A Remarkable 3 Hour Thermonuclear Burst from 4U 1820−30

Abstract

We present a detailed observational and theoretical study of a D3 hr long X-ray burst (the "" superburst ÏÏ) observed by the Rossi X-Ray T iming Explorer (RXT E) from the low-mass X-ray binary 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 superburst is thermonuclear in origin. Its peak luminosity of D3.4 ] 1038 ergs s~1 is consistent with the helium Eddington limit for a neutron star at D7 kpc as well as the peak luminosity of other, shorter, thermonuclear bursts from the same source. The superburst begins in the decaying tail of a more typical (B20 s duration) thermonuclear burst. These shorter, more frequent bursts are well-known helium Ñashes from this source. The level of the accretion-driven Ñux as well as the observed energy release of upward of 1.5 ] 1042 ergs indicate that helium could not be the energy source for the superburst. 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 superburst. Ignition at the temperatures in the deep carbon "" ocean ÏÏ requires more than 30 times the mass of carbon inferred from the observed burst energetics unless the He Ñash is able to trigger a deÑagration 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 Ñows 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 10, making the scenario of burning a large mass of carbon at great depths consistent with the observed Ñuence without invoking any additional trigger. A strong constraint on this scenario is the recurrence time : to accrete an ignition column of 1013 g cm~2 takes g s~1) yr. Spectral analysis during the superburst reveals the D13/(M 0 /3 ] 1017 presence of a broad emission line between 5.8 and 6.4 keV and an edge at 8È9 keV, likely due to reÑec-tion of the burst Ñux from the inner accretion disk in 4U 1820[30. We believe that this is the Ðrst time such a signature has been unambiguously detected in the spectrum of an X-ray burst.

Photospheric radius expansion X-ray bursts as standard candles

“…In this respect it is interesting to note that in the early part of the X-ray burst (which contains the photospheric radius expansion/contraction phase) from A1850−08 black-body models did not provide acceptable fits, whereas they were acceptable during the cooling stage (Hoffman et al 1980). In most of the cases the RXTE/PCA spectra exhibited residuals which were similar to that reported during the photospheric radius expansion phases of the strong X-ray burst from 4U 2129+12 seen by the Ginga/LAC (van Paradijs et al 1990), the long X-ray bursts seen from GX 17+2 (Kuulkers et al 2002), and the superburst from 4U 1820−30 (Strohmayer & Brown 2002). Additional components are necessary to improve the fits, such as a line and an edge feature (see Strohmayer & Brown 2002).…”

“…A re-analysis of the EXOSAT/ME X-ray bursts showed that the bolometric flux at touch down ranges between 4.3 and 5.0 ×10 −8 erg cm −2 s −1 , with a mean of 4.65 ×10 −8 erg cm −2 s −1 (Damen et al 1990). More recently, the RXTE/PCA observed a short (∼25 s) X-ray burst (Zhang et al 1998), which was also a radius expansion burst (Franco & Strohmayer 1999; see also Strohmayer & Brown 2002). Another very long (>2.5 hr) X-ray burst (so-called superburst), also seen with RXTE/PCA, showed strong photospheric radius expansion and reached a bolometric peak flux of 6.5 × 10 −8 erg cm −2 s −1 (Strohmayer & Brown 2002).…”

Anisotropy of X-Ray Bursts From Neutron Stars With Concave Accretion Disks

“…Only in the case of two superbursts could the FeKα line from reflection be detected and could the reflection component be separated from the direct burst component (Ballantyne & Strohmayer 2004;Keek et al 2014aKeek et al , 2014bKeek et al , 2015. Superbursts are exceptionally long bursts that last hours (Cornelisse et al 2000;Strohmayer & Brown 2002;Kuulkers 2004;Keek & in 't Zand 2008). They are, however, rare, and only in those two cases were relatively high-quality spectra obtained.…”

What ignites on the neutron star of 4U 0614+091?

“…This seems to be at odds (see Juett et al 2001;Juett & Chakrabarty 2003) with the fact that we see bursts in 4U 0614+091, which are usually understood as being due to unstable ignition of He or H. Moreover, 4U 0614+091 is an interesting case among the sources showing superbursts. Superbursts are expected to occur only in systems with accretion rates in excess of about 10% of the Eddington rate, i.e., the rate which yields emission at the Eddington limit (Cumming & Bildsten 2001;Strohmayer & Brown 2002). However, the low persistent X-ray luminosity of 4U 0614+091 indicates an accretion rate which is an order of magnitude lower (e.g., Ford et al 2000).…”