SEVERAL globular clusters have low-luminosity X-ray sources at their cores1–3, of which X0021.8–7221 in 47 Tucanae (NGC104) is one of the brightest. The nature of these sources is a mystery, and attempts to detect optical counterparts have been frustrated by the overcrowding of stars in cluster cores. The resolution of ground-based observations is insufficient to pick out the faint blue objects that are likely to be the optical counterparts to the X-ray sources, but the Faint Object Camera (FOC) on the Hubble Space Telescope has proved effective in identifying blue objects in dense clusters such as 47 Tuc4. Thus encouraged to use the FOC to search for the optical counterpart to X0021.8–7221, we have discovered a faint, variable and very blue object located in the error circle5 for X0021.8–7221 from the Einstein satellite High-Resolution Imager. The object has the spectral characteristics of a cataclysmic variable, and the high inferred X-ray to optical brightness ratio suggests that it is a magnetic cataclysmic variable, one of the candidates that has been suggested as the X-ray source in globular clusters.

Possible cataclysmic variable in the core of the globular cluster 47 Tucanae

The 0.1-2.4 keV bandpass of the ROSAT Position Sensitive Proportional Counter (PSPC) offers an opportunity to study the very soft X-ray continuum of bright low-mass X-ray binaries (LMXBs). In 46 pointed observations, 23 LMXBs were observed with count rates between 0.4 and 165.4 counts s-1. The survey identified a total of 29 different luminosity levels, which are compared with observations and identified spectral states from other missions. The atoll source 4U 1705-44 was observed near Eddington luminosities in an unusually high intensity state. Spectral analysis provided a measure of the interstellar column density for all 49 observations. The sensitivity of spectral fits depends strongly on column density. Fits to highly absorbed spectra are merely insensitive toward any particular spectral model. Sources with column densities well below 1022 cm-2 are best fitted by power laws, while the blackbody model gives clearly worse fits to the data. Most single-component fits from sources with low column densities, however, are not acceptable at all. The inclusion of a blackbody component in eight sources can improve the fits significantly. The obtained emission radii of less than 5 km suggest emission from the neutron star surface. In 10 sources acceptable fits can only be achieved by including soft-line components. With a spectral resolution of the PSPC of 320-450 eV, between 0.6 and 1.2 keV unresolved broad-line features were detected around 0.65, 0.85, and 1.0 keV. The line fluxes range within 10-11 and 10-12 ergs cm-2 s-1, with equivalent widths between 24 and 210 eV. In LMC X-2, 2S 0918-549, and 4U 1254-690, line emission is indicated for the first time. The soft emission observed in 4U 0614+091 compares with recent ASCA results, with a new feature indicated at 1.31 keV. The deduced line fluxes in 4U 1820-30 and Cyg X-2 showed variability of a factor of 2 within timescales of 1-2 days. Average fluxes of line components in 4U 1820-30 varied by the same factor over a time span of 3 yr.

https://iopscience.iop.org/article/10.1086/306666/pdf

ROSAT Energy Spectra of Low-Mass X-Ray Binaries

We have performed a detailed study of the broadband spectra of the luminous (>10 36 erg s 1 ) globular cluster X{ray sources using BeppoSAX. With the exception of X 2127+119, located in NGC 7078, all the other spectra are well represented by a two component model consisting of a disk-blackbody and Comptonized emission. The measured low-energy absorptions are in good agreement with those predicted from optical measurements of the host globular clusters. This implies that there is little intrinsic X{ray absorption within the binaries themselves, and that the above spectral model provides a good representation of the low-energy continua. The sources can be divided into two groups. In the rst group, composed of 3 ultra-compact (orbital period <1 hr) sources, the disk-blackbody temperatures and inner-radii appear physically realistic and the Comptonization seed photons temperatures and radii of the emission areas are consistent with the disk temperatures and inner radii. For all the other sources, the disk-blackbody parameters appear not to be physically realistic and the Comptonization parameters are unrelated to those of the disk-blackbody emission. If this is a spectral signature of ultra-compact binaries, this implies that no other ultra-compact binaries are present among those studied here. It is unclear why this dierence between the two types of binaries should exist. One possibility may be related to the mass ratio, which is similar in the ultracompact systems and binaries containing black holes. In the latter systems the soft components are also well-t with disk-blackbody models, which appear to have physically realistic parameters.

/pdf/bepposax-study-of-the-broad-band-properties-of-luminous-2f75obx2xm.pdf

BeppoSAX study of the broad-band properties of luminous globular cluster X{ray sources

If millisecond pulsars (MSPs) are responsible for the excess gamma-ray emission observed from the region surrounding the Galactic Center, the same region should also contain a large population of low-mass X-ray binaries (LMXBs). In this study, we compile and utilize a sizable catalog of LMXBs observed in the the Milky Way's globular cluster system and in the Inner Galaxy, as well as the gamma-ray emission observed from globular clusters, to estimate the flux of gamma rays predicted from MSPs in the Inner Galaxy. From this comparison, we conclude that only up to ~ 4-23% of the observed gamma-ray excess is likely to originate from MSPs. This result is consistent with, and more robust than, previous estimates which utilized smaller samples of both globular clusters and LMXBs. If MSPs had been responsible for the entirety of the observed excess, INTEGRAL should have detected ~ 103 LMXBs from within a 10o radius around the Galactic Center, whereas only 42 LMXBs (and 46 additional LMXB candidates) have been observed.

http://iopscience.iop.org/article/10.1088/1475-7516/2017/05/056/pdf

Low mass X-ray binaries in the Inner Galaxy: implications for millisecond pulsars and the GeV excess

We have detected three new X-ray point sources, in addition to the known low-mass X-ray binary (LMXB) X1832-330, in the globular cluster NGC 6652 with a Chandra 1.6 ks HRC-I exposure. Star 49 (MV ~ 4.7), suggested by Deutsch and coworkers as the optical candidate for the LMXB, is identified (<03) not with the LMXB, but with another, newly detected source (B). Using archival Hubble Space Telescope (HST) images, we identify (<03) the LMXB (A) and one of the remaining new sources (C) with blue variable optical counterparts at MV ~ 3.7 and 5.3, respectively. The other new source (D) remains unidentified in the crowded cluster core. In the 0.5-2.5 keV range, assuming a 5 keV thermal bremsstrahlung spectrum and NH = 5.5 × 1020, source A has intrinsic luminosity LX ~ 5.3 × 1035 ergs s-1. Assuming a 1 keV thermal bremsstrahlung spectrum, B has LX ~ 4.1 × 1033 ergs s-1, while C and D have LX ~ 8 × 1032 ergs s-1. Source B is probably a quiescent LMXB, while source C may be either a luminous cataclysmic variable or a quiescent LMXB.

/pdf/identification-of-the-low-mass-x-ray-binary-and-faint-x-ray-3qqptu5mfm.pdf

K. S. Wood

Papers

The nature of the low-luminosity globular cluster X-ray sources.