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Swift XRT Observations of AXP 4U 0142+61: Evidence for a Modified Pulse Profile

ATEL # 1001; S. K. Patel (NASA/MSFC), M. H. Finger (USRA), S. Campana (OA Brera), E. Gogus (Sabanci U.), C. Kouveliotou (NASA/MSFC)
on 12 Feb 2007; 20:06 UT
Password Certification: S. K. Patel (sandy.patel@nasa.gov)

Subjects: X-ray, Neutron Stars, Pulsars

The anomalous X-ray pulsar 4U 0142+61 was observed with Swift XRT again on 10 Feb 2007 (MJD 54141) in Window Timing mode with 1.766 ms time resolution. At the observed XRT total count rate (5.0 c/s) pile-up was negligible. We find a good fit (chi^2/dof=291.2/272) to the data using an absorbed blackbody plus power law model with N_H=(0.90+/-0.08 ) x 1e22 cm^-2, PL index= 3.8 +/- 0.2 and kT_BB = 0.38 +/- 0.02 (errors are at 90% CL). These results are consistent with our earlier report (Campana et al, 2007, ATEL #998). We find a total unabsorbed flux (2-10 keV) of 7.9 (-0.7,+0.2) x 1e-11 erg s^-1 cm^2 and determine that 31% of the flux is from the thermal black body component.

We determine a pulse frequency (using a Z^2,m=2 test) of 0.1150923(7) Hz (P = 8.68868(5) s), consistent with the extrapolation of the Dib et al. (2006, astro-ph/0610932) ephemeris, which was based in RXTE data from MJD 51610-53787. We find a pulse fraction of 14+/-3 % ([Fmax-Fmin]/[Fmax+Fmin]) using the full XRT energy range (~0.2-10 kev). The pulse profile has two peaks with the second peak slightly larger then the first, and with the dip preceding the first peak deeper than the one following it. For comparison we analyzed the 4U 0142+61 observation made with Swift XRT on 2 July 2006. We find that the dip between the first and second peak in the current observation is significantly deeper then the previous observation, which is counter to the trend reported by Dib et al. of this dip having grown shallower since 2000 in the 2-4 keV band. Since the XRT efficiently probes softer energies below the useful lower limit of RXTE (~2 keV), a direct comparison of the profiles is not possible.


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