AXTAR: mission design concept

Ray, Paul S.; Chakrabarty, Deepto; Wilson-Hodge, C.; Phlips, Bernard F.; Remillard, Ronald A.; Levine, Alan M.; Wood, K. S.; Wolff, M. T.; Gwon, C.; Strohmayer, Tod E.; Baysinger, M.; Briggs, M. S.; Capizzo, Peter; Fabisinski, Leo; Hopkins, Randall C.; Hornsby, Linda; Johnson, C. L.; Maples, C. Dauphne; Miernik, Janie H.; Thomas, Dan; Geronimo, G. De

doi:10.1117/12.857385

Cited by 10 publications

(7 citation statements)

References 41 publications

(37 reference statements)

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“…To exploit them to measure the EOS, however, requires larger area instruments, and various mission concepts are now being proposed. These have included the 3 m 2 Advanced X-ray Timing Array (AXTAR: Ray et al, 2010), and the 8.5-10 m 2 Large Observatory for X-ray Timing, LOFT (Feroci et al, 2012(Feroci et al, , 2014, see also the LOFT ESA M3 Yellow Book http://sci.esa.int/loft/53447-loft-yellow-book).…”

Section: Discussionmentioning

confidence: 99%

Colloquium: Measuring the neutron star equation of state using x-ray timing

et al. 2016

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One of the primary science goals of the next generation of hard X-ray timing instruments is to determine the equation of state of the matter at supranuclear densities inside neutron stars, by measuring the radius of neutron stars with different masses to accuracies of a few percent. Three main techniques can be used to achieve this goal. The first involves waveform modelling. The flux we observe from a hotspot on the neutron star surface offset from the rotational pole will be modulated by the star's rotation, and this periodic modulation at the spin frequency is called a pulsation. As the photons propagate through the curved space-time of the star, information about mass and radius is encoded into the shape of the waveform (pulse profile) via special and general relativistic effects. Using pulsations from known sources (which have hotspots that develop either during thermonuclear bursts or due to channelled accretion) it is possible to obtain tight constraints on mass and radius. The second technique involves characterising the spin distribution of accreting neutron stars. A large collecting area enables highly sensitive searches for weak or intermittent pulsations (which yield spin) from the many accreting neutron stars whose spin rates are not yet known. The most rapidly rotating stars provide a very clean constraint, since the limiting spin rate where the equatorial surface velocity is comparable to the local orbital velocity, at which mass-shedding occurs, is a function of mass and radius. However the overall spin distribution also provides a guide to the torque mechanisms in operation and the moment of inertia, both of which can depend sensitively on dense matter physics. The third technique is to search for quasiperiodic oscillations in X-ray flux associated with global seismic vibrations of magnetars (the most highly magnetized neutron stars), triggered by magnetic explosions. The vibrational frequencies depend on stellar parameters including the dense matter equation of state, and large area X-ray timing instruments would provide much improved detection capability. We illustrate how these complementary X-ray timing techniques can be used to constrain the dense matter equation of state, and discuss the results that might be expected from a 10m 2 instrument. We also discuss how the results from such a facility would compare to other astronomical investigations of neutron star properties.

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Section: Discussionmentioning

confidence: 99%

Colloquium: Measuring the neutron star equation of state using x-ray timing

et al. 2016

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“…In Section 4 we discuss possible mode identifications for the best candidate frequency in J1751. We also briefly discuss how future observations with larger collecting area missions, such as ESA's Large Observatory for X-Ray Timing (LOFT; Feroci et al 2012) and the Advanced X-Ray Timing Array (Ray et al 2010), can improve the sensitivity of such searches. We conclude with a brief summary of our findings in Section 5.…”

Section: Introductionmentioning

confidence: 99%

A Non-Radial Oscillation Mode in an Accreting Millisecond Pulsar?

Strohmayer

Mahmoodifar

2014

ApJ

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We present results of targeted searches for signatures of non-radial oscillation modes (such as r-and g-modes) in neutron stars using RXTE data from several accreting millisecond X-ray pulsars (AMXPs). We search for potentially coherent signals in the neutron star rest frame by first removing the phase delays associated with the star's binary motion and computing fast Fourier transform power spectra of continuous light curves with up to 2 30 time bins. We search a range of frequencies in which both r-and g-modes are theoretically expected to reside. Using data from the discovery outburst of the 435 Hz pulsar XTE J1751−305 we find a single candidate, coherent oscillation with a frequency of 0.5727597 × ν spin = 249.332609 Hz, and a fractional Fourier amplitude of 7.46 × 10 −4 . We estimate the significance of this feature at the 1.6 × 10 −3 level, slightly better than a 3σ detection. Based on the observed frequency we argue that possible mode identifications include rotationally modified g-modes associated with either a helium-rich surface layer or a density discontinuity due to electron captures on hydrogen in the accreted ocean. In the latter case the presence of sufficient hydrogen in this ultracompact system with a likely helium-rich donor would present an interesting puzzle. Alternatively, the frequency could be identified with that of an inertial mode or a core r-mode modified by the presence of a solid crust; however, the r-mode amplitude required to account for the observed modulation amplitude would induce a large spin-down rate inconsistent with the observed pulse timing measurements. For the AMXPs XTE J1814−338 and NGC 6440 X−2 we do not find any candidate oscillation signals, and we place upper limits on the fractional Fourier amplitude of any coherent oscillations in our frequency search range of 7.8 × 10 −4 and 5.6 × 10 −3 , respectively. We briefly discuss the prospects and sensitivity for similar searches with future, larger X-ray collecting area missions.

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“…and the covariant derivative is compatible with the effective post-TOV metric r) , (1 − 2m(r)/r) −1 , r 2 , r 2 sin 2 θ ], (5) with…”

Section: A Executive Summarymentioning

confidence: 99%

“…Neutron stars play a special role among astrophysical objects, because they are excellent laboratories for matter under extreme conditions (unlike black holes) and also excellent laboratories to probe strong gravity (unlike ordinary stars or white dwarfs) [1]. For these reasons neutron stars are among the main targets of future observatories, such as SKA [2], NICER [3], LOFT [4] and AXTAR [5]. These experiments have the potential to measure neutron star masses and radii to unprecedented levels [6][7][8].…”

Section: Introductionmentioning

confidence: 99%

Post-Tolman-Oppenheimer-Volkoff formalism for relativistic stars

et al. 2015

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Besides their astrophysical interest, compact stars also provide an arena for understanding the properties of theories of gravity that differ from Einstein's general relativity. Numerous studies have shown that different modified theories of gravity can modify the bulk properties (such as mass and radius) of neutron stars for given assumptions on the microphysics. What is not usually stressed though is the strong degeneracy in the predictions of these theories for the stellar mass and radius. Motivated by this observation, in this paper we take an alternative route and construct a stellar structure formalism which, without adhering to any particular theory of gravity, describes in a simple parametrized form the departure from compact stars in general relativity. This "post-TOV" formalism for spherical static stars is inspired by the well-known parametrized post-Newtonian theory, extended to second post-Newtonian order by adding suitable correction terms to the fully relativistic Tolman-Oppenheimer-Volkoff (TOV) equations. We show how neutron star properties are modified within our formalism, paying special attention to the effect of each correction term. We also show that the formalism is equivalent to general relativity with an "effective" (gravity-modified) equation of state.

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AXTAR: mission design concept

Cited by 10 publications

References 41 publications

Colloquium: Measuring the neutron star equation of state using x-ray timing

Colloquium: Measuring the neutron star equation of state using x-ray timing

A Non-Radial Oscillation Mode in an Accreting Millisecond Pulsar?

Post-Tolman-Oppenheimer-Volkoff formalism for relativistic stars

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