A superburst candidate in EXO 1745--248 as a challenge to thermonuclear ignition models

Abstract

We report on Chandra, RXTE, Swift/BAT and MAXI observations of a ~1 day X-ray flare and subsequent outburst of a transient X-ray source observed in October-November 2011 in the globular cluster Terzan 5. We show that the source is the same as the transient that was active in 2000, i.e., the neutron star low-mass X-ray binary EXO 1745-248. For the X-ray flare we estimate a 6-11 hr exponential decay time and a radiated energy of 2-9 x 10^42 erg. These properties, together with strong evidence of decreasing blackbody temperature during the flare decay, are fully consistent with what is expected for a thermonuclear superburst. We use the most recent superburst models and estimate an ignition column depth of ~10^12 g cm^-2 and an energy release between 0.1-2 x 10^18 erg g^-1, also consistent with expected superburst values. We conclude therefore that the flare was most probably a superburst. We discuss our results in the context of theoretical models and find that even when assuming a few days of low level accretion before the superburst onset (which is more than what is suggested by the data), the observations of this superburst are very challenging for current superburst ignition models.

9 pages, 4 figures, Accepted for publication in MNRAS

References in corpus (14)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• X-ray time variability across the atoll source states of 4U 1636--53
• Faint X-ray Sources in the Globular Cluster Terzan 5
• Possible Resonances in the 12C + 12C Fusion Rate and Superburst Ignition
• Millihertz quasi-periodic oscillations and thermonuclear bursts from Terzan 5: A showcase of burning regimes
• Distances of the bulge globular clusters Terzan 5, Liller 1, UKS 1 and Terzan 4 based on HST NICMOS photometry
• First superburst from a classical low-mass X-ray binary transient
• Intermediate long X-ray bursts from the ultra-compact binary source SLX 1737-282
• Superburst Models for Neutron Stars with Hydrogen and Helium-Rich Atmospheres
• The spin and orbit of the newly discovered pulsar IGR J17480-2446
• Strong X-ray variability in the quiescent state of the neutron star low-mass X-ray binary EXO 1745-248
• Multi-Zone Models of Superbursts from Accreting Neutron Stars
• Compositionally-driven convection in the oceans of accreting neutron stars
• Hard X-ray Bursts Recorded by the IBIS Telescope of the INTEGRAL Observatory in 2003-2009

Cited by in corpus (25)

• Thermonuclear X-ray bursts
• Nuclear Physics of the Outer Layers of Accreting Neutron Stars
• Accretion disks and coronae in the X-ray flashlight
• Discovery of the Third Transient X-ray Binary in the Galactic Globular Cluster Terzan 5
• Nuclear Reactions in the Crusts of Accreting Neutron Stars
• X-ray emission and absorption features during an energetic thermonuclear X-ray burst from IGR J17062-6143
• X-ray Reflection and An Exceptionally Long Thermonuclear Helium Burst from IGR J17062-6143
• Disc-Jet Coupling in the Terzan 5 Neutron Star X-ray Binary EXO 1745$-$248
• Constraining the properties of neutron star crusts with the transient low-mass X-ray binary Aql X-1
• A signature of chemical separation in the cooling curves of transiently accreting neutron stars
• The Galactic LMXB Population and the Galactic Centre Region
• A catalogue of unusually long thermonuclear bursts on neutron stars
• Carbon production on accreting neutron stars in a new regime of stable nuclear burning
• Time-dependent, compositionally driven convection in the oceans of accreting neutron stars
• The imprint of carbon combustion on a superburst from the accreting neutron star 4U 1636-536
• Maxi observations of long X-ray bursts
• Crust cooling of the neutron star in Aql X-1: Different depth and magnitude of shallow heating during similar accretion outbursts
• An XMM-Newton and INTEGRAL view on the hard state of EXO 1745-248 during its 2015 outburst
• The Thermal Stability of Helium Burning on Accreting Neutron Stars
• A solution of the puzzling symbiotic X-ray system 4U 1700+24
• Model of heat diffusion in the outer crust of bursting neutron stars
• Selfsimilarity relations for torsional oscillations of neutron stars
• Discovery of a strong 6.6 keV emission feature from EXO 1745$-$248 after the superburst in 2011 October
• Thermonuclear superburst of MAXI J1752$-$457 observed with NinjaSat and MAXI
• A deep Chandra study verifies diffuse non-thermal X-ray emission from the globular cluster Terzan 5