Swift detection of a long thermonuclear X-ray burst from 4U 1850-08
ATel #5972; Jean in 't Zand (SRON, Netherlands), Manuel Linares (IAC, Spain), Craig Markwardt (NASA-GSFC, USA)
on 11 Mar 2014; 20:49 UT
Credential Certification: Jean in' t Zand (jeanz@sron.nl)
Subjects: X-ray, Globular Cluster, Neutron Star, Transient
On March 10, 2014, at 21:05:00 UT the Swift Burst Alert Telescope
triggered on an interesting increase of flux from the the low-mass
ultracompact X-ray binary 4U 1850-08 in the globular cluster NGC
6712. The BAT light curve shows an increasing flux starting 2.2 min
before and peaking at 5.8+/-0.8 min after the trigger. The average
15-50 keV spectrum can be modeled by a black body with kT=2.78+/-0.13
keV.
Swift carried out an automatic slew of the narrow field instrument to
the source starting 7.0 min after the trigger and was on target 8.3
min after the trigger. The pointed observation lasted 12.9 min. The
X-ray telescope (XRT) shows a bright source, decaying by a factor of 2
throughout the observation, suggesting an overal e-folding decay time
of roughly 103 s. A time-resolved spectroscopic analysis
shows the spectrum to be consistent with a simple black body, with kT
decaying from 2.65+/-0.07 keV to 1.65+/-0.01 keV and an emission area
consistent with a sphere with a radius varying between 11.8+/-0.4 km
and 15.8+/-0.6 km at a distance of 6.9 kpc (Harris 2010,
arXiv:1012.3224). The bolometric flux varies between
(10.7+/-0.9)x10-8 erg/s/cm2 at the beginning to
(2.53+/-0.05)x10-8 erg/s/cm2 at the end. This is after
ignoring a 23-s long dip which occurs 550 s into the observation. We
note that the typical 2-10 keV flux of 4U 1850-08 is
1x10-10 erg/s/cm2. Clearly, the Eddington limit is reached
of a canonical 1.4 solar-mass neutron star with a hydrogen-poor
photosphere of 7x10-8 erg/s/cm2 at 6.9 kpc. The
burst shows strong flux variations around the general decay trend,
somewhat similar as those seen previously in 2S 0918-549 (in
't Zand et al. 2005, A&A 471, 675, in 't Zand et al. 2011, A&A 525,
A111) and IGR J17062-6143 (Degenaar et al. 2013, ApJ 767, L37).
These properties are all consistent with a long thermonuclear X-ray
burst, lasting until at least 24 min when it is still approximately
half the peak brightness. The rising BAT flux indicates a period of
photospheric expansion lasting 8.0+/-0.8 min. These durations are much
longer than for any other X-ray burst ever detected from 4U 1850-08,
which were always ~< 1 min long. The longevity places this burst
between the class of intermediate duration bursts (thought to be due
to ignition of thick helium-rich layers on relatively cold neutron
stars in ultracompact X-ray binaries; Cumming et al. 2006, ApJ 646,
429) and superbursts (though to be due to ignition of deep carbon-rich
layers; Cumming et al. 2001, ApJ 559, L127, and Strohmayer & Brown
2002, ApJ 566, 1045). The longest intermediate duration burst has an
e-folding decay time of 600 s (SLX 1735-269; Molkov et al. 2005, A&A
434, 1069), the shortest superburst 1800 s (4U 1820-30; in 't Zand et
al. 2011). We favor the intermediate duration hypothesis, because 4U
1850-08 has a relatively low accretion rate (~0.5% of Eddington; in 't
Zand et al. 2007, A&A 665, 953), in contrast to other sources with
short superbursts.