[ Previous | Next | ADS ]

IGR J18245-2452: an accreting neutron star and thermonuclear burster in M28

ATel #4960; Manuel Linares (IAC, Spain)
on 8 Apr 2013; 14:46 UT
Credential Certification: Manuel Linares (linares@mit.edu)

Subjects: X-ray, Binary, Globular Cluster, Neutron Star, Transient, Variables

Referred to by ATel #: 4961, 4964, 4981, 5031, 5068, 5069, 5086

Swift-XRT detected a thermonuclear (type I) X-ray burst from IGR J18245-2452, the new X-ray transient in the globular cluster M28 (ATels #4925, #4927, #4929, #4934), during an observation taken on 2013-04-07. The burst peaked at 22:15:43 UTC, with a maximum count rate of ~300 c/s, a rise time of ~3s and a duration in the XRT band of approximately 100 s. Time-resolved spectroscopy of the burst (correcting for dead-time and fixing nH=4.4E21 cm^-2; ATels #4927, #4929) shows a thermal spectrum with a temperature decreasing from 2.7+/-0.3 keV at the peak down to ~1.1 keV in the tail. Detecting such "cooling tail" allows us to unambiguously identify the burst as thermonuclear. The unabsorbed bolometric [absorbed 2-10 keV] burst peak flux was ~5.8E-8 [3.0E-8] erg/cm2/s, which corresponds to a luminosity of 2.1E38 erg/s at 5.5 kpc (ATel #4927; Harris, 1996).

These results identify IGR J18245-2452 as an accreting neutron star (NS) in a low-mass X-ray binary (LMXB), and as far as we know the first confirmed thermonuclear burster in M28 (see, however, Gotthelf & Kulkarni 1997, ApJ, 490, 161). Note this is consistent with a tentative burst reported by Negoro (MAXI alert 6390849999) which occurred ~8.3 hours later, on 2013-04-08 at 06:35:06 UT. The Swift-XRT burst has been recently reported by Papitto et al. (ATel #4959), and we provide here spectroscopic confirmation and extended discussion.

The persistent (unabsorbed 0.5-10 keV) accretion-powered flux prior to the burst was ~7.5e-10 erg/cm2/s, which corresponds to about 1% of the Eddington luminosity at 5.5kpc. The photon index in the XRT spectrum (1.5+/-0.2) is consistent with that of low-luminosity NS-LMXBs (atoll sources) in the hard state. If the MAXI burst is confirmed as a thermonuclear burst from IGR J18245-2452, the recurrence time (8.3 hr) is shorter than that expected at low (~1% Eddington) mass accretion rates.

We thank the Swift team for making these observations possible.