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Register a new userAn Optical Counterpart to the Anomalous X-ray Pulsar 4U 0142+61
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F. Hulleman
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The energy source of the anomalous X-ray pulsars is not well understood, hence their designation as anomalous. Unlike binary X-ray pulsars, no companions are seen, so the energy cannot be supplied by accretion of matter from a companion star. The loss of rotational energy, which powers radio pulsars, is insufficient to power AXPs. Two models are generally considered: accretion from a large disk left over from the birth process, or decay of a very strong magnetic field (10^15 G) associated with a magnetar. The lack of counterparts at other wavelengths has hampered progress in our understanding of these objects. Here, we present deep optical observations of the field around 4U 0142+61, which is the brightest AXP in X-rays. We find an object with peculiar optical colours at the position of the X-ray source, and argue that it is the optical counterpart. The optical emission is too faint to admit the presence of a large accretion disk, but may be consistent with magnetospheric emission from a magnetar.
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We report on a 25 ks observation of the 8.7 s anomalous X-ray pulsar 4U~0142+61 with the High Energy Transmission Grating Spectrometer (HETGS) on the Chandra X-ray Observatory. The continuum spectrum is consistent with previous measurements and is well fit by an absorbed power-law + blackbody with photon index Gamma=3.3+/-0.4 and kT=0.418+/-0.013 keV. No evidence was found for emission or absorption lines, with an upper limit of ~50 eV on the equivalent width of broad features in the 2.5-13 A (0.95-5.0 keV) range and an upper limit of ~10 eV on the equivalent width of narrow features in the 4.1-17.7 A (0.7-3.0 keV) range. If the source is a magnetar, then the absence of a proton cyclotron line strongly constrains magnetar atmosphere models and hence the magnetic field strength of the neutron star. We find no strong features that are indicative of cyclotron absorption for magnetic field strengths of (1.9-9.8)x10^{14} G. This is still consistent with the dipole field strength of B=1.3x10^{14} G (at the polar cap) estimated from the pulsars spindown.
We have searched for pulsation of the anomalous X-ray pulsar (AXP) 4U 0142+61 in the K band ($lambda_{rm eff} = 2.11$ $mu$m) using the fast-readout mode of IRCS at the Subaru 8.2-m telescope. We found no significant signal at the pulse frequency expected by the precise ephemeris obtained by the X-ray monitoring observation with RXTE. Nonetheless, we obtained a best upper limit of 17% (90% C.L.) for the root-mean-square pulse fraction in the K band. Combined with i band pulsation (Dhillon et al. 2005), the slope of the pulsed component ($F_ u propto u^alpha$) was constrained to $alpha > -0.87$ (90% C.L.) for an interstellar extinction of $A_{V} = 3.5$.
We present results obtained from X-ray observations of the anomalous X-ray pulsar (AXP) 4U 0142+61 taken between 2000-2007 using XMM-Newton, Chandra and Swift. In observations taken before 2006, the pulse profile is observed to become more sinusoidal and the pulsed fraction increased with time. These results confirm those derived using the Rossi X-ray Timing Explorer and expand the observed evolution to energies below 2 keV. The XMM-Newton total flux in the 0.5-10 keV band is observed to be nearly constant in observations taken before 2006, while an increase of ~10% is seen afterwards and coincides with the burst activity detected from the source in 2006-2007. After these bursts, the evolution towards more sinusoidal pulse profiles ceased while the pulsed fraction showed a further increase. No evidence for large-scale, long-term changes in the emission as a result of the bursts is seen. The data also suggest a correlation between the flux and hardness of the spectrum, with brighter observations on average having a harder spectrum. As pointed out by other authors, we find that the standard blackbody plus power-law model does not provide the best spectral fit to the emission from 4U 0142+61. We also report on observations taken with the Gemini telescope after two bursts. These observations show source magnitudes consistent with previous measurements. Our results demonstrate the wide range of X-ray variability characteristics seen in AXPs and we discuss them in light of current emission models for these sources.
We report on our Spitzer observations of the anomalous X-ray pulsar 4U 0142+61, made following a large X-ray burst that occurred on 2007 February 7. To search for mid-infrared flux variations, four imaging observations were carried out at 4.5 and 8.0 $mu$m with the Infrared Array Camera from February 14 to 21. No significant flux variations were detected, and the average fluxes were 32.1$pm$2.0 $mu$Jy at 4.5 $mu$m and 59.8$pm8.5$ $mu$Jy at 8.0 $mu$m, consistent with those obtained in 2005. The non-detection of variability is interesting in light of reported rapid variability from this source in the near-infrared, but consistent with the fact that the source already went back to its quiescent state before our observations began, as indicated by contemporaneous X-ray observations. In order to understand the origin of the near-infrared variability, frequent, simultaneous multi-wavelength observations are needed.
The anomalous X-ray pulsar 4U 0142+61 was observed with Suzaku on 2007 August 15 for a net exposure of -100 ks, and was detected in a 0.4 to ~70 keV energy band. The intrinsic pulse period was determined as 8.68878 pm 0.00005 s, in agreement with an extrapolation from previous measurements. The broadband Suzaku spectra enabled a first simultaneous and accurate measurement of the soft and hard components of this object by a single satellite. The former can be reproduced by two blackbodies, or slightly better by a resonant cyclotron scattering model. The hard component can be approximated by a power-law of photon index Gamma h ~0.9 when the soft component is represented by the resonant cyclotron scattering model, and its high-energy cutoff is constrained as >180 keV. Assuming an isotropic emission at a distance of 3.6 kpc, the unabsorbed 1-10 keV and 10-70 keV luminosities of the soft and hard components are calculated as 2.8e+35 erg s^{-1} and 6.8e+34 erg s^{-1}, respectively. Their sum becomes ~10^3 times as large as the estimated spin-down luminosity. On a time scale of 30 ks, the hard component exhibited evidence of variations either in its normalization or pulse shape.
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