Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636–53

Abstract

We detected millihertz quasi-periodic oscillations (QPOs) in an XMM-Newton observation of the neutron-star low-mass X-ray binary 4U 1636−53. These QPOs have been interpreted as marginally-stable burning on the neutron-star surface. At the beginning of the observation the QPO was at around 8 mHz, together with a possible second harmonic. About 12 ks into the observation a type I X-ray burst occurred and the QPO disappeared; the QPO reappeared ∼ 25 ks after the burst and it was present until the end of the observation. We divided the observation into four segments to study the evolution of the spectral properties of the source during intervals with and without mHz QPO. We find that the temperature of the neutron-star surface increases from the QPO segment to the non-QPO segment, and vice versa. We also find a strong correlation between the frequency of the mHz QPO and the temperature of a blackbody component in the energy spectrum representing the temperature of neutron-star surface. Our results are consistent with previous results that the frequency of the mHz QPO depends on the variation of the heat flux from the neutron star crust, and therefore supports the suggestion that the observed QPO frequency drifts could be caused by the cooling of deeper layers.

Millihertz Quasi-Periodic Oscillations in 4u 1636–536: Putting Possible Constraints on the Neutron Star Size

“…1). In the September observation the mHz QPOs are observed before the occurrence of a type-I burst that is also covered by this observation (Lyu et al 2014). In addition, this observation covers about 20 ks after the type-I burst during which no mHz QPOs have been detected.…”

“…The connection between mHz QPOs and nuclear burning has been fostered by the frequency decrease of the mHz QPO in 4U 1636−53 observed prior to the occurrence of a type-I X-ray burst (Altamirano et al 2008b;Lyu et al 2014Lyu et al , 2015. This frequency drift can be reproduced by models of nuclear burning that take mixing processes and crustal heating into account, and could be caused by the cooling of deeper layers of the NS atmosphere (Keek et al 2009).…”

Spectral and timing analysis of the mHz QPOs in the neutron-star low-mass X-ray binary 4U 1636–53

“…This frequency-temperature correlation agrees well with the results of Keek et al (2009): As the burning layers cool down, the frequency of the oscillations decreases by tens of per cents until a burst happens. Besides, Lyu et al (2014a) found that the QPO in that observation disappeared after one burst, and it subsequently reappeared ∼25.3 ks later, which was the longest reappearance time scale so far measured for a mHz QPO in any LMXBs.…”

“…We defined soft and hard X-ray colours as the 3.5 − 6.0/2.0 − 3.5 keV and 9.7 − 16.0/6.0 − 9.7 keV count rate ratios, respectively (see Zhang et al 2011, for details). We parametrized the position of the source on the colour-colour diagram (CCD) by the length of a solid curve S a (see, Lyu et al (2014a). figure) represents the averaged Crab-normalised colours (see Zhang et al 2011, for details) of a single RXTE observation.…”

“…Recently, Keek et al (2014) investigated the influence of the chemical composition of the accreted fuel and nuclear reaction rates on the marginally stable burning process; their results showed that no allowed variation in accretion fuel composition and nuclear reaction rate was able to produce marginally stable burning at the observed accretion rate. Lyu et al (2014a) found that the frequency of the mHz QPOs correlates with the temperature of the neutron star surface in 4U 1636-53 using an XMM-Newton observation. This frequency-temperature correlation agrees well with the results of Keek et al (2009): As the burning layers cool down, the frequency of the oscillations decreases by tens of per cents until a burst happens.…”