Soft and Hard X-Ray Emissions from the Anomalous X-ray Pulsar 4U 0142+61 Observed with Suzaku

arXiv:1102.1213v1 [astro-ph.HE] 7 Feb 2011 Soft and Hard X-Ray Emissions from the Anomalous X-ray Pulsar 4U 0142+61 Observed with Suzaku T. Enoto1,2, K. Makishima1,3, K. Nakazawa1, M. Kokubun4, M. Kawaharada4 J. Kotoku,5, and N. Shibazaki6 enoto@stanford.edu 1 Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan 2 Current address: Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and SLAC National Accelerator Laboratory, Stanford University, Stanford, CA 94305, USA 3 High Energy Astrophysics Laboratory, Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan 4 Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan 5 Department of Radiological Technology, Faculty of Medical Technology, Teikyo University 2-11-1 Kaga, Itabashi-ku, Tokyo 173-8605, Japan 6 Department of Physics, Faculty of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan (Received ; accepted ) Abstract 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±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 com- ponents 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 Γh ∼0.9 when the soft component is represented by the resonant cyclotron scattering model, and its high-energy cutoffis 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 2.8 × 1035 erg s−1 and 6.8 × 1034 erg s−1, 1 respectively. Their sum becomes ∼103 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. Key words: magnetic fields — neutron stars: individual (4U 0142+61)— X-rays: general, individual (4U 0142+61) 1. Introduction Anomalous X-ray pulsars (AXPs), comprising at present ∼9 objects discovered in the local universe, are characterized by rotational periods in the range P = 5−12 s, and spin down rates as ˙P ∼10−11 s s−1. Together with soft gamma repeaters (SGRs), AXPs are thought to form a subgroup of neutron stars called “magnetars”, of which the surface magnetic field strengths are believed to reach ∼1014−15 G (Thompson & Duncan 1995; Thompson & Duncan 1996; Woods & Thompson 2006). In energies below ∼10 keV, AXPs exhibit spectra which are much softer than those of ordinary binary X-ray pulsars, while harder than those of isolated neutron stars. Their X-ray luminosities (1034−35 erg s−1), which often exceed up to two order of magnitude those available from their spin-down power (∼1032 erg s−1), are usually thought to be sustained by a release of energies stored in their ultra-strong magnetic fields (Duncan & Thompson 1992; Thompson & Duncan 1995). A novel observational window onto magnetars has been opened by the INTEGRAL discoveries of a distinct pulsed hard X-ray component from at least 3 AXPs and 2 SGRs (Kuiper et al. 2004; Mereghetti et al. 2005; den Hartog et al. 2006; Kuiper et al. 2006; G¨otz et al. 2007). This component extends to ∼100 keV or more with a very flat photon index of Γ ∼1, and exhibits a luminosity comparable to or higher than that in the softer X-ray band. Although theoretical accounts for this enigmatic component are far from settled (e.g., Heyl & Hernquist 2005; Beloborodov & Thompson 2007; Baring & Harding 2007), its near-absence in other types of X-ray sources suggests its close relation to the proposed strong magnetic fields of magnetars. The study of magnetars now requires a broad energy band, because their soft and hard components, which are generally variable, must be measured simultaneously and accu- rately. This makes Suzaku (Mitsuda et al. 2007) an ideal observatory. In fact, the X-ray Imaging Spectrometer (XIS), operating in the 0.2–12 keV range (Koyama et al. 2007), provides high-quality data of their soft components. Simultaneously, the Hard X-ray Detector (HXD; Takahashi et al. 2007; Kokubun et al. 2007), consisting of HXD-PIN (10–70 keV) and HXD- GSO (40–600 keV), can conduct detailed spectroscopic studies of their hard components in considerably shorter exposures than are needed by INTEGRAL. Such wide-band spectroscopic observations with Suzaku have allowed detections of the 2 hard-tail component from activated magnetars, including SGR 0501+4516 (Enoto et al. 2009; Rea et al. 2009; Enoto et al. 2010b) and 1E

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