Long-period X-ray Pulsars Research Articles

On the Geometry of the X-ray emission from pulsars a consistent inclination and beaming solution for the Be/X-ray pulsar SXP 1062

ABSTRACT SXP 1062 is a long-period X-ray pulsar (XRP) with a Be optical companion located in the Small Magellanic Cloud. First discovered in 2010 from XMM–Newton data, it has been the target of multiple observational campaigns due to the seeming incongruity between its long spin period and recent birth. In our continuing modelling efforts to determine the inclination angle (i) and magnetic axis angle (θ) of XRPs, we have fitted 19 pulse profiles from SXP 1062 with our pulsar model, Polestar, including three consecutive Chandra observations taken during the trailing end of a Type I outburst. These fittings have resulted in most likely values of i = 76○ ± 2○ and θ = 40○ ± 9○. SXP 1062 mostly displays a stable double-peaked pulse profile with the peaks separated by roughly a third of a phase, but recently the pulsar has spun up and widened to a spacing of roughly half of a phase, yet the Polestar fits for i and θ remain constant. Additionally, we note a possible correlation between the X-ray luminosity and the separation of the peaks in the pulse profiles corresponding to the highest luminosity states.

Understanding the coexistence of spin-up and spin-down behaviours in long-period X-ray pulsars

ABSTRACT Assuming wind-fed accretion magnetars in long-period X-ray pulsars, we calculated the rotational evolution of neutron stars. Our calculations considered the effects of magnetic field decay in magnetars. The results show that wind-fed accretion magnetars can evolve to long-period X-ray pulsars with a spin period much longer than 1000 s. The spin-down trend observed in 4U 2206+54-like sources is expected when young X-ray binary systems are on the way to their equilibrium period. Detailed calculations showed that the spin-down may be affected by accretion with outflows or accretion while spinning down. Due to magnetic field decay in magnetars, wind-fed accretion magnetars will have a decreasing equilibrium period for a constant mass accretion rate. For 2S 0114+65, the spin-up rate due to magnetic field decay is one order of magnitude smaller than observations. The spin-up rate of 2S 0114+65 may be attributed to the formation of a transient disc during wind accretion. The slowest X-ray pulsar AX J1910.7+0917 would be a link source between 4U 2206+54 and 2S 0114+65.

Discovery of a putative supernova remnant around the long-period X-ray pulsar SXP 1323 in the Small Magellanic Cloud

ABSTRACT We report the discovery of a circular shell centred on the Be X-ray binary (BeXB) SXP 1323 in the Small Magellanic Cloud. The shell was detected in an H α image obtained with the Very Large Telescope. Follow-up spectroscopy with the Southern African Large Telescope showed that the shell expands with a velocity of ${\approx }100{\rm \, km\, s^{-1}}$ and that its emission is due to shock excitation. We suggest that this shell is a remnant of the supernova explosion that led to the formation of SXP 1323’s neutron star ${\approx }40\, 000$ yr ago. SXP 1323 represents the second known case of a BeXB associated with a supernova remnant (the first one is SXP 1062). Interestingly, both of these BeXBs harbour long-period pulsars and are located in a low-metallicity galaxy.

A multiwavelength study of SXP 1062, the long-period X-ray pulsar associated with a supernova remnant

SXP 1062 is a Be X-ray binary located in the Small Magellanic Cloud. It hosts a long-period X-ray pulsar and is likely associated with the supernova remnant MCSNR J0127-7332. In this work we present a multi-wavelength view on SXP 1062 in different luminosity regimes. We consider monitoring campaigns in optical (OGLE survey) and X-ray (SWIFT telescope). During these campaigns a tight coincidence of X-ray and optical outbursts is observed. We interpret this as typical Type I outbursts as often detected in Be X-ray binaries at periastron passage of the neutron star. To study different X-ray luminosity regimes in depth, during the source quiescence we observed it with XMM-Newton while Chandra observations followed an X-ray outburst. Nearly simultaneously with Chandra observations in X-rays, in optical the RSS/SALT telescope obtained spectra of SXP 1062. On the basis of our multi-wavelength campaign we propose a simple scenario where the disc of the Be star is observed face-on, while the orbit of the neutron star is inclined with respect to the disc. According to the model of quasi-spherical settling accretion our estimation of the magnetic field of the pulsar in SXP 1062 does not require an extremely strong magnetic field at the present time.

How young the accretion-powered pulsars could be?

A question about the age of accretion-powered X-ray pulsars has recently been reopened by a discovery of the X-ray pulsar SXP 1062 in the SMC. This High Mass X-ray Binary (HMXB) contains a neutron star rotating with the period of 1062 s and is associated with a supernova remnant of the age ∼ 104 yr. An attempt to explain the origin of this young long-period X-ray pulsar within the traditional scenario of three basic states (ejector, propeller and accretor) encounters difficulties. Even if this pulsar were born as a magnetar the spin-down time during the propeller stage would exceed 104 yr. Here we explore a more circuitous way of the pulsar spin evolution in HMXBs, in which the propeller stage in the evolutionary track is avoided. We find this way to be possible if the stellar wind of the massive companion to the neutron star is magnetized. The geometry of plasma flow captured by the neutron star in this case differs from spherically symmetrical and the magnetospheric radius of the neutron star is smaller than that evaluated in the convention accretion scenarios. We show that the age of an accretion-powered pulsar in this case can be as small as ∼ 104 years without the need of invoking initial magnetic field in excess of 1013 G.

Spin and spectral variations of a peculiar high-mass X-ray binary 4U 2206+54

Spin properties and spectral variations of high mass X-ray binary 4U 2206+54 are studied with long-term hard X-ray monitoring observations by INTEGRAL. A long-period X-ray pulsar of P_spin\sim 5558 s has been identified in 4U 2206+54. The spin evolution of the neutron star in 4U 2206+54 is detected with the INTEGRAL/IBIS data. From 2005 to 2011, the spin period of the neutron star in 4U 2206+54 varies from \sim 5558 s to 5588 s. The average spin-down rate in the last 20 years is derived as \sim 5\times 10^{-7} s s^{-1}. 4U 2206+54 is a variable source with luminosities of \sim 10^{35} - 10^{36} erg s^{-1} in the range of 3 -- 100 keV. Its spectrum can be described by an absorbed power-law model with exponential rolloff. The hydrogen column density and photon index show the anti-correlations with hard X-ray luminosity: low column density and small photon index at maximum of luminosity. This spectral variation pattern suggests that 4U 2206+54 would be a highly obscured binary system. Furthermore, the possible cyclotron line features are searched for in the spectra of different luminosities. The possible cyclotron absorption feature around 30 keV is not confirmed in our work. The physical origin and evolutional track of this very slow pulsation neutron star are still unclear. We suggest that 4U 2206+54 would be a young system with the long-term spin-down trend, would evolve to a longer spin period range and transit to the spin-up phase similar to 2S 0114+65. These very slow pulsation X-ray pulsars would belong to a new class of compact objects -- accreting magnetars.

Is there a highly magnetized neutron star in GX 301–2?

We present the results of an in-depth study of the long-period X-ray pulsar GX 301-2. Using archival data of INTEGRAL, RXTE ASM, and CGRO BATSE, we study the spectral and timing properties of the source. Comparison of our timing results with previously published work reveals a secular decay of the orbital period at a rate of \simeq -3.25 \times 10^{-5} d yr^{-1}, which is an order of magnitude faster than for other known systems. We argue that this is probably result either of the apsidal motion or of gravitational coupling of the matter lost by the optical companion with the neutron star, although current observations do not allow us to distinguish between those possibilities. We also propose a model to explain the observed long pulse period. We find that a very strong magnetic field B \sim 10^{14} G can explain the observed pulse period in the framework of existing models for torques affecting the neutron star. We show that the apparent contradiction with the magnetic field strength B_{CRSF} \sim 4 \times 10^{12} G derived from the observed cyclotron line position may be resolved if the line formation region resides in a tall accretion column of height \sim 2.5 - 3 R_{NS}. The color temperature measured from the spectrum suggests that such a column may indeed be present, and our estimates show that its height is sufficient to explain the observed cyclotron line position.

The origin of long-period X-ray pulsars

Several relatively bright, persistent X-ray sources display regular pulses, with periods in the range of (0.7–10) × 103 s. These sources are identified with massive close binaries in which a neutron star accretes material on to its surface. The observed pulsations in all of them, but one, are unambiguously associated with the spin period of the neutron star. Analysing the possible history of these pulsars, I conclude that the neutron stars in these systems undergo spherical accretion, and their evolutionary tracks in a previous epoch contained three instead of two states, namely ejector, supersonic propeller and subsonic propeller. An assumption about a supercritical value of the initial magnetic field of the neutron stars within this scenario is not necessary. Furthermore, I show that the scenario in which the neutron star in 2S 0114+650 is assumed to be a magnetar descendant encounters major difficulties in explaining the evolution of the massive companion. An alternative interpretation of the spin evolution of the neutron star in this system is presented and the problem raised by the association of the 104 s pulsations with the spin period of the neutron star is briefly discussed.

Rapid multiperiodic variability in a Small Magellanic Cloud high-mass X-ray binary

The positions of high-mass X-ray binaries are often known precisely enough to identify the optical component unambiguously, and a number of those stars are or have been monitored by the Optical Gravitational Lensing Experiment (OGLE) and MACHO collaborations. The light curves of two such candidates are examined for evidence of Be-star behaviour and for periodicity. These stars are likely companions to long-period X-ray pulsars in the Small Magellanic Cloud. One of the stars exhibits two periods of 6.832 and 15.587 h, much shorter and more stable than periods of Be/X-ray binaries that are attributed to the disc of the Be star, but consistent with short-term Be variability attributed to pulsations. The multiperiodicity is quantified with Fourier techniques and examined for phase stability. The phase of both variations has wandered by more than a cycle during the 11 years of optical monitoring. Stellar pulsation is the most likely physical mechanism.

Two long-period X-ray pulsars detected in the SMC field around

An XMM-Newton target of opportunity observation of the field around the transient 18.37 s pulsar in the Small Magellanic Cloud (SMC) revealed two bright, long-period X-ray pulsars in the EPIC data. A new pulsar, , with a pulse period of s was discovered and s pulsations were detected from (=CXOU J005455.6–724510), confirming the period found in Chandra data. We derive X-ray positions of , and , (J2000.0) with an uncertainty of 02 utilizing optical identification with OGLE stars. For both objects, the optical brightness and colours and the X-ray spectra are consistent with Be/X-ray binary systems in the SMC.

A phase-dependent absorption line in the spectrum of the X-ray pulsar RX J0720.4-3125

The XMM-Newton spectra of the isolated neutron star RX J0720.4-3125 show deviations from a Planckian energy distribution below 400 eV, similar to the spectra of RBS1223, another long-period X-ray pulsar, as reported recently by Haberl et al. (2003). For a Gaussian-shaped absorption line we derive an energy of 271 eV and an equivalent width of -40 eV from the phase-averaged spectra of RX J0720.4-3125. We investigate the spectral variations seen in hardness ratios as function of pulse phase and find that they are best described by changes in the depth of the absorption line. The line equivalent width changes between -31 eV around intensity maximum of the pulse and -58 eV at the declining part of the pulse. Small variations (<20 eV) of the line energy with pulse phase may still be caused by statistical fluctuations. On the other hand, the black-body temperature varies significantly by 2.5 eV (statistical 90% errors typically 0.7 eV) reaching the highest value at pulse maximum. One possible interpretation for the absorption line is cyclotron resonance scattering of protons in a magnetic field with B about 5x10^13 G. This field strength is compatible with estimates inferred from recent spin down measurements of the pulsar.

X-ray binaries and stellar evolution

Observational evidence suggests that most — if not all — binary X-ray sources are neutron stars. The evolutionary status and possible formation mechanisms of the type I (massive) and type II (low-mass) X-ray binaries are discussed. The difference between the “standard” massive X-ray binaries and the Be/X-ray binaries is ascribed to a somewhat different evolutionary history and status, and possible reasons for the existence of short- and long-period X-ray pulsars are discussed. Type II X-ray sources in globular clusters were most probably formed by capture processes; their formation rate inferred from the observations indicates that only a small fraction (≲ 1 to 10 percent) of the originally formed neutron stars have remained in their clusters. Type II sources in the galactic bulge may also have formed from cataclysmic binaries in which a white dwarf was driven over the Chandrasekhar limit by accretion.