Welcome to the Swift-XRT Globular Cluster Monitor

Four Galactic globular clusters observed with Swift twice a month.

(Click on the Chandra X-ray image of a cluster below to move to the corresponding data product page)
47Tuc  (Heinke et al. 2005)                  M28  (Becker et al. 2003)

M62  (Pooley et al. 2003)                  Ter5  (Heinke et al. 2006)

Globular clusters (GCs) are efficient factories of X-ray binaries and millisecond pulsars (MSPs). High angular resolution Chandra observations have uncovered numerous faint X-ray sources lurking in the cores of Galactic GCs. Many of these are quiescent neutron stars in low-mass X-ray binaries (NS LMXBs) and binary MSPs, and definitive proof of the connection between these two types of neutron stars has been recently found.

We initiated in November 2014 a Swift-XRT monitoring of selected GCs in order to: i) find new low-luminosity transient LMXBs, ii) detect short and faint outbursts from known transient NS LMXBs and iii) monitor the X-ray activity of the known MSP population to search for more MSP-LMXB transition pulsars. Even if the angular resolution of Swift-XRT cannot resolve the X-ray sources in the core of GCs, with typical luminosities Lx~1E29-1E32 erg/s, their integrated emission can reveal subtle changes in the unexplored Lx = 1E33-1E34 erg/s regime.

We have selected the following GCs, which have large numbers of known MSPs, no persistent bright X-ray sources and are relatively nearby and not strongly absorbed (comparing Harris' and Freire's catalogs). The data products are made public here after the observations take place.

47Tuc: Contains 23 known MSPs. D=4.5 kpc; NH=0.01E22 cm2. REFS: Heinke et al. (2005ApJ...625..796H).

Ter5: Contains 34 known MSPs and three known neutron star transients. D=6.9 kpc; NH=1.2E22 cm2. REFS: Heinke et al. (2006ApJ...651.1098H).

M28: Contains 12 known MSPs and two known neutron star transients. D=5.5 kpc; NH=0.24E22 cm2. REFS: Becker et al. (2003ApJ...594..798B).

M62(since March 2016): Contains 6 known MSPs and one binary BH candidate. D=6.9kpc; NH=0.28E22 cm2. REFS: Pooley et al. (2003ApJ...591L.131P), Chomiuk et al. (2013ApJ...777...69C).


If these data have been useful to you please include the following acknowledgement: "This research has made use of the Swift-XRT Globular Cluster Monitoring page (http://www.iac.es/proyecto/SwiftGloClu) maintained by M. Linares & J. Chenevez."

We thank the Swift team for having made these observations possible

We acknowledge support from the COST Action MP1304 "NewCompStar"  


July 2015 (automatic updates; last upgrade April 2016)