Burst Oscillation Periods from 4U 1636−53: A Constraint on the Binary Doppler Modulation

Abstract

The burst oscillations seen during Type I X-ray bursts from low mass X-ray binaries (LMXB) typically evolve in period towards an asymptotic limit that likely reflects the spin of the underlying neutron star. If the underlying period is stable enough, measurement of it at different orbital phases may allow a detection of the Doppler modulation caused by the motion of the neutron star with respect to the center of mass of the binary system. Testing this hypothesis requires enough X-ray bursts and an accurate optical ephemeris to determine the binary phases at which they occurred. We present here a study of the distribution of asymptotic burst oscillation periods for a sample of 26 bursts from 4U 1636-53 observed with the Rossi X-ray Timing Explorer (RXTE). The burst sample -2includes both archival and proprietary data and spans more than 4.5 years. We also present new optical light curves of V801 Arae, the optical counterpart of 4U 1636-53, obtained during 1998-2001. We use these optical data to refine the binary period measured by Augusteijn et al. (1998) to 3.7931206(152) hours. We show that a subset of ∼ 70% of the bursts form a tightly clustered distribution of asymptotic periods consistent with a period stability of ∼ 1 × 10 −4 . The tightness of this distribution, made up of bursts spanning more than 4 years in time, suggests that the underlying period is highly stable, with a time to change the period of ∼ 3 × 10 4 yr. This is comparable to similar numbers derived for X-ray pulsars. We investigate the period and orbital phase data for our burst sample and show that it is consistent with binary motion of the neutron star with v ns sin i < 55 and 75 km s −1 at 90 and 99% confidence, respectively. We use this limit as well as previous radial velocity data to constrain the binary geometry and component masses in 4U 1636-53. Our results suggest that unless the neutron star is significantly more massive than 1.4 M ⊙ the secondary is unlikely to have a mass as large as 0.36 M ⊙ , the mass estimated assuming it is a main sequence star which fills its Roche lobe. We show that a factor of 2-3 increase in the number of bursts with asymptotic period measurements should allow a detection of the neutron star velocity.

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

“…X-ray studies of 4U 1636−53 have shown a variety of variability on different time scales ranging form mHz to kHz. The NS nature of the compact object has been confirmed through the detection of thermonuclear X-ray bursts (Hoffman et al 1977;Galloway et al 2006;Zhang et al 2011), which show asymptotic millisecond burst oscillations, indicating a spin frequency of ∼581 Hz (Zhang et al 1997;Giles et al 2002;Strohmayer & Markwardt 2002). In addition to variability studies, the X-ray energy spectra, which show broad, asymmetric Fe emission lines, have been investigated (Pandel et al 2008;Cackett et al 2010;Ng et al 2010;Sanna et al 2013).…”

“…Since then the source has been observed over a wide range of wavelengths. Optical photometry revealed an orbital period of ∼3.8 h (van Paradijs et al 1990), and a companion star with a mass of ∼0.4 M , assuming an NS mass of 1.4 M (Giles et al 2002). X-ray studies of 4U 1636−53 have shown a variety of variability on different time scales ranging form mHz to kHz.…”

Broad iron line in the fast spinning neutron-star system 4U 1636−53

“…X-ray studies of 4U 1636-53 have shown a variety of rapid time variability, including i) thermonuclear X-ray bursts (e.g. Hoffman et al 1977;Galloway et al 2006;Zhang et al 2011), confirming the presence of a NS as the compact object, ii) millisecond oscillations during the X-ray bursts, indicating a spin frequency of ∼581 Hz (Zhang et al 1997;Giles et al 2002;Strohmayer & Markwardt 2002), and iii) kHz quasi-periodic oscillations (kHz QPOs, e.g. Wijnands et al 1997;Belloni et al 2007;Altamirano et al 2008;Sanna et al 2012).…”

Signature of Temporary Burning Front Stalling from a Non-Photospheric Radius Expansion Double-peaked Burst

“…2.18 # 10 the oscillations have been searched at the known frequency (which does not evolve by more than ≈6 Hz; Giles et al 2002), multiplying by the number (p3) of trials, we get a significance of , which implies a ∼4 j detection. We also fit…”