Transient Anomalous X-ray Pulsar XTE Research Articles

Constraints on the Emission and Viewing Geometry of the Transient Anomalous X‐Ray Pulsar XTE J1810−197

The temporal decay of the flux components of Transient Anomalous X-ray Pulsar XTE J1810-197 following its 2002 outburst presents a unique opportunity to probe the emission geometry of a magnetar. Toward this goal, we model the magnitude of the pulsar's modulation in narrow spectral bands over time. Following previous work, we assume that the post-outburst flux is produced in two distinct thermal components arising from a hot spot and a warm concentric ring. We include general relativistic effects on the blackbody spectra due to gravitational redshift and light bending near the stellar surface, which strongly depend on radius. This affects the model fits for the temperature and size of the emission regions. For the hot spot, the observed temporal and energy-dependent pulse modulation is found to require an anisotropic, pencil-beamed radiation pattern. We are able to constrain an allowed range for the angles that the line-of-sight (psi) and the hot spot pole (xi) make with respect to the spin-axis. Within errors, this is defined by the locus of points in the xi-psi plane that lie along the line (xi+beta(R))(psi+beta(R)) ~ constant, where beta(R) is a function of the radius R of the star. For a canonical value of R=12 km, the viewing parameters range from psi=xi=37 deg to (psi,xi)=(85 deg,15 deg). We discuss our results in the context of magnetar emission models.

Outburst of the 2 s Anomalous X‐Ray Pulsar 1E 1547.0−5408

Following our discovery of radio pulsations from the newly recognized anomalous X-ray pulsar (AXP) 1E 1547.0–5408, we initiated X-ray monitoring with the Swift X-ray telescope and obtained a single target-of-opportunity observation with the Newton X-ray Multi-Mirror Mission (XMM-Newton). In comparison with its historic minimum flux of 3 × 10−13 ergs cm−2 s−1, the source was found to be in a record high state, fX(1–8 keV) = 5 × 10−12 ergs cm−2 s−1, or LX = 1.7 × 1035(d/9 kpc)2 ergs s−1, and declining by 25% in 1 month. Extrapolating the decay, we bound the total energy in this outburst to 1042 ergs < E < 1043 ergs. The spectra (fitted with a Comptonized blackbody) show that an increase in the temperature and area of a hot region, to 0.5 keV and ~16% of the surface area of the neutron star, respectively, are primarily responsible for its increase in luminosity. The energy, spectrum, and timescale of decay are consistent with a deep crustal heating event, similar to an interpretation of the X-ray turn-on of the transient AXP XTE J1810–197. Simultaneous with the 4.6 hr XMM-Newton observation, we observed at 6.4 GHz with the Parkes telescope, measuring the phase relationship of the radio and X-ray pulse. The X-ray pulsed fraction of 1E 1547.0–5408 is only ~7%, while its radio pulse is relatively broad for such a slow pulsar, which may indicate a nearly aligned rotator. As also inferred from the transient behavior of XTE J1810–197, the only other AXP known to emit in the radio, the magnetic field rearrangement responsible for this X-ray outburst of 1E 1547.0–5408 is probably the cause of its radio turn-on.

A Study of the Pulsed Radio Emission from XTE J1810−197

We present observations of pulsed radio emission from the transient anomalous X-ray pulsar XTE J1810-197. Data were obtained over a period of 100 days beginning in 2006 May using a baseband recording system at the 26 m Mount Pleasant radio telescope operated by the University of Tasmania. Pulsed emission was detected in three frequency bands centered on 1.4, 4.8, and 8.4 GHz. The mean flux density varied significantly within all bands, but we obtained fewer detections toward the end of the observing period, and the monitoring program was eventually abandoned. Most interestingly, we observed changes in the pulse-to-pulse emission characteristics. The variance of the flux in 10 s integrations changed significantly during the span of our observations, but we did not have sufficient data to identify a clear secular trend. In addition, timing measurements confirmed that the neutron star's spin period derivative changed significantly on or around MJD 53935.

Chandra monitoring of the candidate anomalous X-ray pulsar AX J1845.0-0258

The population of clearly identified anomalous X-ray pulsars has recently grown to seven, however, one candidate anomalous X-ray pulsar (AXP) still eludes re-confirmation. Here, we present a set of seven Chandra ACIS-S observations of the transient pulsar AX J1845.0-0258, obtained during 2003. Our observations reveal a faint X-ray point source within the ASCA error circle of AX J1845.0-0258’s discovery, which we designate CXOU J184454.6-025653 and tentatively identify as the quiescent AXP. Its spectrum is well described by an absorbed single-component blackbody (kT∼2.0 keV) or power law (Γ∼1.0) that is steady in flux on timescales of at least months, but fainter than AX J1845.0-0258 was during its 1993 period of X-ray enhancement by at least a factor of 13. Compared to the outburst spectrum of AX J1845.0-0258, CXOU J184454.6-025653 is considerably harder: if truly the counterpart, then its spectral behavior is contrary to that seen in the established transient AXP XTE J1810-197, which softened from kT∼0.67 keV to ∼0.18 keV in quiescence. This unexpected result prompts us to examine the possibility that we have observed an unrelated source, and we discuss the implications for AXPs, and magnetars in general.

The anatomy of a magnetar: XMM monitoring of the transient anomalous X-ray pulsar XTE J1810–197

We present the latest results from a multi-epoch timing and spectral study of the Transient Anomalous X-ray Pulsar XTE J1810–197. We have acquired seven observations of this pulsar with the Newton X-ray Multi-mirror Mission (XMM-Newton) over the course of two and a half years, to follow the spectral evolution as the source fades from outburst. The spectrum is arguably best characterized by a two-temperature blackbody whose luminosities are decreasing exponentially with τ 1=870 d and τ 2=280 d, respectively. The temperatures of these components are currently cooling at a rate of 22% per year from a nearly constant value recorded at earlier epochs of kT 1=0.25 keV and kT 2=0.67 keV, respectively. The new data show that the temperature T 1 and luminosity of that component have nearly returned to their historic quiescent levels and that its pulsed fraction, which has steadily decreased with time, is now consistent with the previous lack of detected pulsations in quiescence. We also summarize the detections of radio emission from XTE J1810–197, the first confirmed for any AXP. We consider possible models for the emission geometry and mechanisms of XTE J1810–197.

The Spectral Evolution of Transient Anomalous X‐Ray Pulsar XTE J1810−197

We present a multiepoch spectral study of the transient anomalous X-ray pulsar XTE J1810-197 obtained with the X-Ray Multi-Mirror Mission (XMM-Newton). Four observations taken over the course of a year reveal strong spectral evolution as the source fades from outburst. The origin of this is traced to the individual decay rates of the pulsar's spectral components. A two-temperature fit at each epoch requires that the temperatures remain nearly constant at kT1 = 0.25 keV and kT2 = 0.67 keV, while the luminosities of these components decrease exponentially with τ1 = 900 and τ2 = 300 days, respectively. The integrated outburst energy is E1 = 1.3 × 1042d ergs and E2 = 3.9 × 1042d ergs for the two spectral components, respectively. One possible interpretation of the XMM-Newton observations is that the slowly decaying cooler component is the radiation from a deep heating event that affected a large fraction of the crust, while the hotter component is powered by external surface heating at the footpoints of twisted magnetic field lines by magnetospheric currents that are decaying more rapidly. The energy-dependent pulse profile of XTE J1810-197 is well modeled at all epochs by the sum of a broad pulse that dominates in the soft X-rays and a narrower one at higher energies. These profiles peak at the same phase, suggesting a concentric emission geometry on the neutron star surface. The spectral and pulse evolution together argue against the presence of a significant power-law contribution to the X-ray spectrum below 8 keV. The extrapolated flux is projected to return to the historic quiescent level, characterized by an even cooler blackbody spectrum, by the year 2007.

The Fading of Transient Anomalous X‐Ray Pulsar XTE J1810−197

Three observations of the 5.54 s transient anomalous X-ray pulsar XTE J1810-197 obtained over 6 months with the Newton X-ray Multi-Mirror (XMM-Newton) mission are used to study its spectrum and pulsed light curve as the source fades from outburst. The decay is consistent with an exponential of time constant ~300 days but not a power law as predicted in some models of sudden deep crustal heating events. All spectra are well fitted by a blackbody plus a steep power law, a problematic model that is commonly fitted to anomalous X-ray pulsars (AXPs). A two-temperature blackbody fit is also acceptable and better motivated physically in view of the faint optical/IR fluxes, the X-ray pulse shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting areas that are less than or equal to the surface area of a neutron star. The fitted temperatures remained the same while the flux declined by 46%, which can be interpreted as a decrease in area of the emitting regions. The pulsar continues to spin down, albeit at a reduced rate of (5.1 ± 1.6) × 10-12 s s-1. The inferred characteristic age τc ≡ P/2 ≈ 17,000 yr, magnetic field strength Bs ≈ 1.7 × 1014 G, and outburst properties are consistent with both the outburst and quiescent X-ray luminosities being powered by magnetic field decay, i.e., XTE J1810-197 is a magnetar.

Correlated Infrared and X-ray variability of the transient Anomalous X-ray Pulsar XTE J1810-197

We report on observations aimed at searching for flux variations from the proposed IR counterpart of the Anomalous X-ray Pulsar (AXP) XTE J1810−197. These data, obtained in March 2004 with the adaptive optics camera NAOS-CONICA at the ESO VLT, show that the candidate proposed by Israel et al. (2004a, ApJ, 603, L97) was fainter by ∆H = 0.7 ± 0.2 and ∆Ks = 0.5 ± 0.1 with respect to October 2003, confirming it as the IR counterpart of XTE J1810−197. We also report on an XMM-Newton observation carried out the day before the VLT observations. The 0.5−10 keV absorbed flux of the source was 2.2 × 10 −11 erg cm −2 s −1 , which is less by a factor of about two compared to the previous XMM-Newton observation on September 2003. Therefore, we conclude that a similar flux decrease took place in the X-ray and IR bands. We briefly discuss these results in the framework of the proposed mechanism(s) responsible for the IR variable emission of AXPs.