We present a detailed observational and theoretical study of a \sim 3 hr long X-ray burst ( the “ super burst ” ) observed by the Rossi X-ray Timing Explorer ( RXTE ) from the low mass X-ray binary ( LMXB ) 4U 1820-30 .
This is the longest X-ray burst ever observed from this source , and perhaps one of the longest ever observed in great detail from any source .
We show that the super burst is thermonuclear in origin .
Its peak luminosity of \sim 3.4 \times 10 ^ { 38 } ergs s ^ { -1 } is consistent with the helium Eddington limit for a neutron star at \sim 7 kpc , as well as the peak luminosity of other , shorter , thermonuclear bursts from the same source .
The super burst begins in the decaying tail of a more typical ( \approx 20 s duration ) thermonuclear burst .
These shorter , more frequent bursts are well known helium flashes from this source .
The level of the accretion driven flux as well as the observed energy release of upwards of 1.5 \times 10 ^ { 42 } ergs indicate that helium could not be the energy source for the super burst .
We outline the physics relevant to carbon production and burning on helium accreting neutron stars and present calculations of the thermal evolution and stability of a carbon layer and show that this process is the most likely explanation for the super burst .
Ignition at the temperatures in the deep carbon “ ocean ” requires > 30 times the mass of carbon inferred from the observed burst energetics unless the He flash is able to trigger a deflagration from a much smaller mass of carbon .
We show , however , that for large columns of accreted carbon fuel , a substantial fraction of the energy released in the carbon burning layer is radiated away as neutrinos , and the heat that is conducted from the burning layer in large part flows inward , only to be released on timescales longer than the observed burst .
Thus the energy released during the event possibly exceeds that observed in X-rays by more than a factor of ten , making the scenario of burning a large mass of carbon at great depths consistent with the observed fluence without invoking any additional trigger .
A strong constraint on this scenario is the recurrence time : to accrete an ignition column of 10 ^ { 13 } g cm ^ { -2 } takes \sim 13 / ( \dot { M } / 3 \times 10 ^ { 17 } g s ^ { -1 } ) yr. Spectral analysis during the super burst reveals the presence of a broad emission line between 5.8 - 6.4 keV and an edge at 8 - 9 keV likely due to reflection of the burst flux from the inner accretion disk in 4U 1820-30 .
We believe this is the first time such a signature has been unambiguously detected in the spectrum of an X-ray burst .