Discovery of a new pulsating X-ray source with a 1549.1-s period, AX J183220-0840

A new pulsating X-ray source, AX J183220-0840, with a 1549.1-s period was discovered at R.A.= 18h32m20s and Dec.=-8d40'30'' (J2000, uncertainty=0.6degree) during an ASCA observation on the Galactic plane. The source was observed two times, in 1997 and in 1999. A phase-averaged X-ray flux of 1.1E-11 ergs cm-2 s-1 and pulsation period of 1549.1+/-0.4 s were consistently obtained from these two observations. The X-ray spectrum was represented by a flat absorbed power-law with a photon-index of =~0.8 and an absorption column density of =~1.3E22 cm-2. Also, a signature of iron K-shell line emission with a centroid of 6.7 keV and an equivalent width of approximately 450 eV was detected. From the pulsation period and the iron-line feature, AX J183220-0840 is likely to be a magnetic white dwarf binary with a complexly absorbed thermal spectrum with a temperature of about 10 keV.Comment: 13 pages, 4 figures, accepted for publication in ApJ Letter

Probing the stellar wind environment of Vela X-1 with MAXI

Vela X-1 is among the best studied and most luminous accreting X-ray pulsars. The supergiant optical companion produces a strong radiatively-driven stellar wind, which is accreted onto the neutron star producing highly variable X-ray emission. A complex phenomenology, due to both gravitational and radiative effects, needs to be taken into account in order to reproduce orbital spectral variations. We have investigated the spectral and light curve properties of the X-ray emission from Vela X-1 along the binary orbit. These studies allow to constrain the stellar wind properties and its perturbations induced by the compact object. We took advantage of the All Sky Monitor MAXI/GSC data to analyze Vela X-1 spectra and light curves. By studying the orbital profiles in the $4-10$ and $10-20$ keV energy bands, we extracted a sample of orbital light curves (${\sim}15$% of the total) showing a dip around the inferior conjunction, i.e., a double-peaked shape. We analyzed orbital phase-averaged and phase-resolved spectra of both the double-peaked and the standard sample. The dip in the double-peaked sample needs $N_H\sim2\times10^{24}\,$cm$^{-2}$ to be explained by absorption solely, which is not observed in our analysis. We show how Thomson scattering from an extended and ionized accretion wake can contribute to the observed dip. Fitted by a cutoff power-law model, the two analyzed samples show orbital modulation of the photon index, hardening by ${\sim}0.3$ around the inferior conjunction, compared to earlier and later phases, hinting a likely inadequacy of this model. On the contrary, including a partial covering component at certain orbital phase bins allows a constant photon index along the orbital phases, indicating a highly inhomogeneous environment. We discuss our results in the framework of possible scenarios.Comment: 10 pages, 9 figures, accepted for publication in A&

Footprints in the wind of Vela X-1 traced with MAXI

The stellar wind around the compact object in luminous wind-accreting high mass X-ray binaries is expected to be strongly ionized with the X-rays coming from the compact object. The stellar wind of hot stars is mostly driven by light absorption in lines of heavier elements, and X-ray photo-ionization significantly reduces the radiative force within the so-called Stroemgren region leading to wind stagnation around the compact object. In close binaries like Vela X-1 this effect might alter the wind structure throughout the system. Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study the observed dependence of the photoelectric absorption as function of orbital phase in Vela X-1, and find that it is inconsistent with expectations for a spherically-symmetric smooth wind. Taking into account previous investigations we develop a simple model for wind structure with a stream-like photoionization wake region of slower and denser wind trailing the neutron star responsible for the observed absorption curve.Comment: 5 pages, 3 figures, accepted in A&

Further evidence that 1RXS J170849.0-400910 is an Anomalous X-ray pulsar

We report the results of two ROSAT HRI observations of the recently discovered 11s X-ray pulsar 1RXS J170849.0-400910. A refined position with a smaller error radius (10" uncertainty) and a new spin period measurement were obtained. These results allowed to derive a period derivative of about 7 times 10^-4 s yr^-1 and to perform a photometric and spectroscopic study of the possible optical counterparts of the source. The limits derived from the optical to X-ray flux ratio exclude the presence of a massive OB companion. These findings, together with the nearly constant X-ray flux, the stability of the pulse shape and pulsed fraction across observations spanning three years, strongly support the inclusion of this 11s pulsar in the class of Anomalous X-ray Pulsars (AXPs).Comment: 4 pages plus 4 postscript figures. emulateapj style. Accepted for publication in Astrophysical Journal Letter

Correlations Between Spectral Properties and Spin-Down Rate in Soft Gamma-Ray Repeaters and Anomalous X-ray Pulsars

Anomalous x-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are x-ray sources with unusual properties distinguishing them from both rotation-powered and most accretion-powered pulsars. Using archival ASCA data over the energy range 0.5-10.0 keV, we have studied the spectra of the persistent emission from these sources and their variation with spin-down rate. Using a single power law spectral model, we find that the overall hardness of the spectra increase with increasing spin-down rate, and therefore the spectral and spin-down mechanism are inextricably linked in these objects. In terms of the two-component blackbody plus power law spectral models, this correlation is seen as an increasing hardness of the high energy component with increasing spin-down rate, with the temperature of the low energy blackbody component remaining essentially constant. Also for the two component spectral model: the ratio of the 2-10 keV power law and bolometric blackbody luminosities gradually increases with the spin-down rate. We discuss these results in terms of the various theoretical models for SGRs and AXPs.Comment: 12 pages with 2 figures. Accepted by ApJ Letter

Emission Spectra of Fallback Disks Around Young Neutron Stars

The nature of the energy source powering anomalous X-ray pulsars is uncertain. Proposed scenarios involve either an ultramagnetized neutron star, or accretion onto a neutron star. We consider the accretion model proposed recently by Chatterjee, Hernquist & Narayan, in which a disk is fed by fallback material following a supernova. We compute the optical, infrared, and submillimeter emission expected from such a disk, including both viscous dissipation and reradiation of X-ray flux impinging on the disk from the pulsar. We find that it is possible with current instruments to put serious constraints on this and on other accretion models of AXPs. Fallback disks could also be found around isolated radio pulsars and we compute the corresponding spectra. We show that the excess emission in the R and I bands observed for the pulsar PSR 0656+14 is broadly consistent with emission from a disk.Comment: 12 pages, 1 table, 4 figures, submitted to Ap

Photon Propagation Around Compact Objects and the Inferred Properties of Thermally Emitting Neutron Stars

Anomalous X-ray pulsars, compact non-pulsing X-ray sources in supernova remnants, and X-ray bursters are three distinct types of sources for which there are viable models that attribute their X-ray emission to thermal emission from the surface of a neutron star. Inferring the surface area of the emitting regions in such systems is crucial in assessing the viability of different models and in providing bounds on the radii of neutron stars. We show that the inferred areas of the emitting regions may be over- or under-estimated by a factor of <=2, because of the geometry of the system and general relativistic light deflection, combined with the effects of phase averaging. Such effects make the determination of neutron-star radii uncertain, especially when compared to the ~5% level required for constraining the equation of state of neutron-star matter. We also note that, for a given spectral shape, the inferred source luminosities and pulse fractions are anticorrelated because they depend on the same properties of the emitting regions, namely their sizes and orientations, i.e., brighter sources have on average weaker pulsation amplitudes than fainter sources. We argue that this property can be used as a diagnostic tool in distinguishing between different spectral models. As an example, we show that the high inferred pulse fraction and brightness of the pulsar RXS J1708-40 are inconsistent with isotropic thermal emission from a neutron-star surface. Finally, we discuss the implication of our results for surveys in the soft X-rays for young, cooling neutron stars in supernova remnants and show that the absence of detectable pulsations from the compact source at the center of Cas A (at a level of >=30%) is not a strong argument againts its identification with a spinning neutron star.Comment: 6 pages, 6 figures, to appear in the Astrophysical Journal; minor change

General Relativistic Constraints on Emission Models of Anomalous X-ray Pulsars

Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray spectra and pulsations by means of radiation processes that occur on the surfaces of neutron stars. For any such model, general relativistic deflection of light severely suppresses the amplitude of the observed pulsations. We calculate the expected pulsation amplitudes of AXPs according to various models and compare the results with observations. We show that the high (<= 70%) pulse amplitudes observed in some AXPs can be accounted for only if the surface emission is localized (spot radius <40 degrees) and strongly beamed (cos^n[theta'] with n>2, where theta' is the angle to the normal). These constraints are incompatible with those cooling and magnetar models in which the observed X-rays originate as thermal emission from the neutron-star surface. Accretion models, on the other hand, are compatible with observations for a wide range of parameters. Finally, definitive conclusions cannot be reached on magnetospheric models, since their localization and beaming properties are not well understood.Comment: 7 pages, 9 figures, submitted to The Astrophysical Journa

Future X-ray timing missions

Thanks to the Rossi X-ray Timing Explorer (RXTE), it is now widely recognized that fast X-ray timing can be used to probe strong gravity fields around collapsed objects and constrain the equation of state of dense matter in neutron stars. We first discuss some of the outstanding issues which could be solved with an X-ray timing mission building on the great successes of RXTE and providing an order of magnitude better sensitivity. Then we briefly describe the 'Experiment for X-ray timing and Relativistic Astrophysics' (EXTRA) recently proposed to the European Space Agency as a follow-up to RXTE and the related US mission 'Relativistic Astrophysics Explorer' (RAE).Comment: To be published in `Proceedings of the Third Microquasar Workshop: Granada Workshop on galactic relativistic jet sources', Eds A. J. Castro-Tirado, J. Greiner and J. M. Paredes, Astrophysics and Space Science, in press. More about EXTRA can be found at: http://www.cesr.fr/~barret/extra.htm