The strongest cosmic magnets: soft gamma-ray repeaters and anomalous X-ray pulsars

Two classes of X-ray pulsars, the anomalous X-ray pulsars and the soft gamma-ray repeaters, have been recognized in the last decade as the most promising candidates for being magnetars: isolated neutron stars powered by magnetic energy. I review the observational properties of these objects, focussing on the most recent results, and their interpretation in the magnetar model. Alternative explanations, in particular those based on accretion from residual disks, are also considered. The possible relations between these sources and other classes of neutron stars and astrophysical objects are also discussed.     

The broad-band noise characteristics of selected anomalous X-ray pulsars and soft gamma-ray repeaters

We present the broad-band noise structure of selected anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) in the 2–60 keV energy band. We have analysed Rossi X-Ray Timing Explorer Proportional Counter Array archival light curves for four AXPs and one SGR. We detect that the persistent emission of these sources shows band-limited noise at low frequencies in the range 0.005–0.05 Hz varying from 2.5 to 70 per cent integrated rms in times of prolonged quiescence and following outbursts. We discovered band-limited red noise in 1E 2259+586 only for ∼2 yr after its major 2002 outburst. The system shows no broad-band noise otherwise. Although this rise in noise in 1E 2259+586 occurred following an outburst which included a rotational glitch, the other glitching AXPs showed no obvious change in broad-band noise, thus it does not seem that this noise is correlated with glitches. The only source that showed significant variation in broad-band noise was 1E   1048.1−5937  , where the noise gradually rose for 1.95 yr at a rate of ∼3.6 per cent per year. For this source the increases in broad-band noise was not correlated with the large increases in persistent and pulsed flux, or its two short SGR-like bursts. This rise in noise did commence after a long burst, however, given the sparsity of this event, and the possibility that similar bursts went unnoticed the trigger for the rise is noise in 1E   1048.1−5937  is not as clear as for 1E 2259+586. The other three sources indicate a persistent band-limited noise at low levels in comparison.     

Are radio pulsars the progenitors of anomalous X-ray pulsars and soft gamma repeaters?

We analyze the possibility that anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) originate from radio pulsars subjected to considerable and prolonged glitches. The observed characteristics of such pulsars, their association with supernova remnants, and their evolution in the P-? diagram with allowance made for the actual age of the possible AXP and SGR progenitors are shown to be in conflict with the suggested scenario.     

Unveiling soft gamma-ray repeaters with INTEGRAL

Thanks to INTEGRAL’s long exposures of the Galactic Plane, the two brightest Soft Gamma-Ray Repeaters, SGR 1806-20 and SGR 1900+14, have been monitored and studied in detail for the first time at hard-X/soft gamma rays. This has produced a wealth of new scientific results, which we will review here. Since SGR 1806-20 was particularly active during the last two years, more than 300 short bursts have been observed with INTEGRAL and their characteristics have been studied with unprecedented sensitivity in the 15–200 keV range. A hardness-intensity anticorrelation within the bursts has been discovered and the overall Number-Intensity distribution of the bursts has been determined. In addition, a particularly active state, during which 100 bursts were emitted in 10 minutes, has been observed on October 5 2004, indicating that the source activity was rapidly increasing. This eventually led to the Giant Flare of December 27th 2004, for which a possible soft gamma-ray (>80 keV) early afterglow has been detected. The deep observations allowed us to discover the persistent emission in hard X-rays (20–150 keV) from 1806-20 and 1900+14, the latter being in a quiescent state, and to directly compare the spectral characteristics of all Magnetars (two SGRs and three Anomalous X-ray Pulsars) detected with INTEGRAL. D.G. acknowledges the French Space Agency (CNES) for financial support. Based on observations with INTEGRAL, an ESA project with instruments and the science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA. ISGRI has been realized and maintained in flight by CEA-Saclay/DAPNIA with the support of CNES. K.H. is grateful for support under NASA’s INTEGRAL U.S. Guest Investigator program, Grants NAG5-13738 and NNG05GG35G.     

XMM–Newton observations of soft gamma-ray repeaters

All the confirmed Soft Gamma-ray Repeaters have been observed with the EPIC instrument on the XMM–Newton satellite. We review the results obtained in these observations, providing the most accurate spectra on the persistent X-ray emission in the 1–10 keV range for these objects, and discuss them in the context of the magnetar interpretation.      

Recent progress on anomalous X-ray pulsars

I review recent observational progress on Anomalous X-ray Pulsars, with an emphasis on timing, variability, and spectra. Highlighted results include the recent timing and flux stabilization of the notoriously unstable AXP 1E 1048.1–5937, the remarkable glitches seen in two AXPs, and the newly recognized variety of AXP variability types, including outbursts, bursts, flares, and pulse profile changes. I also discuss recent discoveries regarding AXP spectra, including their surprising hard X-ray and far-infrared emission, as well as the pulsed radio emission seen in one source. Much has been learned about these enigmatic objects over the past few years, with the pace of discoveries remaining steady. However additional work on both observational and theoretical fronts is needed before we have a comprehensive understanding of AXPs and their place in the zoo of manifestations of young neutron stars.      

On the X-ray spectra of soft gamma repeaters

Radiation transfer in a scattering medium in a superstrong magnetic field is considered. Because cross-sections depend on frequency, photons with different energies escape layers with different temperatures and therefore the spectrum of the outgoing radiation differs significantly from the equilibrium blackbody or Bose–Einstein spectrum. It is shown that the emergent spectrum (the photon flux per unit energy band) is flat at low energies. Applications of the result to soft gamma repeaters (SGRs) are discussed. Even though the spectrum is strongly distorted when the radiation propagates through the magnetosphere, a flat segment may be observed in the outgoing spectrum if the surface magnetic field of the neutron star is not too high,   B <10¹⁵ G  .     

Resonant Compton upscattering in anomalous X-ray pulsars

A significant new development in the study of Anomalous X-ray Pulsars (AXPs) has been the recent discovery by INTEGRAL and RXTE of flat, hard X-ray components in three AXPs. These non-thermal spectral components differ dramatically from the steeper quasi-power-law tails seen in the classic X-ray band in these sources. A prime candidate mechanism for generating this new component is resonant, magnetic Compton upscattering. This process is very efficient in the strong magnetic fields present in AXPs. Here an introductory exploration of an inner magnetospheric model for upscattering of surface thermal X-rays in AXPs is offered, preparing the way for an investigation of whether such resonant upscattering can explain the 20–150 keV spectra seen by INTEGRAL. Characteristically flat emission spectra produced by non-thermal electrons injected in the emission region are computed using collision integrals. A relativistic QED scattering cross section is employed so that Klein–Nishina reductions are influential in determining the photon spectra and fluxes. Spectral results depend strongly on the magnetospheric locale of the scattering and the observer’s orientation, which couple directly to the angular distributions of photons sampled.     

Magnetic deformation of magnetars for the giant flares of the soft gamma-ray repeaters

We present one possible mechanism for the giant flares of the soft gamma-ray repeaters (SGRs) within the framework of the magnetar (superstrongly magnetized neutron star) model, motivated by the positive period increase associated with the August 27 event from SGR 1900+14. From second-order perturbation analysis of the equilibrium of the magnetic polytrope, we find that there exist different equilibrium states separated by the energy of the giant flares and the shift in the moment of inertia to cause the period increase. This suggests that, if we assume that global reconfiguration of the internal magnetic field of     suddenly occurs, the positive period increase     as well as the energy ≳10⁴⁴ erg of the giant flares may be explained. The moment of inertia can increase with a release of energy, because the star shape deformed by the magnetic field can be prolate rather than oblate. In this mechanism, since oscillation of the neutron star will be excited, a ∼ ms-period pulsation of the burst profile and an emission of gravitational waves are expected. The gravitational waves could be detected by planned interferometers such as LIGO, VIRGO and LCGT.     

Search for radio pulsations in four anomalous X-ray pulsars and discovery of two new pulsars

We have performed deep searches for radio pulsations from four southern anomalous X-ray pulsars (AXPs) to investigate their physical nature in comparison with the rotation powered pulsars. The data were acquired using the Parkes radio telescope with the 1.4 GHz multibeam receiver. No pulsed emission with periodicity matching the X-ray ephemeris have been found in the observed targets down to a limit of ∼0.1 mJy. A blind search has also been performed on all the 13 beams of the multibeam receiver (the central beam being pointed on the target AXP), leading to the serendipitous discovery of two new radio pulsars and to the further detection of 18 pulsars. Also a search for single dispersed pulses has been performed in the aim to detect signals similar to those of the recently discovered rotating radio transients.      

The contribution to the Galactic diffuse gamma-ray spectrum from unresolved rotation-powered pulsars

We consider the contribution to the Galactic diffuse γ-ray emission from unresolved γ-ray pulsars. Based on the thick outer gap model, Monte Carlo methods are used to simulate the properties (period, distance, magnetic field, etc.) of the Galactic population of rotation-powered pulsars the gamma-ray flux of which is lower than the threshold sensitivity of the EGRET detector on the Compton Gamma-Ray Observatory . Furthermore, the contribution to the Galactic diffuse γ-ray spectrum from the unresolved γ-ray pulsars is calculated. Our results indicate that the unresolved γ-ray pulsars contribute ∼5 to ∼10 per cent to the measured Galactic diffuse γ-ray emission if the birth rate of neutron stars in the Galaxy is 1 to 2 per century, and that these pulsars contribute significantly to the observed Galactic diffuse γ-ray emission above 1 GeV. Comparing the model spectrum with the observed spectrum, we show that the unresolved γ-ray pulsars contribute very little to the diffuse emission at lower energies but can account for ∼50 per cent of the observed spectrum above 1 GeV if the product of the birth rate of neutron stars and the γ-ray beaming fraction is about unity. Such a large pulsar contribution can explain the difference (∼60 per cent) between the intensity of the Galactic diffuse emission as measured by EGRET above 1 GeV and model predictions based on cosmic ray–matter interaction only.     

On the soft X-ray spectrum of cooling flows

Strong evidence for cooling flows has been found in low-resolution X-ray imaging and spectra of many clusters of galaxies. However, high-resolution X-ray spectra of several clusters from the Reflection Grating Spectrometer on XMM-Newton now show a soft X-ray spectrum inconsistent with a simple cooling flow. The main problem is a lack of the emission lines expected from gas cooling below 1–2 keV. Lines from gas at about 2–3 keV are observed, even in a high-temperature cluster such as A1835, indicating that gas is cooling down to about 2–3 keV, but is not found at lower temperatures. Here we discuss several solutions to the problem: heating, mixing, differential absorption and inhomogeneous metallicity. Continuous or sporadic heating creates further problems, including the targeting of the heat at the cooler gas and also the high total energy required. So far there is no clear observational evidence for widespread heating, or shocks, in cluster cores, except in radio lobes which occupy only part of the volume. Alternatively, if the metals in the intracluster medium are not uniformly spread but are clumped, then little line emission is expected from the gas cooling below 1 keV. The low-metallicity part cools without line emission, whereas the strengths of the soft X-ray lines from the metal-rich gas depend on the mass fraction of that gas and not on the abundance, since soft X-ray line emission dominates the cooling function below 2 keV.     

The efficiency of gamma-ray emission from pulsars

We present a modified scenario of gamma-ray emission from pulsars within the framework of polar cap models. Our model incorporates the possible acceleration of electron–positron pairs created in magnetospheres, and their subsequent contribution to the gamma-ray luminosity L _γ. It also reproduces the empirical trend in L _γ for seven pulsars detected with Compton Gamma-Ray Observatory ( CGRO ) experiments. At the same time it avoids basic difficulties faced by theoretical models when confronted with observational constraints.   We show that the classical and millisecond pulsars form two distinct branches in the L _γ— L _sd diagram (where L _sd is the spin-down luminosity). In particular, we explain why the millisecond pulsar J0437−4715 has not been detected with any of the CGRO instruments despite its very high position in the ranking list of spin-down fluxes (i.e. L _sd/ D ², where D is a distance). The gamma-ray luminosity predicted for this particular object is about one order of magnitude below the upper limit set by EGRET.     

The gamma-ray conversion efficiency of rotation-powered pulsars

We use a new self-consistent model to derive the conversion efficiency from rotation power to γ-ray power for pulsars (ε_th). Our result indicates that ε_th∝τ^6/7 P ², where τ and P are the characteristic age and period of the pulsar, which shows that although the efficiency increases with the characteristic age of the pulsar, it also depends on the pulsar period. We test our model results with the survey of high-energy γ-rays from pulsars by EGRET. Our model not only successfully explains the efficiency of the confirmed γ-ray pulsars but also explains why the γ-ray efficiency of millisecond pulsars is so low.     

A search for soft X-ray radiation from pulsars with the Astronomical Netherlands Satellite

We present the results of a search for X-ray emission in the energy range 0.2–0.28 keV and 1–7 keV from a number of radio pulsars, including Crab, Vela and the binary pulsar PSR 1913+16, using the soft X-ray experiment aboard ANS. Except for the Crab no pulsed flux has been found. From the Vela pulsar we have detected continuous flux in agreement with earlier observations. Upper limits are given.Paper presented at the COSPAR Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

Radio pulsars with expected gamma radiation and gamma-ray pulsars as pulsating radio emitters

Pulsars play a crucial astrophy sical role as highly energetic compact radio, X-ray and gammaray sources. Our previous works show that radio pulsars identified as pulsing gamma-ray sources by the Large Area Telescope(LAT) on board the Fermi Gamma-Ray Space Telescope have high values of magnetic field near the light cylinder, two-three orders of magnitude stronger compared with the magnetic fields of radio pulsars: log B_(lc)(G) are 3.60-3.95 and 1.75 correspondingly. Moreover,their losses of rotational energy are also three orders higher than the corresponding values for the main group of radio pulsars on average: log E(erg s~(-1)) = 35.37-35.53 and 32.64. The correlation between gammaray luminosities and radio luminosities is found. It allows us to select those objects from all sets of known radio pulsars that can be detected as gamma-ray pulsars with high probability. We provide a list of such radio pulsars and propose to search for gamma emission from these objects. On the other hand,the known catalog of gamma-ray pulsars contains some sources which are not currently identified as radio pulsars. Some of them have large values of gamma-ray luminosities and according to the obtained correlation, we can expect marked radio emission from these objects. We give the list of such pulsars and expected flux densities to search for radiation at frequencies 1400 and 111 MHz.     

The hard X-ray spectrum of soft X-ray-selected AGN

I use ASCA data to investigate the 2–10 keV X-ray emission of active galactic nuclei (AGN) taken from the ROSAT International X-ray Optical Survey (RIXOS). I find that the integrated spectrum of these faint, soft X-ray-selected AGN in the 2–10 keV band is harder (best-fitting α = 0.8 ± 0.1) than the slope measured with ROSAT between 0.1 and 2 keV, but softer than the 2–10 keV X-ray background, and consistent with the average 2–10 keV spectrum of bright, nearby Seyfert galaxies. With this spectral slope and using measurements of the AGN contribution to the 1–2 keV X-ray background, I estimate that the AGN percentage contribution to the 2–10 keV background is 0.60 ^+0.19_−0.14 times the AGN percentage contribution to the 1–2 keV background. Hence AGN produce between 12 and 32 per cent of the 2–10 keV X-ray background. This is only the contribution from the types of AGN which are found in soft X-ray surveys; a population of absorbed AGN could represent an additional component of the 2–10 keV X-ray background.