226 research outputs found
Thermonuclear Bursts with Short Recurrence Times from Neutron Stars Explained by Opacity-Driven Convection
Thermonuclear flashes of hydrogen and helium accreted onto neutron stars
produce the frequently observed Type I X-ray bursts. It is the current paradigm
that almost all material burns in a burst, after which it takes hours to
accumulate fresh fuel for the next burst. In rare cases, however, bursts are
observed with recurrence times as short as minutes. We present the first
one-dimensional multi-zone simulations that reproduce this phenomenon. Bursts
that ignite in a relatively hot neutron star envelope leave a substantial
fraction of the fuel unburned at shallow depths. In the wake of the burst,
convective mixing events driven by opacity bring this fuel down to the ignition
depth on the observed timescale of minutes. There, unburned hydrogen mixes with
the metal-rich ashes, igniting to produce a subsequent burst. We find burst
pairs and triplets, similar to the observed instances. Our simulations
reproduce the observed fraction of bursts with short waiting times of ~30%, and
demonstrate that short recurrence time bursts are typically less bright and of
shorter duration.Comment: 11 pages, 15 figures, accepted for publication in Ap
Photospheric radius expansion in superburst precursors from neutron stars
Thermonuclear runaway burning of carbon is in rare cases observed from
accreting neutron stars as day-long X-ray flares called superbursts. In the few
cases where the onset is observed, superbursts exhibit a short precursor burst
at the start. In each instance, however, the data was of insufficient quality
for spectral analysis of the precursor. Using data from the propane
anti-coincidence detector of the PCA instrument on RXTE, we perform the first
detailed time resolved spectroscopy of precursors. For a superburst from 4U
1820-30 we demonstrate the presence of photospheric radius expansion. We find
the precursor to be 1.4-2 times more energetic than other short bursts from
this source, indicating that the burning of accreted helium is insufficient to
explain the full precursor. Shock heating would be able to account for the
lacking energy. We argue that this precursor is a strong indication that the
superburst starts as a detonation, and that a shock induces the precursor.
Furthermore, we employ our technique to study the superexpansion phase of the
same superburst in greater detail.Comment: 9 pages, 6 figures, submitted to Ap
Superburst oscillations: ocean and crustal modes excited by Carbon-triggered Type I X-ray bursts
Accreting neutron stars (NS) can exhibit high frequency modulations in their
lightcurves during thermonuclear X-ray bursts, known as burst oscillations. The
frequencies can be offset from the spin frequency of the NS by several Hz, and
can drift by 1-3 Hz. One possible explanation is a mode in the bursting ocean,
the frequency of which would decrease (in the rotating frame) as the burst
cools, hence explaining the drifts. Most burst oscillations have been observed
during H/He triggered bursts, however there has been one observation of
oscillations during a superburst; hours' long Type I X-ray bursts caused by
unstable carbon burning deeper in the ocean. This paper calculates the
frequency evolution of an oceanic r-mode during a superburst. The rotating
frame frequency varies during the burst from 4-14 Hz, and is sensitive to the
background parameters, in particular the temperature of the ocean and ignition
depth. This calculation is compared to the superburst oscillations observed on
4U-1636-536. The predicted mode frequencies ( 10 Hz) would require a spin
frequency of 592 Hz to match observations; 6 Hz higher than the spin
inferred from an oceanic r-mode model for the H/He triggered burst
oscillations. This model also over-predicts the frequency drift during the
superburst by 90 %.Comment: Accepted for publication in MNRA
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