A deep search for faint Chandra X-ray sources, radio sources, and optical counterparts in NGC 6752

We report the results of a deep search for faint Chandra X-ray sources, radio sources, and optical counterparts in the nearby, core-collapsed globular cluster, NGC 6752. We combined new and archival Chandra imaging to detect 51 X-ray sources (12 of which are new) within the 1.9 arcmin half-light radius. Three radio sources in deep ATCA 5 and 9 GHz radio images match with Chandra sources. We have searched for optical identifications for the expanded Chandra source list using deep Hubble Space Telescope photometry in B435, R625, H α, UV275, and U336. Among the entire sample of 51 Chandra sources, we identify 18 cataclysmic variables (CVs), 9 chromospherically active binaries (ABs), 3 red giants (RGs), 3 galaxies (GLXs), and 6 active galactic nuclei (AGNs). Three of the sources are associated with millisecond pulsars (MSPs). As in our previous study of NGC 6752, we find that the brightest CVs appear to be more centrally concentrated than the faintest CVs, although the effect is no longer statistically significant as a consequence of the inclusion in the faint group of two intermediate brightness CVs. This possible difference in the radial distributions of the bright and faint CV groups appears to indicate that mass segregation has separated them. We note that photometric incompleteness in the crowded central region of the cluster may also play a role. Both groups of CVs have an inferred mass above that of the main-sequence turnoff stars. We discuss the implications for the masses of the CV components

Extrapolating SMBH correlations down the mass scale: the case for IMBHs in globular clusters

Empirical evidence for both stellar mass black holes M_bh<10^2 M_sun) and supermassive black holes (SMBHs, M_bh>10^5 M_sun) is well established. Moreover, every galaxy with a bulge appears to host a SMBH, whose mass is correlated with the bulge mass, and even more strongly with the central stellar velocity dispersion sigma_c, the `M-sigma' relation. On the other hand, evidence for "intermediate-mass" black holes (IMBHs, with masses in the range 1^2 - 10^5 M_sun) is relatively sparse, with only a few mass measurements reported in globular clusters (GCs), dwarf galaxies and low-mass AGNs. We explore the question of whether globular clusters extend the M-sigma relationship for galaxies to lower black hole masses and find that available data for globular clusters are consistent with the extrapolation of this relationship. We use this extrapolated M-sigma relationship to predict the putative black hole masses of those globular clusters where existence of central IMBH was proposed. We discuss how globular clusters can be used as a constraint on theories making specific predictions for the low-mass end of the M-sigma relation.Comment: 14 pages, 3 figures, accepted for publication in Astrophysics and Space Science; fixed typos and a quote in Sec.

The Global Kinematics of the Globular Cluster M92

We report the determination of high-accuracy radial velocities for 306 members of the globular cluster M92 using the Hydra multiobject spectrograph on the WIYN telescope. We have concentrated on stars outside of the central region of the cluster, located up to 14.4&arcmin; from the cluster center. Candidate members were selected for spectroscopy based on a photometric metallicity index determined from three-band Washington photometry, also obtained with the WIYN telescope. The median error in the velocities is 0.35 km s-1. We find the heliocentric radial velocity of the cluster to be -121.2 ± 0.3 km s-1. We have used an improved Bayesian analysis to determine the velocity dispersion profile of M92. The most probable profile is a cored power law with a scale radius of 2&arcmin;, a velocity dispersion at 1&arcmin; of 6.3 km s-1, and an outer power law with a slope of -0.6. We have also reanalyzed the M15 radial velocities of Drukier et al. and find that a pure power law with a 1&arcmin; velocity dispersion of 8 km s-1 and a slope of -0.5 and the combination of a power law with a slope of -0.4 and a scale of 7.5 km s-1 inside 9&arcmin; and a dispersion of 4 km s-1 outside are equally likely. In both clusters there is evidence that the samples include escaping stars. We present results from a GRAPE-based N-body simulation of an isolated cluster that demonstrates this effect. We suggest additional tests to determine the relative importance of tidal heating and stellar ejection for establishing the velocity field in globular cluster halos

HST search of the region around IGR J18245-2452

We searched the region around the new INTEGRAL transient source IGR J18245-2452 (ATel #4925 #4927 #4929 #4934 #4959 #4960 #4961 #4964 #4981 #5003) using HST WFC3 and ACS/WFC imaging to characterize possible optical counterparts. The ATCA position of the transient source is R.A. 18h24m32.51s Dec. -24° 52' 07.9" (with a 90% confidence error of 0.5"; ATel #4981). Comparing the ATCA 8.5 GHz flux to the Swift/XRT measured 2-10 keV flux at the time (0.5-1E-10 erg/cm^2/s), we find that the radio/X-ray flux ratio is similar to that seen in e.g

The radius of the quiescent neutron star in the globular cluster M13

International audienceX-ray spectra of quiescent low-mass X-ray binaries containing neutron stars can be fit with atmosphere models to constrain the mass and the radius. Mass-radius constraints can be used to place limits on the equation of state of dense matter. We perform fits to the X-ray spectrum of a quiescent neutron star in the globular cluster M13, utilizing data from ROSAT, Chandra, and XMM–Newton, and constrain the mass–radius relation. Assuming an atmosphere composed of hydrogen and a 1.4 M_⊙ neutron star, we find the radius to be $$R_{\rm NS}=12.2^{+1.5}_{-1.1}$$ km, a significant improvement in precision over previous measurements. Incorporating an uncertainty on the distance to M13 relaxes the radius constraints slightly and we find $$R_{\rm NS}=12.3^{+1.9}_{-1.7}$$ km (for a 1.4M_⊙ neutron star with a hydrogen atmosphere), which is still an improvement in precision over previous measurements, some of which do not consider distance uncertainty. We also discuss how the composition of the atmosphere affects the derived radius, finding that a helium atmosphere implies a significantly larger radius