Anomalous X-ray Pulsar XTE J1810-197 Research Articles

Simultaneous multifrequency single-pulse properties of AXP XTE J1810-197

We have used the 76-m Lovell, 94-m equivalent Westerbork Synthesis Radio Telescope (WSRT) and 100-m Effelsberg radio telescopes to investigate the simultaneous single-pulse properties of the radio emitting magnetar Anomalous X-ray Pulsar (AXP) XTE J1810-197 at frequencies of 1.4, 4.8 and 8.35 GHz during 2006 May and July. We study the magnetar's pulse-energy distributions which are found to be very peculiar as they are changing on time-scales of days and cannot be fit by a single statistical model. The magnetar exhibits strong spiky single giant-pulse-like subpulses, but they do not fit the definition of the giant pulse or giant micropulse phenomena. Measurements of the longitude-resolved modulation index reveal a high degree of intensity fluctuations on day-to-day time-scales and dramatic changes across pulse phase. We find the frequency evolution of the modulation index values differs significantly from what is observed in normal radio pulsars. We find that no regular drifting subpulse phenomenon is present at any of the observed frequencies at any observing epoch. However, we find a quasi-periodicity of the subpulses present in the majority of the observing sessions. A correlation analysis indicates a relationship between components from different frequencies. We discuss the results of our analysis in light of the emission properties of normal radio pulsars and a recently proposed model which takes radio emission from magnetars into consideration.

Neutral Hydrogen Absorption toward XTE J1810−197: The Distance to a Radio‐emitting Magnetar

We have used the Green Bank Telescope to measure HI absorption against the anomalous X-ray pulsar XTE J1810-197. Assuming a flat rotation curve, we find that XTE J1810-197 is located at a distance of 3.4(+0.5,-0.7) kpc. For a rotation curve that incorporates a model of the Galactic bar, we obtain a distance of 4.0(+0.3,-0.8) kpc. Using a rotation curve that incorporates a model of the Galactic bar and the spiral arms of the Galaxy, the distance is 3.7(+/-0.6) kpc. These values are consistent with the distance to XTE J1810-197 of about 3.3 kpc derived from its dispersion measure, and estimates of 2-5 kpc obtained from fits to its X-ray spectra. Overall, we determine that XTE J1810-197 is located at a distance of 3.5(+/-0.5) kpc, possibly not far in front of the infrared dark cloud G10.74-0.13. We also used the GBT in an attempt to measure absorption in the OH 1612, 1665, 1667, and 1720 MHz lines against XTE J1810-197. We were unsuccessful in this, mainly because of its declining radio flux density. Analysis of HI 21 cm, OH, and CO(2-1) emission toward XTE J1810-197 allows us to place a lower limit of $N_{\rm H} \ga 4.6 \times 10^{21}$ cm$^{-2}$ on the non-ionized hydrogen column density to \magnetar, consistent with estimates obtained from fits to its X-ray spectra.

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.

The Variable Radio–to–X‐Ray Spectrum of the Magnetar XTE J1810−197

We have observed the 5.54 s anomalous X-ray pulsar XTE J1810-197 at radio, millimeter, and infrared (IR) wavelengths, with the aim of learning about its broadband spectrum. At the IRAM 30 m telescope, we have detected the magnetar at ν = 88 and 144 GHz, the highest radio-frequency emission ever seen from a pulsar. At 88 GHz we detected numerous individual pulses, with typical widths ~2 ms and peak flux densities up to 45 Jy. Together with nearly contemporaneous observations with the Parkes, Nancay, and Green Bank telescopes, we find that in late 2006 July the spectral index of the pulsar was -0.5 α 0 (with flux density Sν ∝ να) over the range 1.4-144 GHz. Nine dual-frequency Very Large Array and Australia Telescope Compact Array observations in 2006 May-September are consistent with this finding, while showing variability of α with time. We infer from the IRAM observations that XTE J1810-197 remains highly linearly polarized at millimeter wavelengths. Also, toward this pulsar, the transition frequency between strong and weak scattering in the interstellar medium may be near 50 GHz. At Gemini, we detected the pulsar at 2.2 μm in 2006 September, at the faintest level yet observed, Ks = 21.89 ± 0.15. We have also analyzed four archival IR Very Large Telescope observations (two unpublished), finding that the brightness fluctuated within a factor of 2-3 over a span of 3 years, unlike the monotonic decay of the X-ray flux. Thus, there is no correlation between IR and X-ray flux, and it remains uncertain whether there is any correlation between IR and radio flux.

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.

The Magnetar XTE J1810−197: Variations in Torque, Radio Flux Density, and Pulse Profile Morphology

We report on 9 months of observations of the radio-emitting anomalous X-ray pulsar XTE J1810-197 starting in 2006 May using the Nancay, Parkes, GBT, and VLA telescopes mainly at a frequency of 1.4 GHz. The torque experienced by the neutron star during this period, as inferred from a measurement of its rotational frequency derivative, decreased by 60%, although not in a steady manner. We have also observed very large ongoing fluctuations in flux density and pulse shape. Superimposed on these, a general diminution of flux density and a broadening of the pulse profile components occurred nearly contemporaneously with a decrease in torque of about 10% that took place in late 2006 July over an interval of 2 weeks. After a slight increase in average flux density, since 2006 October the flux density has continued to decline and the pulse profiles, while still varying, appear more uniform. In addition, a simultaneous observation of the pulsar with the Chandra X-ray Observatory and the GBT allows us to show how the X-ray and radio profiles are aligned. We discuss briefly the implications of these results for the magnetospheric currents in this remarkable object.

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.

Polarized Radio Emission from the Magnetar XTE J1810-197

We have used the Parkes radio telescope to study the polarized emission from the anomalous X-ray pulsar XTE J1810-197 at frequencies of 1.4, 3.2, and 8.4 GHz. We find that the pulsed emission is nearly 100% linearly polarized. The position angle of linear polarization varies gently across the observed pulse profiles, varying little with observing frequency or time, even as the pulse profiles have changed dramatically over a period of 7 months. In the context of the standard pulsar rotating vector model, there are two possible interpretations of the observed position angle swing coupled with the wide profile. In the first, the magnetic and rotation axes are substantially misaligned and the emission originates high in the magnetosphere, as seen for other young radio pulsars, and the beaming fraction is large. In the second interpretation, the magnetic and rotation axes are nearly aligned, and the line of sight remains in the emission zone over almost the entire pulse phase. We deprecate this possibility because of the observed large modulation of thermal X-ray flux. We have also measured the Faraday rotation caused by the Galactic magnetic field, RM = +77 rad m-2, implying an average magnetic field component along the line of sight of 0.5 μG.

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.

Accurate X-Ray Position of the Anomalous X-Ray Pulsar XTE J1810-197 and Identification of Its Likely Infrared Counterpart

We report the accurate subarcsecond X-ray position of the new anomalous X-ray pulsar (AXP) XTE J1810-197, derived with a Chandra High Resolution Camera Target of Opportunity observation carried out in 2003 November. We also report the discovery of a likely IR counterpart based on a Very Large Telescope (IR band) Target of Opportunity observation carried out in 2003 October. Our proposed counterpart is the only IR source (Ks = 20.8) in the X-ray error circle. Its IR colors as well as the X-ray/IR flux ratio are consistent with those of the counterparts of all other AXPs (at variance with field star colors). Deep Gunn i-band images obtained at the 3.6 m ESO telescope detected no sources down to a limiting magnitude of 24.3. Moreover, we find that the pulsed fraction and count rates of XTE J1810-197 remained nearly unchanged since the previous Chandra and XMM-Newton observations (2003 August 27 and September 8, respectively). We briefly discuss the implications of these results. In particular, we note that the transient (or at least highly variable) nature of this AXP might imply a relatively large number of hidden members of this class.