High-Energy Gamma-ray Emission Properties of an Anomalous X-ray Pulsar, 4U 0142+61

arXiv:1111.0954v1 [astro-ph.HE] 3 Nov 2011 2011 Fermi Symposium, Roma., May. 9-12 1 High-Energy Gamma-ray Emission Properties of an Anomalous X-ray Pulsar, 4U 0142+61 S. S¸a¸smaz Mu¸s, E. G¨o˘g¨u¸s Sabancı University, FENS, Orhanlı- Tuzla, 34956, Istanbul, Turkey Anomalous X-ray Pulsars (AXPs) are bright X-ray sources. Few AXPs emit highly pulsed emission in hard X-rays. Using data collected with the Large Area Telescope on board Fermi Gamma-ray Space Telescope, we explored high-energy gamma-ray emission from the brightest AXP, 4U 0142+61. We do not detect any significant emission from the source. Here, we present the upper limits to the persistent and pulsed emission of 4U 0142+61 in the high-energy gamma-ray domain. I. INTRODUCTION Anomalous X-ray Pulsars (AXPs) are bright X-ray emitters; their luminosities (below 10 keV) range from 1033 to 1036 erg s−1 that exceed their spin-down lumi- nosity. Their spin periods are clustered between 2−12 s and spin-down rates are relatively large, i.e., between 10−10−10−13 s s−1 (see [1] for a review). Their obser- vational properties are explained either with a fallback disk [2, 3] or by the decay of their strong magnetic field (magnetar model) [4]. Until the discovery of hard X-ray emission from AXPs 1E 1841-045 [5], 4U 0142+61 [6] and 1RXS J170849.0-400910 [7] they were known to be emitting soft X-rays only (< 10 keV). The origin of hard X-ray emission is still not well understood. Emis- sion from corona [8], breakdown of fast-modes via quantum electrodynamics effects [9, 10] and resonant Compton upscattering of soft photons [11] are pro- posed to explain the hard-X ray emission. Construction of spectral behaviors of these sources in a broad energy band is important for understand- ing the origin of the hard X-ray emission. Here we present the upper limits to the persistent and pulsed high-energy gamma-ray emission from 4U 0142+61 at the GeV range. We estimated an upper limit to the spectral break energy by extending the νFν spectrum of the source to GeV range. II. OBSERVATIONS AND DATA ANALYSIS The LAT observations between 2008 August 4 to 2010 April 29 with an exposure time of ∼31.7 Ms were used to investigate the persistent emission from 4U 0142+61. We obtained and analyzed the data from 15◦radius around the source and also, a 2◦radius region was selected and analyzed in order to avoid contamination from nearby bright sources. Spectral fits and flux calculations were done with the Python version of gtlike, pyLikelihood, for the 0.2−1.0 and 1.0−10.0 GeV energy bands. See [21] for the details of LAT data calibration. To perform timing analysis, all event photon arrival times were extracted from a 2◦region and converted by gtbary to the arrival times at the solar system barycenter. We used 2−10 keV RXTE observations and found the spin ephemeris of the source between 2008 August 4 and 2010 April 30 which also covers the extracted LAT observations. To search for pulsed high-energy gamma-ray emis- sion from 4U 0142+61, first we obtained the spin ephemeris of the source using contemporaneous RXTE/PCA observations in the 2−10 keV range with a total exposure of 196 ks. A Fourier based epoch fold- ing technique was applied to the data to obtain the spin ephemeris. We generated the pulse profiles of the source using three consecutive PCA observations around the epoch (MJD 54713.5) which are grouped such as they are separated at least 0.2 days from each other. We determined the phase shift of each pointing with respect to the template by cross correlating the pulse profiles of each group of pointings with the tem- plate profile and fitted the phase shifts with a polyno- mial. In Table 1 we present the best fit spin ephemeris parameters of 4U 0142+61. We used the precise PCA spin ephemeris that we obtained to search for pulsed high-energy gamma-ray emission from 4U 0142+61. We generated the LAT pulse profiles in the 0.2−1.0 GeV and 1.0−10.0 GeV energy ranges and found that both LAT profiles are consistent with random fluctu- ations with respect to its mean. TABLE I: Spin ephemeris of 4U 0142+61 as determined using RXTE/PCA observations. Parameter Value Range (MJD) 54682.6 −55315.1 Epoch (MJD) 54713.5 ν (Hz) 0.1150900026(9) ˙ν (10−14 Hz s−1) −2.745(8) ¨ν (10−23 Hz s−2) 3.6(3) III. RESULTS After processing the data as explained in §2, we fitted a power-law to the data obtained from 15◦ eConf C110509 2 2011 Fermi Symposium, Roma., May. 9-12 FIG. 1: A wide band νFν spectrum of 4U 0142+61: INTEGRAL/ISGRI (20-300 keV) in black (stars), INTEGRAL/SPI (20-1000 keV) in red (open squares) and CGRO/COMPTEL (0.75-30 MeV) 2σ upper limits in black (data obtained from [12]). Blue diamonds are the Fermi/LAT upper limits in the 0.2−1.0 GeV and 1.0−10.0 GeV obtained using 2◦ extraction region. Red triangles are upper limits for the 15◦extraction region. Dashed line is the best fit power-law model to the ISGRI data points [12]. Solid line shows the power-law upper limit trend of the 2◦Fermi/LAT region. Figure

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