slew Survey (XMMSL), which covers 84% of the sky and counting. In this work Freund and collaborators have used the XMMSL source catalog and correlated it with the Gaia optical catalog to pinpoint X-ray emitting stars. They identified 6815 stars of all spectral types with a majority of late-type stars, providing the largest census so far.
- Published on Friday, 22 June 2018 08:00
In section 6. Interstellar and circumstellar matter
Interstellar bromine abundance is consistent with cometary ices from Rosetta
This paper presents a unique, intriguing study of the abundance of a rare halogen, bromine, in the gas phase by the Rosetta mission in comet 67P/Churyumov-Gerasimenko. Obtaining the abundance of rare heavy elements has been a challenge for interstellar medium studies for decades and untill now, it has not been possible to measure this element in the diffuse gas phase. For comet 67P, the detection of HBr and the measured elemental ratio Br/O = (1 − 7) × 10^−6 is consistent with the ratios found for its cousin Cl (for which the Br/Cl ratio is consistent with terrestrial values), and consistent with all of the Br being sequestered in cometary ice mantles. The derived abundances are compared with upper limits for HBr and HBr^+ from infrared spectra of several star forming regions and employ chemical network modeling to understand the molecular abundances. This study represents the first direct measurement of the bromine abundance in a cosmic environment other than meteorites.
- Published on Tuesday, 12 June 2018 08:00
In section 4. Extragalactic astronomy
Constraints on submicrojansky radio number counts based on evolving VLA-COSMOS luminosity functions
Radio emission in galaxies at frequencies below 30 GHz is mostly synchrotron radiation from cosmic electrons gyrating in magnetic fields. The electrons are accelerated by supernovae in star forming (SF) galaxies, and by an accreting supermassive black hole (SMBH) in active galactic nuclei (AGN). Comparison with other wavelengths, such as optical, IR, or X-ray, is necessary to disentangle the two possibilities. This paper presents the results of the Large Program COSMOS-3GHz with the VLA. The total radio luminosity function (LF) since z~5.7 has been derived for the radio selected sample of 7826 galaxies with robust optical/near-infrared (NIR) counterparts. The populations of SF galaxies and AGN are separated on the basis of presence or absence of a radio excess with respect to the star-formation rates (SFRs) derived from the infrared (IR) emission. The redshift dependent pure luminosity evolution (PLE) model fitted to the data with an MCMC algorithm is in very good agreement with the previously published VLA-COSMOS LFs. Different models of evolving LFs are able to reproduce the observed radio sky brightness, despite relying on extrapolations toward the faint end. The results imply that no new radio emitting galaxy population is present below 1 microJy. Selecting galaxies with radio flux densities between 0.1 and 10 microJy will yield a starforming galaxy in 90-95% of the cases with a large percentage of these galaxies existing around a redshift of z ~ 2, thus providing useful constraints for planned surveys with the Square Kilometer Array (SKA) and its precursors.