Neutron Star Magnetic Field Strength Research Articles

Be X-Ray Binaries as Progenitors of Double Neutron Stars: Evidence of Similar bi-modal Classifications

Abstract We explore the evolutional links between Be X-ray binaries (BeXBs) and double neutron stars (DNSs) by analyzing their physical parameters and classifications. We find that both BeXBs and DNSs show positive correlation trends between the neutron star (NS) spin period–P and binary orbital period–P orb, which may relate to the influence of binary accretion. In addition, BeXBs show a bi-modal classification divided by P ∼ 40 s/P orb ∼ 60 days, where BeXBs with P < 40 s/P orb < 60 days exhibit stronger accretion-induced properties, e.g., the lower average NS magnetic field strength, than those with P > 40 s/P orb > 60 days. Similarly, DNSs exhibit a bi-modal classification divided by P orb ∼ 1 day, where DNSs with P orb < 1 day share the stronger accretion-induced properties, e.g., the higher average accretion rate of the recycled NSs, than those with P orb > 1 day. The multi-parameter analysis with the new aggregated data set improves the knowledge of the bi-modal classifications of BeXBs/DNSs. In addition, we consider the influence of the common envelope phase/supernova kick of the donor star on the evolution from BeXBs to DNSs, and further discuss their compatibility with the observed bi-modal classifications of BeXBs/DNSs. Last, we propose a potential evolutional relation between the bi-modal BeXBs and DNSs.

Confirmation of the presence of a CRSF in the NICER spectrum of X 1822-371

Aims. X 1822-371 is an eclipsing binary system with a period close to 5.57 h and an orbital period derivative Ṗorb of 1.42(3) × 10−10 s s−1. The extremely high value of its Ṗorb is compatible with a super-Eddington mass transfer rate from the companion star and, consequently, an intrinsic luminosity at the Eddington limit. The source is also an X-ray pulsar, it shows a spin frequency of 1.69 Hz and is in a spin-up phase with a spin frequency derivative of 7.4 × 10−12 Hz s−1. Assuming a luminosity at the Eddington limit, a neutron star magnetic field strength of B = 8 × 1010 G is estimated. However, a direct measure of B could be obtained observing a CRSF in the energy spectrum. Analysis of XMM-Newton data suggested the presence of a cyclotron line at 0.73 keV, with an estimated magnetic field strength of B = (8.8 ± 0.3)×1010 G. Methods. Here we analyze the 0.3–50 keV broadband spectrum of X 1822-371 combining a 0.3–10 keV NICER spectrum and a 4.5–50 keV NuSTAR spectrum to investigate the presence of a cyclotron absorption line and the complex continuum emission spectrum. Results. The NICER spectrum confirms the presence of a cyclotron line at 0.66 keV. The continuum emission is modeled with a Comptonized component, a thermal component associated with the presence of an accretion disk truncated at the magnetospheric radius of 105 km and a reflection component from the disk blurred by relativistic effects. Conclusions. We confirm the presence of a cyclotron line at 0.66 keV inferring a NS magnetic field of B = (7.9 ± 0.5)×1010 G and suggesting that the Comptonized component originates in the accretion columns.

Evidence of disc reflection in the X-ray spectrum of the neutron star low-mass X-ray binary 4U 1636−536

ABSTRACT We present a broad-band spectral analysis of the atoll source 4U 1636−536 observed for ∼92 ks with NuSTAR. The source was found to be in a low-luminosity state during this observation with 3–79 keV X-ray luminosity of L3–79 keV = (1.03 ± 0.01) × 1037 erg s−1, assuming a distance of 6 kpc. We have identified and removed 12 type-I X-ray bursts during this observation to study the persistent emission. The continuum is well described by a thermal Comptonization model nthcomp with Γ ∼ 1.9, kTe ∼ 28 keV, and kTs ∼ 0.9 keV. The NuSTAR data reveal a clear signature of disc reflection, a significantly broad Fe–K emission line (around 5–8 keV), and the corresponding reflection hump (around 15–30 keV). We have modelled the data with two relativistically blurred reflection models. Both families of reflection models prefer truncated inner disc radii prior to the ISCO. We find that the inner disc is truncated with an inner radius of Rin = 3.2–4.7 RISCO ($\simeq 16\!-\!24\, R_{\mathrm{ g}}\: \text{or}\: 36\!-\!54$ km). This inner disc radius suggests that the neutron star magnetic field strength is B ≤ 2.0 × 109 G.

Observational constraints on the magnetic field of the bright transient Be/X-ray pulsar SXP 4.78

ABSTRACT We report results of the spectral and timing analysis of the Be/X-ray pulsar SXP 4.78 using the data obtained during its recent outburst with NuSTAR, Swift, Chandra, and NICER observatories. Using an overall evolution of the system luminosity, spectral analysis, and variability power spectrum we obtain constraints on the neutron star magnetic field strength. We found a rapid evolution of the variability power spectrum during the rise of the outburst, and absence of the significant changes during the flux decay. Several low frequency quasi-periodic oscillation features are found to emerge on the different stages of the outburst, but no clear clues on their origin were found in the energy spectrum and overall flux behaviour. We use several indirect methods to estimate the magnetic field strength on the neutron star surface and found that most of them suggest magnetic field B ≲ 2 × 1012 G. The strictest upper limit comes from the absence of the cyclotron absorption features in the energy spectra and suggests relatively weak magnetic field B < 6 × 1011 G.)

Bright transients from strongly-magnetized neutron star-black hole mergers

Direct detection of black hole-neutron star pairs is anticipated with the advent of aLIGO. Electromagnetic counterparts may be crucial for a confident gravitational-wave detection as well as for extraction of astronomical information. Yet black hole-neutron star pairs are notoriously dark and so inaccessible to telescopes. Contrary to this expectation, a bright electromagnetic transient can occur in the final moments before merger as long as the neutron star is highly magnetized. The orbital motion of the neutron star magnet creates a Faraday flux and corresponding power available for luminosity. A spectrum of curvature radiation ramps up until the rapid injection of energy ignites a fireball, which would appear as an energetic blackbody peaking in the x ray to $\gamma$ rays for neutron star field strengths ranging from $10^{12}$G to $10^{16}$G respectively and a $10M_{\odot} $ black hole. The fireball event may last from a few milliseconds to a few seconds depending on the neutron star magnetic-field strength, and may be observable with the Fermi Gamma-Ray Burst Monitor with a rate up to a few per year for neutron star field strengths $\gtrsim 10^{14}$G. We also discuss a possible decaying post-merger event which could accompany this signal. As an electromagnetic counterpart to these otherwise dark pairs, the black-hole battery should be of great value to the development of multi-messenger astronomy in the era of aLIGO.

Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9–342058

1RXS J180408.9-342058 is a transient neutron star low-mass X-ray binary that exhibited a bright accretion outburst in 2015. We present Nustar, Swift, and Chandra observations obtained around the peak of this outburst. The source was in a soft X-ray spectral state and displayed an X-ray luminosity of Lx~(2-3)E37 (D/5.8 kpc)^2 erg cm-2 (0.5-10 keV). The Nustar data reveal a broad Fe-K emission line that we model as relativistically broadened reflection to constrain the accretion geometry. We found that the accretion disk is viewed at an inclination of i~27-35 degrees and extended close to the neutron star, down to Rin~5-7.5 gravitational radii (~11-17 km). This inner disk radius suggests that the neutron star magnetic field strength is B<2E8 G. We find a narrow absorption line in the Chandra/HEG data at an energy of ~7.64 keV with a significance of ~4.8 sigma. This feature could correspond to blue-shifted Fe xxvi and arise from an accretion disk wind, which would imply an outflow velocity of v~0.086c (~25800 km s-1). However, this would be extreme for an X-ray binary and it is unclear if a disk wind should be visible at the low inclination angle that we infer from our reflection analysis. Finally, we discuss how the X-ray and optical properties of 1RXS J180408.9-342058 are consistent with a relatively small (Porb<3 hr) binary orbit.

FAST RADIO BURSTS AND RADIO TRANSIENTS FROM BLACK HOLE BATTERIES

Most black holes (BHs) will absorb a neutron star (NS) companion fully intact, without tidal disruption, suggesting the pair will remain dark to telescopes. Even without tidal disruption, electromagnetic luminosity is generated from the battery phase of the binary when the BH interacts with the NS magnetic field. Originally the luminosity was expected in high-energy X-rays or gamma-rays, however we conjecture that some of the battery power is emitted in the radio bandwidth. While the luminosity and timescale are suggestive of fast radio bursts (FRBs; millisecond-scale radio transients) NS--BH coalescence rates are too low to make these a primary FRB source. Instead, we propose the transients form a FRB sub-population, distinguishable by a double peak with a precursor. The rapid ramp-up in luminosity manifests as a precursor to the burst which is $20\%-80\%$ as luminous, given 0.5~ms timing resolution. The main burst is from the peak luminosity before merger. The post-merger burst follows from the NS magnetic field migration to the BH, causing a shock. NS--BH pairs are especially desirable for ground-based gravitational wave (GW) observatories since the pair might not otherwise be detected, with electromagnetic counterparts greatly augmenting the scientific leverage beyond the GW signal. Valuably, the electromagnetic signal can break degeneracies in the parameters encoded in the GW as well as probe the NS magnetic field strength, yielding insights into open problems in NS magnetic field decay.

The critical accretion luminosity for magnetized neutron stars

The accretion flow around X-ray pulsars with a strong magnetic field is funnelled by the field to relatively small regions close to the magnetic poles of the neutron star (NS), the hotspots. During strong outbursts regularly observed from some X-ray pulsars, the X-ray luminosity can be so high, that the emerging radiation is able to stop the accreting matter above the surface via radiation-dominated shock, and the accretion column begins to rise. This border luminosity is usually called the "critical luminosity". Here we calculate the critical luminosity as a function of the NS magnetic field strength $B$ using exact Compton scattering cross section in strong magnetic field. Influence of the resonant scattering and photon polarization is taken into account for the first time. We show that the critical luminosity is not a monotonic function of the B-field. It reaches a minimum of a few 10^{36} erg s^{-1} when the cyclotron energy is about 10 keV and a considerable amount of photons from a hotspot have energy close to the cyclotron resonance. For small B, this luminosity is about 10^{37} erg s^{-1}, nearly independent of the parameters. It grows for the B-field in excess of 10^{12} G because of the drop in the effective cross-section of interaction below the cyclotron energy. We investigate how different types of the accretion flow and geometries of the accretion channel affect the results and demonstrate that the general behaviour of the critical luminosity on B-field is very robust. The obtained results are shown to be in a good agreement with the available observational data and provide a necessary ground for the interpretation of upcoming high quality data from the currently operating and planned X-ray telescopes.

A highly optimized code for calculating atomic data at neutron star magnetic field strengths using a doubly self-consistent Hartree–Fock–Roothaan method

A highly optimized code for calculating atomic data at neutron star magnetic field strengths using a doubly self-consistent Hartree–Fock–Roothaan method

Accreting magnetars: a new type of high-mass X-ray binaries?

The discovery of very slow pulsations (Pspin=5560s) has solved the long-standing question of the nature of the compact object in the high-mass X-ray binary 4U 2206+54 but has posed new ones. According to spin evolutionary models in close binary systems, such slow pulsations require a neutron star magnetic field strength larger that the quantum critical value, suggesting the presence of a magnetar. We present the first XMM-Newton observations of 4U 2206+54 and investigate its spin evolution. We find that the observed spin-down rate agrees with the magnetar scenario. We analyse ISGRI/INTEGRAL observations of 4U 2206+54 to search for the previously suggested cyclotron resonance scattering feature at ~30 keV. We do not find a clear indication of the presence of the line, although certain spectra display shallow dips, not always at 30 keV. The association of these dips with a cyclotron line is very dubious because of its apparent transient nature. We also investigate the energy spectrum of 4U 2206+54 in the energy range 0.3-10 keV with unprecedented detail and report for the first time the detection of very weak 6.5 keV fluorescence iron lines. The photoelectric absorption is consistent with the interstellar value, indicating very small amount of local matter, which would explain the weakness of the florescence lines. The lack of matter locally to the source may be the consequence of the relatively large orbital separation of the two components of the binary. The wind would be too tenuous in the vicinity of the neutron star.

A highly optimized code for calculating atomic data at neutron star magnetic field strengths using a doubly self-consistent Hartree–Fock–Roothaan method

Our previously published code for calculating energies and bound–bound transitions of medium-Z elements at neutron star magnetic field strengths [D. Engel, M. Klews, G. Wunner, Comput. Phys. Comm. 180 (2009) 302–311] was based on the adiabatic approximation. It assumes a complete decoupling of the (fast) gyration of the electrons under the action of the magnetic field and the (slow) bound motion along the field under the action of the Coulomb forces. For the single-particle orbitals this implied that each is a product of a Landau state and an (unknown) longitudinal wave function whose B-spline coefficients were determined self-consistently by solving the Hartree–Fock equations for the many-electron problem on a finite-element grid. In the present code we go beyond the adiabatic approximation, by allowing the transverse part of each orbital to be a superposition of Landau states, while assuming that the longitudinal part can be approximated by the same wave function in each Landau level. Inserting this ansatz into the energy variational principle leads to a system of coupled equations in which the B-spline coefficients depend on the weights of the individual Landau states, and vice versa, and which therefore has to be solved in a doubly self-consistent manner. The extended ansatz takes into account the back-reaction of the Coulomb motion of the electrons along the field direction on their motion in the plane perpendicular to the field, an effect which cannot be captured by the adiabatic approximation. The new code allows for the inclusion of up to 8 Landau levels. This reduces the relative error of energy values as compared to the adiabatic approximation results by typically a factor of three (1/3 of the original error), and yields accurate results also in regions of lower neutron star magnetic field strengths where the adiabatic approximation fails. Further improvements in the code are a more sophisticated choice of the initial wave functions, which takes into account the shielding of the core potential for outer electrons by inner electrons, and an optimal finite-element decomposition of each individual longitudinal wave function. These measures largely enhance the convergence properties compared to the previous code, and lead to speed-ups by factors up to two orders of magnitude compared with the implementation of the Hartree–Fock–Roothaan method used by Engel and Wunner in [D. Engel, G. Wunner, Phys. Rev. A 78 (2008) 032515].

A fast parallel code for calculating energies and oscillator strengths of many-electron atoms at neutron star magnetic field strengths in adiabatic approximation

We have developed a new method for the fast computation of wavelengths and oscillator strengths for medium- Z atoms and ions, up to iron, at neutron star magnetic field strengths. The method is a parallelized Hartree–Fock approach in adiabatic approximation based on finite-element and B-spline techniques. It turns out that typically 15–20 finite elements are sufficient to calculate energies to within a relative accuracy of 10 −5 in 4 or 5 iteration steps using B-splines of 6th order, with parallelization speed-ups of 20 on a 26-processor machine. Results have been obtained for the energies of the ground states and excited levels and for the transition strengths of astrophysically relevant atoms and ions in the range Z = 2 … 26 in different ionization stages. Program summary Program title: HFFEM Catalogue identifier: AECC_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AECC_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 3845 No. of bytes in distributed program, including test data, etc.: 27 989 Distribution format: tar.gz Programming language: MPI/Fortran 95 and Python Computer: Cluster of 1–26 HP Compaq dc5750 Operating system: Fedora 7 Has the code been vectorised or parallelized?: Yes RAM: 1 GByte Classification: 2.1 External routines: MPI/GFortran, LAPACK, PyLab/Matplotlib Nature of problem: Calculations of synthetic spectra [1] of strongly magnetized neutron stars are bedevilled by the lack of data for atoms in intense magnetic fields. While the behaviour of hydrogen and helium has been investigated in detail (see, e.g., [2]), complete and reliable data for heavier elements, in particular iron, are still missing. Since neutron stars are formed by the collapse of the iron cores of massive stars, it may be assumed that their atmospheres contain an iron plasma. Our objective is to fill the gap and to provide a program which allows users to calculate as comprehensively as possible energies, wavelengths, and oscillator strengths of medium- Z atoms and ions up to Z = 26 in neutron star magnetic field strengths. Obviously, the method for achieving this goal must be highly efficient since for the calculation of synthetic spectra data of many thousands or even millions of atomic transitions may be required. Solution method: As in previous work on the problem (cf. [3,7]) we exploit the fact that a strong magnetic field results in an approximate decoupling of the dynamics of the electrons parallel and perpendicular to the field. In this adiabatic approximation the single-particle wave functions take the form: ψ i ( ρ , φ , z ) = ϕ n m ( ρ , φ ) ⋅ P n m ν ( z ) , where ϕ n m ( ρ , φ ) are Landau wave functions, describing the (fast) motion perpendicular to the field, and the P n m ν ( z ) are the longitudinal wave functions, describing the (slow) bound motion along the direction of the field. The spins of the electrons are all aligned antiparallel to the magnetic field and need not be accounted for explicitly. The total N-electron wave function is constructed as a Slater determinant of the single-particle wave functions, and the unknown longitudinal wave functions are determined from the Hartree–Fock equations, which follow from inserting the total N-electron wave function into Schrödinger's variational principle for the total energy. The novel feature of our approach [8] is to use finite-element and B-spline techniques to solve the Hartree–Fock equations for atoms in strong magnetic fields. This is accomplished through the following steps: 1) decomposition of the z-axis into finite elements with quadratically widening element borders; 2) sixth-order B-spline expansion of the single-particle wave functions on the individual finite elements; 3) formulation of the variational principle equivalent to the Hartree–Fock equations in terms of the expansion coefficients. This leads to a simple system of linear equations for the expansion coefficients, which is solved numerically, and, since the direct and exchange interaction potential terms depend on the wave functions, in a self consistent way. The iteration procedure is initialized by distributing the electrons on magnetic sublevels according to the level scheme of the hydrogen atom in intense magnetic fields. To speed up the calculations, the code is parallelized. The parallelization strategy is: a) each processor calculates one or several electrons, depending on the total number of processors, b) single-particle wave functions are broadcast from each processor to every other processor. As the coefficient vectors in the B-spline basis are small ( dim ≈ 20 – 25 ), there is only little communication between the nodes. Typical speedups by a factor of 20 are obtained on a 26-processor cluster of HP Compaq dc57750. Running time: The test runs provided only require a few seconds using 2 processors.

Hartree-Fock-Roothaan calculations for many-electron atoms and ions in neutron-star magnetic fields

The quantitative analysis of the electromagnetic spectra of isolated neutron stars by means of model atmosphere calculations requires extensive data sets of atomic energy values and transition probabilities in intense magnetic fields. We present a method for the fast computation of wave functions, energies, and oscillator strengths of medium-$Z$ atoms and ions at neutron star magnetic field strengths $B\ensuremath{\gtrsim}{10}^{7}\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ which strikes a balance between numerical accuracy and computing times. We use a Hartree-Fock ansatz in which each single-electron orbital is expanded in terms of Landau states with one longitudinal expansion function, and each Landau level contributes with a different weight to the orbital. Both the longitudinal expansion functions and the Landau weights are determined in a doubly self-consistent way. Hartree-Fock equations are solved by decomposing the $z$ axis in finite elements and expanding the longitudinal wave functions in terms of sixth-order $B$-splines. The contributions of the eight lowest Landau levels are taken into account. The procedure can be efficiently parallelized. Results are presented for the ground states and different excited states of atoms and ions for nuclear charges $Z=2,\dots{},26$ and $N=2,\dots{},26$ electrons, and for oscillator strengths. Wherever possible, a comparison with the results of previous calculations is made.

Order in the Chaos: Spin-up and Spin-down during the 2002 Outburst of SAX J1808.4-3658

We present a timing analysis of the 2002 outburst of the accreting millisecond pulsar SAX J1808.4-3658. A study of the phase delays of the entire pulse profile shows a behavior that is surprising and difficult to interpret: superposed to a general trend, a big jump by about 0.2 in phase is visible, starting at day 14 after the beginning of the outburst. An analysis of the pulse profile indicates the presence of a significant first harmonic. Studying the fundamental and the first harmonic separately, we find that the phase delays of the first harmonic are more regular, with no sign of the jump observed in the fundamental. The fitting of the phase delays of the first harmonic with a model that takes into account the observed exponential decay of the X-ray flux (and therefore of the mass accretion rate onto the neutron star) gives important information on the torque acting on the neutron star during the outburst. We find that the source shows spin-up in the first part of the outburst, while a spin-down dominates at the end. From these results we derive an estimate of the neutron star magnetic field strength.

ABeppoSAXObservation of KS 1731−260 in Its Quiescent State: Constraints on the Magnetic Field of the Neutron Star

We report here the results of a 90 ks BeppoSAX observation of the low-mass X-ray binary and atoll source KS 1731-260 during a quiescent phase. From this observation we derive a source X-ray luminosity of ~1033 ergs s-1 (for a source distance of 7 kpc). If the neutron star is spinning at a period of a few milliseconds, as inferred from the nearly coherent oscillations observed during type I X-ray bursts, the quiescent X-ray luminosity constrains the neutron star magnetic field strength. We consider all the mechanisms that have been proposed to explain the quiescent X-ray emission of neutron star X-ray transients and compare the corresponding expectations with the measured upper limit on the X-ray luminosity. We find that, in any case, the neutron star magnetic field is most probably less than ~109 G. We have also observed KS 1731-260, still in its quiescent state, at 1.4 GHz with the Parkes radio telescope to search for radio pulses. We found that no radio signals with millisecond periods are present with an upper limit on the flux of 0.60 mJy using a 4 minute integration time (optimal for a close system with an orbital period smaller than a few hours) and of 0.21 mJy using a 35 minute integration time (optimal for a wide-orbit system).

An Accretion Model for Anomalous X‐Ray Pulsars

We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star towards a ``tracking'' solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time, and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties.

BeppoSAX observations of the X-ray binary pulsar 4U1626-67

We report on observations of the low-mass X-ray binary 4U1626-67 performed during the BeppoSAX Science Verification Phase. We present the broad-band 0.1–100 keV pulse averaged spectrum, that is well fit by a two-component function: a 0.27 ± 0.02 keV blackbody and an absorbed power law with a photon index of 0.89 ± 0.02. A very deep and narrow absorption feature at 38 keV, attributable to electron cyclotron resonance, is clearly visible in the broad-band spectrum. It corresponds to a neutron star magnetic field strength of 3.3 × 10 12 G. The 4U1626-67 pulse profiles show a dramatic dependance on energy: the transition between the low energy ( E < 10 keV) bi-horned shape to the high-energy ( E > 10 keV) sinusoidal profile is clearly visible in our data. The modulation index shows a monotomic increase with energy.

ITAL]BeppoSAX[/ITAL] Observation of 4U 1626−67: Discovery of an AbsorptionCyclotron Resonance Feature

We report on an observation of the low-mass X-ray binary 4U 1626-67 performed during the BeppoSAX science verification phase. An absorption feature at ~37 keV, which is attributable to electron cyclotron resonance, has been discovered in its pulse-averaged spectrum. The inferred neutron star magnetic field strength is 3.2(1+z)×1012 G, where z is the gravitational redshift. The feature is deep and narrow and is resolved in both the broadband fit and in the ratio of observed counts to those seen from the Crab pulsar. The cyclotron resonance energy is in good agreement with the empirical relation between cyclotron energy and high-energy cutoff, while its width is in agreement with the expected Doppler broadening of thermal electrons at the cyclotron resonance frequency. The broadband 0.1-200 keV spectrum is well fit by a two-component model: a 0.27±0.02 keV blackbody and a power law with a photon index of 0.89±0.02. This is the first broadband observation made after the change from spin-up to spin-down that occurred in mid-1990: it confirms the harder spectrum with respect to those observed in the 2-10 keV range.

Spectral State Transitions in Aquila X-1: Evidence for “Propeller” Effects

Aquila X-1 is a soft X-ray transient source and emits type I X-ray bursts. A spectral state transition was observed with RXTE during its outburst decay in 1997 February and March. Its 10-30 keV and 5-10 keV count rate ratio increased suddenly when its luminosity was between 4 and 12×1035 ergs s-1, assuming a 2.5 kpc distance. Spectral fitting with a model composed of a blackbody and a power-law component showed that its blackbody component decreased and that the power-law component became much harder significantly and simultaneously. We interpret this transition to be caused by the centrifugal barrier, more commonly known as the effect. We thus infer that the magnetic field strength of the neutron star is around 1×108 G, if the neutron star spin period is 1.8 ms. Similarly, we infer the neutron star magnetic field strength in another soft X-ray transient Cen X-4 is about 2×109 G. We also propose a unified scheme for spectral state transitions in soft X-ray transients, from soft high state to hard low state and further to quiescent state. With this scheme accretion onto neutron star may take place even during the propeller regime.

Photon Splitting in Strongly Magnetized Cosmic Objects—Revisited

We reassess the importance of photon splitting to spectral formation in cosmic X-ray and $\gamma$-ray sources with magnetic fields of several $10^{12}$ gauss. Our analysis is based on the recent numerically accurate evaluation of the full relativistic expression for photon splitting at neutron star magnetic field strengths (Mentzel et al. 1994). Using these results we demonstrate that Adler's (1971) widely used approximation formula for photon splitting underestimates the correct splitting rate by {\em several} orders of magnitude, at these magnetic field strengths. Even down at X-ray energies of a few tens of keV magnetic photon splitting is found to be an efficient photon conversion process, producing mean free paths as short as several tens of centimeters for fields on the order of $5 \times 10^{12}$ gauss. We conclude that previous analyses of the contributions of magnetic photon splitting to spectral formation based on Adler's approximation formula are physically unreliable, and, in particular, that photon splitting must be included in realistic modelings of spectra of both strongly magnetized cosmic X-ray and $\gamma$-ray sources.