4 research outputs found
A search for the superburst oscillation signal in the regular thermonuclear bursts of 4U 1636-536
Burst oscillations are brightness asymmetries that develop in the burning
ocean during thermonuclear bursts on accreting neutron stars. They have been
observed during H/He-triggered (Type I) bursts and Carbon-triggered
superbursts. The mechanism responsible is not unknown, but the dominant burst
oscillation frequency is typically within a few Hz of the spin frequency, where
this is independently known. One of the best-studied burst oscillation sources,
4U 1636-536, has oscillations at in both its regular Type I
bursts and in one superburst. Recently however, Strohmayer \& Mahmoodifar
reported the discovery of an additional signal at a higher frequency,
, during the superburst. This higher frequency is consistent
with the predictions for several types of global ocean mode, one of the
possible burst oscillation mechanisms. If this is the case then the same
physical mechanism may operate in the normal Type I bursts of this source. In
this paper we report a stacked search for periodic signals in the regular Type
I bursts: we found no significant signal at the higher frequency, with upper
limits for the single trial root mean square (rms) fractional amplitude of
0.57(6)\%. Our analysis did however reveal that the dominant
burst oscillation signal is present at a weak level even in the sample of
bursts where it cannot be detected in individual bursts. This indicates that
any cutoff in the burst oscillation mechanism occurs below the detection
threshold of existing X-ray telescopes.Comment: 6 pages, 2 figures. Accepted for publication by Ap
The lofar tied-array all-sky survey: Timing of 35 radio pulsars and an overview of the properties of the lofar pulsar discoveries
The LOFAR Tied-Array All-Sky Survey (LOTAAS) is the most sensitive untargeted radio pulsar survey performed at low radio
frequencies (119−151 MHz) to date and has discovered 76 new radio pulsars, including the 23.5-s pulsar J0250+5854, which up until
recently was the slowest spinning radio pulsar known. In this paper, we report on the timing solutions of 35 pulsars discovered by
LOTAAS, which include a nulling pulsar and a mildly recycled pulsar, and thereby complete the full timing analysis of the LOTAAS
pulsar discoveries. We give an overview of the findings from the full LOTAAS sample of 76 pulsars, discussing their pulse profiles,
radio spectra, and timing parameters. We found that the pulse profiles of some of the pulsars show profile variations in time or
frequency, and while some pulsars show signs of scattering, a large majority display no pulse broadening. The LOTAAS discoveries
have on average steeper radio spectra and longer spin periods (1.4×), as well as lower spin-down rates (3.1×) compared to the known
pulsar population. We discuss the cause of these differences and attribute them to a combination of selection effects of the LOTAAS
survey as well as previous pulsar surveys, though we cannot rule out that older pulsars tend to have steeper radio spectra