Optical spectroscopy of the high-mass gamma-ray binary 1FGL J1018.6-5856: A probable neutron star primary

We present medium-resolution optical spectroscopy with the SOAR telescope of the O star secondary of the high-mass gamma-ray binary 1FGL J1018.6-5856 to help determine whether the primary is a neutron star or black hole. We find that the secondary has a low radial velocity semi-amplitude of 11-12 km/s, with consistent values obtained for H and He absorption lines. This low value strongly favors a neutron star primary: while a black hole cannot be excluded if the system is close to face on, such inclinations are disallowed by the observed rotation of the secondary. We also find the high-energy (X-ray and gamma-ray) flux maxima occur when the star is behind the compact object along our line of sight, inconsistent with a simple model of anisotropic inverse Compton scattering for the gamma-ray photons.Comment: ApJL in pres

Star Clusters in M31: V. Internal Dynamical Trends: Some Troublesome, Some Reassuring

We present internal velocity dispersions and precise radial velocities for 200 globular clusters (GCs) in M31 that are derived using new high-resolution spectra from MMT/Hectochelle. Of these, 163 also have King model structural parameters that allow us to estimate their mass-to-light ratios. This is, by far, the largest such dataset available for any galaxy, including the Milky Way. These data strongly confirm earlier suggestions that the optical and near-infrared mass-to-light ratios of M31 GCs decline with increasing metallicity. This behavior is the opposite of that predicted by stellar population models for a standard initial mass function. We show that this phenomenon does not appear to be caused by standard dynamical evolution. A shallower mass function for metal-rich GCs (with dN/dM ~ M^-0.8 to M^-1.3 below one solar mass) can explain the bulk of extant observations. We also observe a consistent, monotonic correlation between mass-to-light ratio and cluster mass. This correlation, in contrast to the correlation with metallicity, is well-explained by the accepted model of dynamical evolution of GCs through mass segregation and the preferential loss of low-mass stars, and these data are among the best available to constrain this process.Comment: AJ in press. 11 pages, 7 figures (not including tables) in emulate forma

Limits on thermal variations in a dozen quiescent neutron stars over a decade

In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the origin of the thermal X-ray component may be either release of heat from the core of the neutron star, or continuing low-level accretion. In general, heat from the core should be stable on timescales $<10^4$ years, while continuing accretion may produce variations on a range of timescales. While some quiescent neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal components on a range of timescales, several others, particularly those in globular clusters with no detectable nonthermal hard X-rays (fit with a powerlaw), have shown no measurable variations. Here, we constrain the spectral variations of 12 low mass X-ray binaries in 3 globular clusters over $\sim10$ years. We find no evidence of variations in 10 cases, with limits on temperature variations below 11% for the 7 qLMXBs without powerlaw components, and limits on variations below 20% for 3 other qLMXBs that do show non-thermal emission. However, in 2 qLMXBs showing powerlaw components in their spectra (NGC 6440 CX 1 & Terzan 5 CX 12) we find marginal evidence for a 10% decline in temperature, suggesting the presence of continuing low-level accretion. This work adds to the evidence that the thermal X-ray component in quiescent neutron stars without powerlaw components can be explained by heat deposited in the core during outbursts. Finally, we also investigate the correlation between hydrogen column density (N$_H$) and optical extinction (A$_V$) using our sample and current models of interstellar X-ray absorption, finding $N_H ({\rm cm}^{-2}) = (2.81\pm0.13)\times10^{21} A_V$.Comment: 16 pages, 5 figures, MNRAS, in pres

Further Definition of the Mass-Metallicity Relation in Globular Cluster Systems Around Brightest Cluster Galaxies

We combine the globular cluster data for fifteen Brightest Cluster Galaxies and use this material to trace the mass-metallicity relations (MMR) in their globular cluster systems (GCSs). This work extends previous studies which correlate the properties of the MMR with those of the host galaxy. Our combined data sets show a mean trend for the metal-poor (MP) subpopulation which corresponds to a scaling of heavy-element abundance with cluster mass Z ~ M^(0.30+/-0.05). No trend is seen for the metal-rich (MR) subpopulation which has a scaling relation that is consistent with zero. We also find that the scaling exponent is independent of the GCS specific frequency and host galaxy luminosity, except perhaps for dwarf galaxies. We present new photometry in (g',i') obtained with Gemini/GMOS for the globular cluster populations around the southern giant ellipticals NGC 5193 and IC 4329. Both galaxies have rich cluster populations which show up as normal, bimodal sequences in the colour-magnitude diagram. We test the observed MMRs and argue that they are statistically real, and not an artifact caused by the method we used. We also argue against asymmetric contamination causing the observed MMR as our mean results are no different from other contamination-free studies. Finally, we compare our method to the standard bimodal fitting method (KMM or RMIX) and find our results are consistent. Interpretation of these results is consistent with recent models for globular cluster formation in which the MMR is determined by GC self-enrichment during their brief formation period.Comment: 35 pages, 20 figures. Accepted by Astronomical Journal. Complete preprint including high resolution figures available at http://www.physics.mcmaster.ca/~cockcroft/MMRpape

Thickness-dependent spontaneous dewetting morphology of ultrathin Ag films

We show here that the morphological pathway of spontaneous dewetting of ultrathin Ag films on SiO2 under nanosecond laser melting is found to be film thickness dependent. For films with thickness h between 2 <= h <= 9.5 nm, the morphology during the intermediate stages of dewetting consisted of bicontinuous structures. For films 11.5 <= h <= 20 nm, the intermediate stages consisted of regularly-sized holes. Measurement of the characteristic length scales for different stages of dewetting as a function of film thickness showed a systematic increase, which is consistent with the spinodal dewetting instability over the entire thickness range investigated. This change in morphology with thickness is consistent with observations made previously for polymer films [A. Sharma et al, Phys. Rev. Lett., v81, pp3463 (1998); R. Seemann et al, J. Phys. Cond. Matt., v13, pp4925, (2001)]. Based on the behavior of free energy curvature that incorporates intermolecular forces, we have estimated the morphological transition thickness for the intermolecular forces for Ag on SiO2 . The theory predictions agree well with observations for Ag. These results show that it is possible to form a variety of complex Ag nanomorphologies in a consistent manner, which could be useful in optical applications of Ag surfaces, such as in surface enhanced Raman sensing.Comment: 20 pages, 5 figure

Antlia B: A faint dwarf galaxy member of the NGC 3109 association

We report the discovery of Antlia B, a faint dwarf galaxy at a projected distance of $\sim$72 kpc from NGC 3109 ($M_{V}$$\sim$$-$15 mag), the primary galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The tip of the red giant branch distance to Antlia B is $D$=1.29$\pm$0.10 Mpc, which is consistent with the distance to NGC 3109. A qualitative analysis indicates the new dwarf's stellar population has both an old, metal poor red giant branch ($\gtrsim$10 Gyr, [Fe/H]$\sim$$-$2), and a younger blue population with an age of $\sim$200-400 Myr, analogous to the original Antlia dwarf, another likely satellite of NGC 3109. Antlia B has \ion{H}{1} gas at a velocity of $v_{helio,HI}$=376 km s$^{-1}$, confirming the association with NGC 3109 ($v_{helio}$=403 km s$^{-1}$). The HI gas mass (M$_{HI}$=2.8$\pm$0.2$\times$10$^{5}$ M$_{\odot}$), stellar luminosity ($M_{V}$=$-$9.7$\pm$0.6 mag) and half light radius ($r_{h}$=273$\pm$29 pc) are all consistent with the properties of dwarf irregular and dwarf spheroidal galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy. The discovery of Antlia B is the initial result from a Dark Energy Camera survey for halo substructure and faint dwarf companions to NGC 3109 with the goal of comparing observed substructure with expectations from the $\Lambda$+Cold Dark Matter model in the sub-Milky Way regime.Comment: 7 pages, 3 figures. Submitted to ApJ