Fermi-GBM detection of a thermonuclear burst from 4U 1608-52

ATel #4091; P. Jenke (MSFC/NPP), M. Linares(MIT), V. Connaughton (UAH), A. J. van der Horst (UvA), A. Camero-Arranz ((IEEC/ICE)), M. Finger (USRA), C. Wilson-Hodge (NASA/MSFC), C. Kouveliotou (NASA/MSFC) for the GBM X-ray burst collaboration.
on 4 May 2012; 15:57 UT
Credential Certification: Peter Jenke (peter.a.jenke@nasa.gov)

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We report the detection with Fermi-GBM (daily CTIME data, 12-25 keV band) of an X-ray burst from a location consistent with the neutron star low-mass X-ray binary and thermonuclear burster 4U 1608-52. The burst peak occurred on May 2, 2012 at 06:47:54 UTC. The Fermi-GBM location of the burst is R.A.(J2000) = 241.3 deg, DEC(J2000) = -51.1 deg (1.8 deg from 4U 1608-52) with a 1 sigma error of 4.7 deg. There is no other known burster within 6 deg of the burst location. Spectral analysis confirms the thermonuclear nature of the burst. The integrated spectrum across the burst is best fit (ChiSq/DOF = 0.97) with a black body model with a temperature of 3.1 +/- 0.2 keV. Time resolved spectroscopy reveals an initial temperature of 3.4 +/- 0.3 keV cooling to 2.0 +/- 0.6 keV along the burst decay. The burst had a duration in the GBM band of 16.4 seconds, with a bolometric peak flux of (7.8 +/- 0.5)e-8 erg/s/cm^2 and an integrated fluence of (7.3 +/- 0.3)e-7 erg/cm^2. Assuming a distance of 3.3 kpc the corresponding peak luminosity and radiated energy are 1e38 erg/s and 1e39 erg, respectively. 4U 1608-52 was not significantly detected by the Swift-BAT and MAXI all-sky monitors on 2012 May 2, the date of the GBM burst. From the BAT non-detection and assuming a typical hard state spectrum (photon index 1.5-2) we estimate an upper limit of 2.7e35 erg/s on the 2-20 keV persistent luminosity (at 3.3 kpc). We conclude that 4U 1608-52 was still accreting on May 2 at a luminosity intermediate between outburst and quiescent levels, and that the thermonuclear burst took place when the mass accretion rate was below 0.1% of the Eddington limit. The high peak burst temperature and luminosity are consistent with energetic, pure He bursts expected at the lowest mass accretion rates.