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Table 1

Known ultracompact X-ray binaries and candidates.

Name Porb  (min) d (kpc) Location Composition Accretor References

4U 1820–30 11.42 7.6 ± 0.4 NGC 6624 He (burst-model) burst 1-p, 2-d, 3, 4, 5-c, 6-a
4U 0513–40 17 12.2 NGC 1851 He burst 7-pc, 8-a
2S 0918–549 17.4 4.8 ± 0.6 Disk C, O? (opt), He (burst) burst 9-p, 10, 11-d, 12, 13-c, 14-a
4U 1543–624 18.2  ~7? Disk C, O? (opt), O (X) 15-pd, 12, 16-c
4U 1850–087 20.6 8.2 ± 0.6 NGC 6712 Ne-excess burst 17-p, 2-d, 18-c, 19, 20-a
M 15 X-2 22.58 10.4 M 15 He, C (UV) burst 21-pc, 22-d, 23, 24-a
XTE J1807–294 40.07 8.3 ± 1.5? Bulge? msp 25-p, 26-a
4U 1626–67 41.4  ~8 C, O (opt), O (UV, X) pulsar 27-pa, 28-d, 29, 30, 31, 32-c
XTE J1751–305 42.42 8.3 ± 1.5? Bulge? msp 33-pa
XTE J0929–314 43.58 10 ± 5 He, C, N msp 34-pca, 35, 36-d, 29-c
4U 1916–05 49.48 8.9 ± 1.3 He, N (opt) burst 37, 38-p, 39-d, 29-c, 40, 41-a
SWIFT J1756.9–2508 54.70 8.3 ± 1.5? Bulge? He (model) msp 42-pca
NGC 6440 X-2 57.3 8.5 ± 0.4 NGC 6440 msp 43-pa, 44-d

4U 1728–34 (GX 354–0) 10.77? 5.3 ± 0.8 burst, msp 45, 46-p, 11-d, 47, 48, 49-a
NGC 6652 B 43.6? 10.0 NGC 6652 burst 50-p, 22-d, 51-a
4U 0614+091 51.3? 3.2 C, O (opt), O (X) burst, msp 52, 53, 54-p, 55-d, 12, 30, 56-c, 57, 58, 59, 60-a

1A 1246–588 4.3 61, 62-d
4U 1812–12 61
4U 1822–000 63
4U 1905+000 8 64
ω Cen qLMXB NGC 5139 65

1RXS J170854.4-321857 66
SAX J1712.6-3739 63
1RXS J171824.2-402934 66
4U 1722-30 Terzan 2 63
1RXS J172525.2-325717 63
SLX 1735-269 63
SLX 1737-282 63
SLX 1744-299 63

Notes. The four groups separated by horizontal lines are confirmed UCXBs, UCXB candidates with tentative orbital period, UCXB candidates with a low ratio of optical to X-ray luminosity and UCXB candidates with persistent behavior at low luminosity, respectively. The columns list source name, orbital period Porb, distance d, location within the Galaxy, chemical composition and indication of accretor type (“msp” = millisecond pulsar, “burst” = Type 1 X-ray burst). Sources with coordinates very near the Galactic bulge are assumed to be located in the bulge. Distances are taken from Liu et al. (2007) unless indicated otherwise. In the references column, “p” refers to orbital period, “c” to composition, “d” to distance and “a” to accretor signal.

References. 1) Stella et al. (1987); 2) Kuulkers et al. (2003); 3) Bildsten (1995); 4) Podsiadlowski et al. (2002); 5) Cumming (2003); 6) Grindlay et al. (1976); 7) Zurek et al. (2009); 8) Forman & Jones (1976); 9) Zhong & Wang (2011); 10) Cornelisse et al. (2002); 11) Jonker & Nelemans (2004); 12) Nelemans et al. (2004); 13) in’t Zand et al. (2005b); 14) Jonker et al. (2001); 15) Wang & Chakrabarty (2004); 16) Madej & Jonker (2011); 17) Homer et al. (1996); 18) Juett et al. (2001); 19) Swank et al. (1976); 20) Hoffman et al. (1980); 21) Dieball et al. (2005); 22) Harris (1996, 2010 edition); 23) Dotani et al. (1990); 24) van Paradijs et al. (1990); 25) Markwardt et al. (2003a); 26) Markwardt et al. (2003b); 27) Middleditch et al. (1981); 28) Chakrabarty (1998); 29) Nelemans et al. (2006); 30) Werner et al. (2006); 31) Schulz et al. (2001); 32) Homer et al. (2002); 33) Markwardt et al. (2002); 34) Galloway et al. (2002); 35) Wijnands et al. (2005); 36) Liu et al. (2007); 37) White & Swank (1982); 38) Walter et al. (1982); 39) Galloway et al. (2008); 40) Becker et al. (1977); 41) Lewin et al. (1977); 42) Krimm et al. (2007); 43) Altamirano et al. (2010); 44) Ortolani et al. (1994); 45) Galloway et al. (2010); 46) Wilson-Hodge et al. (2012); 47) Lewin et al. (1976); 48) Hoffman et al. (1976); 49) Strohmayer et al. (1996); 50) Deutsch et al. (2000); 51) in’t Zand et al. (1998); 52) Shahbaz et al. (2008); 53) Hakala et al. (2011); 54) Zhang et al. (2012); 55) Kuulkers et al. (2010); 56) Madej et al. (2010); 57) Swank et al. (1978); 58) Brandt et al. (1992); 59) Ford et al. (1997); 60) Strohmayer et al. (2008); 61) Bassa et al. (2006); 62) in’t Zand et al. (2008); 63) in’t Zand et al. (2007); 64) Jonker et al. (2006); 65) Haggard et al. (2004); 66) in’t Zand et al. (2005a).

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