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Register a new userChandra Observation of Quiescent Low-Mass X-ray Binaries in the Globular Cluster NGC 6304
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This paper presents the analysis of candidate quiescent low mass xray binarie (qLMXBs) observed during a short Chandra/ACIS observation of the globular cluster (GC) NGC 6304. Two out of the three candidate qLMXBs of this cluster, XMMU 171433-292747 and XMMU 171421-292917, lie within the field of view. This permits comparison with the discovery observation of these sources. The one in the GC core -- XMMU 171433-292747 -- is spatially resolved into two separate X-ray sources, one of which is consistent with a pure H-atmosphere qLMXB, and the other is an X-ray power-law spectrum source. These two spectral components separately account for those observed from XMMU 171433-292747 in its discovery observation. We find that the observed flux and spectral parameters of the H-atmosphere spectral components are consistent with the previous observation, as expected from a qLMXB powered by deep crustal heating. XMMU 171421-292917 also has neutron star atmosphere spectral parameters consistent with those in the XMM-Newton observation and the observed flux has decreased by a factor 0.54^{+0.30}_{-0.24}.
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We report the search for low-mass X-ray binaries in quiescence (qLMXBs) in the globular cluster NGC 6304 using XMM observations. We present the spectral analysis leading to the identification of three candidate qLMXBs within the field of this globular cluster (GC), each consistent with the X-ray spectral properties of previously identified qLMXBs in the field and in other globular clusters -- specifically, with a hydrogen atmosphere neutron star with radius between 5--20km. One (source 4, with R=11.7^{+8.3}_{-0.4} (D/5.97 kpc) km and kT_eff=117^{+59}_{-44} eV) is located within one core radius (r_c) of the centre of NGC 6304. This candidate also presents a spectral power-law component contributing 49 per cent of the 0.5-10 keV flux. A second one (source 9 with R=15.3^{+11.2}_{-6.5} (D/5.97 kpc) km and kT_eff=100^{+24}_{-19} eV) is found well outside the optical core (at 32 r_c) but still within the tidal radius. From spatial coincidence, we identify a bright 2MASS infrared counterpart which, at the distance of NGC 6304, seems to be a post-asymptotic giant branch star. The third qLMXB (source 5 with R=23^{+38}_{-14} (D/5.97 kpc) km and kT_eff=70^{+28}_{-20} eV) is a low signal-to-noise candidate for which we also identify from spatial coincidence a bright 2MASS infrared counterpart, with 99.916 per cent confidence. Three qLMXBs from this GC is marginally consistent with that expected from the encounter rate of NGC 6304. We also report a low signal-to-noise source with an unusually hard photon index (alpha=-2.0^{+1.2}_{-2.2}). Finally, we present an updated catalogue of the X-ray sources lying in the field of NGC 6304, and compare this with the previous catalogue compiled from ROSAT observations.
This paper reports the search for quiescent low-mass X-ray binaries (qLMXBs) in the globular cluster (GC) NGC 6553 using an XMM-Newton observation designed specifically for that purpose. We spectrally identify one candidate qLMXB in the core of the cluster, based on the consistency of the spectrum with a neutron star H-atmosphere model at the distance of NGC 6553. Specifically, the best-fit radius found using the three XMM European Photon Imaging Camera spectra is R_NS=6.3(+2.3)(-0.8) km (for M_NS=1.4 Msun) and the best-fit temperature is kTeff=136 (+21)(-34) eV. Both physical parameters are in accordance with typical values of previously identified qLMXBs in GC and in the field, i.e., R_NS~5-20 km and kTeff~50-150 eV. A power-law (PL) component with a photon index Gamma=2.1(+0.5)(-0.8) is also required for the spectral fit and contributes to ~33% of the total flux of the X-ray source. A detailed analysis supports the hypothesis that the PL component originates from nearby sources in the core, unresolved with XMM. The analysis of an archived Chandra observation provides marginal additional support to the stated hypothesis. Finally, a catalog of all the sources detected within the XMM field of view is presented here.
We present a wide field study of the Globular Clusters/Low Mass X-ray Binaries connection in the cD elliptical NGC1399, combining HST/ACS and Chandra high resolution data. We find evidence that LMXB formation likelihood is influenced by GCs structural parameters, in addition to the well known effects of mass and metallicity, independently from galactocentric distance.
We present a recent Chandra observation of the quiescent low-mass X-ray binary containing a neutron star, located in the globular cluster M30. We fit the thermal emission from the neutron star to extract its mass and radius. We find no evidence of flux variability between the two observations taken in 2001 and 2017, nor between individual 2017 observations, so we analyse them together to increase the signal to noise. We perform simultaneous spectral fits using standard light-element composition atmosphere models (hydrogen or helium), including absorption by the interstellar medium, correction for pile-up of X-ray photons on the detector, and a power-law for count excesses at high photon energy. Using a Markov-chain Monte Carlo approach, we extract mass and radius credible intervals for both chemical compositions of the atmosphere: $R_{textrm{NS}}=7.94^{+0.76}_{-1.21}$ km and $M_{textrm{NS}}<1.19$ M$_{odot}$ assuming pure hydrogen, and $R_{textrm{NS}}=10.50^{+2.88}_{-2.03}$ km and $M_{textrm{NS}}<1.78$ M$_{odot}$ for helium, where the uncertainties represent the 90% credible regions. For H, the small radius is difficult to reconcile with most current nuclear physics models (especially for nucleonic equations of state) and with other measurements of neutron star radii, with recent preferred values generally in the 11-14 km range. Whereas for He, the measured radius is consistent with this range. We discuss possible sources of systematic uncertainty that may result in an underestimation of the radius, identifying the presence of surface temperature inhomogeneities as the most relevant bias. According to this, we conclude that either the atmosphere is composed of He, or it is a H atmosphere with a significant contribution of hot spots to the observed radiation.
Galactic and extragalactic studies have shown that metal-rich globular clusters (GCs) are approximately three times more likely to host bright low-mass X-ray binaries (LMXBs) than metal-poor GCs. There is no satisfactory explanation for this metallicity effect. We tested the hypothesis that the number density of red giant branch (RGB) stars is larger in metal-rich GCs, and thus potentially the cause of the metallicity effect. Using Hubble Space Telescope photometry for 109 unique Milky Way GCs, we investigated whether RGB star density was correlated with GC metallicity. Isochrone fitting was used to calculate the number of RGB stars, which were normalized by the GC mass and fraction of observed GC luminosity, and determined density using the volume at the half-light radius $r_{h}$. The RGB star number density was weakly correlated with metallicity [Fe/H], giving Spearman and Kendall Rank test $p$-values of 0.00016 and 0.00021 and coefficients $r_{s} = 0.35$ and $tau = 0.24$ respectively. This correlation may be biased by a possible dependence of $r_{h}$ on [Fe/H], although studies have shown that $r_{h}$ is correlated with Galactocentric distance and independent of [Fe/H]. The dynamical origin of the $r_{h}$-metallicity correlation (tidal stripping) suggests that metal-rich GCs may have had more active dynamical histories, which would promote LMXB formation. No correlation between the RGB star number density and metallicity was found when using only the GCs that hosted quiescent LMXBs. A complete census of quiescent LMXBs in our Galaxy is needed to further probe the metallicity effect, which will be possible with the upcoming launch of eROSITA.
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