The XMM‐Newton view of magnetars

Anomalous X‐ray Pulsars and Soft Gamma‐ray Repeaters are believed to be magnetars: isolated neutron stars powered by the decay of extremely high magnetic fields. We review some of the main results obtained with XMM‐Newton and discuss the prospects for future observations of this small but extremely interesting class of objects. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectra of the spreading layers on the neutron star surface and constraints on the neutron star equation of state

Spectra of the spreading layers on the neutron star surface are calculated on the basis of the Inogamov–Sunyaev model taking into account general relativity correction to the surface gravity and considering various chemical composition of the accreting matter. Local (at a given latitude) spectra are similar to the X-ray burst spectra and are described by a diluted blackbody. Total spreading layer spectra are integrated accounting for the light bending, gravitational redshift and the relativistic Doppler effect and aberration. They depend slightly on the inclination angle and on the luminosity. These spectra also can be fitted by a diluted blackbody with the colour temperature depending mainly on a neutron star compactness. Owing to the fact that the flux from the spreading layer is close to the critical Eddington, we can put constraints on a neutron star radius without the need to know precisely the emitting region area or the distance to the source. The boundary layer spectra observed in the luminous low-mass X-ray binaries, and described by a blackbody of colour temperature   T _c= 2.4 ± 0.1 keV  , restrict the neutron star radii to   R = 14.8 ± 1.5 km  (for a  1.4-M_⊙  star and solar composition of the accreting matter), which corresponds to the hard equation of state.     

Statistical properties of twin kilohertz quasi‐periodic oscillations in neutron star low‐mass X‐ray binaries

We collect the data of twin kilohertz quasi‐periodic oscillations (kHz QPOs) published before 2012 from 26 neutron star (NS) low‐mass X‐ray binary (LMXB) sources, then we analyze the centroid frequency (ν) distribution of twin kHz QPOs (lower frequency ν₁ and upper frequency ν₂) both for Atoll and Z sources. For the data without shift‐and‐add, we find that Atoll and Z sources show different distributions of ν₁, ν₂ and ν₂/ν₁, but the same distribution of Δν (difference of twin kHz QPOs), which indicates that twin kHz QPOs may share the common properties of LXMBs and have the same physical origins. The distribution of Δν is quite different from a constant value, so is ν 2/ν₁ from a constant ratio. The weighted mean values and maxima of ν₁ and ν₂ in Atoll sources are slightly higher than those in Z sources. We also find that shift‐and‐add technique can reconstruct the distributions of ν₁ and Δν. The K‐S test results of ν₁ and Δν between Atoll and Z sources from data with shift‐and‐add are quite different from those without it, and we think that this may be caused by the selection biases of the sample. We also study the properties of the quality factor (Q) and the root‐meansquared (rms) amplitude of 4U 0614+09 with data from the two observational methods, but the errors are too big to make a robust conclusion. The NS spin frequency (ν_s) distribution of 28 NS‐LMXBs show a bigger mean value (∼408 Hz) than that (∼281 Hz) of the radio binary millisecond pulsars (MSPs), which may be due to the lack of the spin detections from Z sources (systematically lower than 281 Hz). Furthermore, on the relations between the kHz QPOs and NS spin frequency ν_s, we find the approximate correlations of the mean values of Δν with NS spin and its half, respectively. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The estimations of neutron star mass and radius by the kHz QPOs

The kHz quasi‐periodic oscillations (QPOs) have been detected by the RXTE satellite in about thirty neutron stars (NSs) in low mass X‐ray binaries (LMXBs), which are usually interpreted to be related to the Keplerian motions in the orbit close to NS surface where the accreted matter is sucked onto the star. Based on the MHD Alfvén wave oscillation model and the relativistic precession model for the neutron star (NS) kHz QPOs, estimations of mass M and radius R of some NSs are given, which can give clues to evaluate the models. Furthermore, comparisons with theoretical M ‐R relations by stellar equations of state (EOSs) are presented (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

XMM‐Newton: The next decade for cool star research

In this article I will highlight selected results from XMM‐Newton observations of stellar coronae, emphasizing the specific XMM‐Newton capabilities in terms of high‐resolution spectroscopy, its long‐look capability and its optical monitor. I will focus on results on “normal”, cool stars and present science areas hitherto largely unexploired by XMM‐Newton. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A scientific case for XMM‐Newton continuation

The presentations made at the workshop “XMM‐Newton: The Next Decade”, held at ESAC from 4th to the 6th of July 2007, contained an overwhelming amount of new results and well justified scientific questions that can be addressed by observations with XMM‐Newton. XMM‐Newton has over the next decade a solid scientific case. Given the high impact of X‐ray observations, XMM‐Newton operations are not only a matter for “X‐ray astrophysics”, but also of fundamental importance for astrophysics in general. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An XMM–Newton observation of the neutron star X-ray transient 2S 1803−245 in quiescence

We observed the neutron star X-ray transient 2S 1803−245 in quiescence with the X-ray satellite XMM–Newton , but did not detect it. An analysis of the X-ray bursts observed during the 1998 outburst of 2S 1803−245 gives an upper limit to the distance of ≤7.3 kpc, leading to an upper limit on the quiescent 0.5–10 keV X-ray luminosity of  ≤2.8 × 10³² erg s⁻¹  (3σ). Since the expected orbital period of 2S 1803−245 is several hours, this limit is not much higher than those observed for the quiescent black hole transients with similar orbital periods.     

XMM‐Newton and broad iron lines

Iron line emission is common in the X‐ray spectra of accreting black holes. When the line emission is broad or variable then it is likely to originate from close to the black hole. X‐ray irradiation of the accretion flow by the power‐law X‐ray continuum produces the X‐ray ‘reflection’ spectrum which includes the iron line. The shape and variability of the iron lines and reflection can be used as a diagnostic of the radius, velocity and nature of the flow. The inner radius of the dense flow corresponds to the innermost stable circular orbit and thus can be used to determine the spin of the black hole. Studies of broad iron lines and reflection spectra offer much promise for understanding how the inner parts of accretion flows (and outflows) around black holes operate. There remains great potential for XMM‐Newton to continue to make significant progress in this work. The need for high quality spectra and thus for long exposure times is paramount. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The applications of the MHD Alfvén wave oscillation model for kHz quasi‐periodic oscillations

In this paper, we improve the previous work on the MHD Alfvén wave oscillation model for the neutron star (NS) kHz quasi‐periodic oscillations (QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS X‐ray sources with the simultaneously detected twin kHz QPO frequencies, the stellar mass M and radius R constraints are given by means of the derived parameter A in the model, which is associated with the averaged mass density of the star as 〈ρ 〉 = 3M /(4πR³) ≃ 2.4 × 10¹⁴ (A /0.7)² g/cm³, and we also compare the M ‐R constraints with the stellar equations of state. Moreover, we also discuss the theoretical maximum kHz QPO frequency and maximum twin peak separation, and some expectations on SAX J1808.4–3658 are mentioned, such as its highest kHz QPO frequency ∼ 870 Hz, which is about 1.4–1.5 times less than those of the other known kHz QPO sources. The estimated magnetic fields for both Z sources (about Eddington accretion rate ) and Atoll sources (∼ 1% ) are approximately ∼10⁹ G and ∼10⁸ G, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Statistical analysis for the Q‐factor of twin kHz QPOs

Using the recently published data of twin kHz quasi‐period oscillations (QPOs) in neutron stars of low‐mass X‐ray binaries (LMXBs), we study the different profiles between bright Z sources and less luminous Atoll sources. The quality factors of upper kHz QPOs show a narrow distribution both for Z sources and Atoll sources, which concentrate at 7.98 and 9.75, respectively. The quality factors of lower kHz QPOs show a narrow distribution for Z sources and a broader distribution for Atoll sources, which concentrate at 5.25 and 86.22, respectively. In order to investigate the relation between the quality factor and the peak frequency of kHz QPOs, we fit the data with power‐law, linear, and exponential functions, respectively. There is an obvious trend that the quality factors increase with the peak frequencies both for upper and lower QPOs. The implications of our results are discussed (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quasi-periodic oscillations in low-mass X-ray binaries and constraints on the equation of state of neutron star matter

Recently discovered quasi-periodic oscillations in the X-ray brightness of low-mass X-ray binaries are used to derive constraints on the mass of the neutron star component and the equation of state of neutron star matter. The observations are compared with models of rapidly rotating neutron stars which are calculated by means of an exact numerical method in full relativity. For the equations of state we select a broad collection of models representing different assumptions about the many-body structure and the complexity of the composition of superdense matter. The mass constraints differ from their values in the approximate treatment by ∼10 per cent. Under the assumption that the maximum frequency of the quasi-periodic oscillations originates from the innermost stable orbit, the mass of the neutron star is in the range M ∼1.92–2.25 M_⊙. The quasi-periodic oscillation in the Atoll-source 4U 1820−30 in particular is only consistent with equations of state that are rather stiff at high densities, which is explainable, so far, only with pure nucleonic/leptonic composition. This interpretation contradicts the hypothesis that the protoneutron star formed in SN 1987A collapsed to a black hole, since this would demand a maximum neutron star mass below 1.6 M_⊙. The recently suggested identification of quasi-periodic oscillations with frequencies of about 10 Hz with the Lense–Thirring precession of the accretion disc is found to be inconsistent with the models studied in this work, unless it is assumed that the first overtone of the precession is observed.     

XMM‐Newton: The next decade

Some key numbers about the XMM‐Newton users, usage of the mission and the scientific outcome are provided. The background for selecting the topic, the preparation work, the organization and the corresponding committees are briefly described. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Boundary layer on the surface of a neutron star

In an attempt to model the accretion on to a neutron star in low-mass X-ray binaries, we present 2D hydrodynamical models of the gas flow in close vicinity of the stellar surface. First, we consider a gas pressure-dominated case, assuming that the star is non-rotating. For the stellar mass we take   M _star= 1.4 × 10⁻² M_⊙  and for the gas temperature   T = 5 × 10⁶ K  . Our results are qualitatively different in the case of a realistic neutron star mass and a realistic gas temperature of T ≃ 10⁸ K, when the radiation pressure dominates. We show that to get the stationary solution in a latter case, the star most probably has to rotate with the considerable velocity.     

Burial of the polar magnetic field of an accreting neutron star – II. Hydromagnetic stability of axisymmetric equilibria

The theory of polar magnetic burial in accreting neutron stars predicts that a mountain of accreted material accumulates at the magnetic poles of the star, and that, as the mountain spreads equatorward, it is confined by, and compresses, the equatorial magnetic field. Here, we extend previous, axisymmetric, Grad–Shafranov calculations of the hydromagnetic structure of a magnetic mountain up to accreted masses as high as   M _a= 6 × 10⁻⁴ M_⊙  , by importing the output from previous calculations (which were limited by numerical problems and the formation of closed bubbles to   M _a < 10⁻⁴ M_⊙  ) into the time-dependent, ideal-magnetohydrodynamic code zeus-3d and loading additional mass on to the star dynamically. The rise of buoyant magnetic bubbles through the accreted layer is observed in these experiments. We also investigate the stability of the resulting hydromagnetic equilibria by perturbing them in zeus-3d . Surprisingly, it is observed that the equilibria are marginally stable for all   M _a≤ 6 × 10⁻⁴ M_⊙  ; the mountain oscillates persistently when perturbed, in a combination of Alfvén and acoustic modes, without appreciable damping or growth, and is therefore not disrupted (apart from a transient Parker instability initially, which expels <1 per cent of the mass and magnetic flux).     

Resistive relaxation of a magnetically confined mountain on an accreting neutron star

Three-dimensional numerical magnetohydrodynamic (MHD) simulations are performed to investigate how a magnetically confined mountain on an accreting neutron star relaxes resistively. No evidence is found for non-ideal MHD instabilities on a short time-scale, such as the resistive ballooning mode or the tearing mode. Instead, the mountain relaxes gradually as matter is transported across magnetic surfaces on the diffusion time-scale, which evaluates to  τ_I∼ 10⁵–10⁸ yr  (depending on the conductivity of the neutron star crust) for an accreted mass of   M _a= 1.2 × 10⁻⁴ M_⊙  . The magnetic dipole moment simultaneously re-emerges as the screening currents dissipate over  τ_I  . For non-axisymmetric mountains, ohmic dissipation tends to restore axisymmetry by magnetic reconnection at a filamentary neutral sheet in the equatorial plane. Ideal-MHD oscillations on the Alfvén time-scale, which can be excited by external influences, such as variations in the accretion torque, compress the magnetic field and hence decrease  τ_I  by one order of magnitude relative to its standard value (as computed for the static configuration). The implications of long-lived mountains for gravitational wave emission from low-mass X-ray binaries are briefly explored.     

Planned and serendipitous surveys with XMM‐Newton

This article compares and contrasts the virtues of planned (and normally contiguous) surveys with XMM‐Newton and the XMM‐Newton serendipitous sky survey and discusses various ways in which both the quality and efficiency of contiguous surveys with XMM‐Newton could potentially be improved. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Continuous frequency spectrum of the global hydromagnetic oscillations of a magnetically confined mountain on an accreting neutron star

We compute the continuous part of the ideal-magnetohydrodynamic (ideal-MHD) frequency spectrum of a polar mountain produced by magnetic burial on an accreting neutron star. Applying the formalism developed by Hellsten & Spies, extended to include gravity, we solve the singular eigenvalue problem subject to line-tying boundary conditions. This spectrum divides into an Alfvén part and a cusp part. The eigenfunctions are chirped and anharmonic with an exponential envelope, and the eigenfrequencies cover the whole spectrum above a minimum ω_low. For equilibria with accreted mass  1.2 × 10⁻⁶≲ M _a/M_⊙≲ 1.7 × 10⁻⁴  and surface magnetic fields  10¹¹≲ B _*/G ≲ 10¹³, ω_low  is approximately independent of   B _*  , and increases with M _a. The results are consistent with the Alfvén spectrum excited in numerical simulations with the zeus-mp solver. The spectrum is modified substantially by the Coriolis force in neutron stars spinning faster than ∼100 Hz. The implications for gravitational-wave searches for low-mass X-ray binaries are considered briefly.