Spectral Variation of Hard X-Ray Emission from the Crab Nebula with the Suzaku Hard X-Ray Detector

The Crab Nebula is one of the brightest and most stable sources in the X-ray sky. Year-scale flux variation from the object was recently revealed in the hard X-ray band by four satellites. This marked the first detection of year-scale variability from pulsar wind nebulae in the hard X-ray band. The Crab Nebula has been observed at least once a year for calibration purposes with the Suzaku Hard X-ray Detector (HXD) since its launch in 2005. In order to investigate possible spectral changes as well as flux variation, archival data of the HXD were analyzed. The flux variation reported by other instruments was confirmed in the 25–100 keV band by the HXD at a few percent level, but flux above 100 keV did not follow the trend in variation below 100 keV. The hardness ratios produced utilizing the PIN and GSO sensors installed in the HXD exhibit significant scattering, thereby indicating spectral variations in the hard X-ray band. The spectral changes were quantified by spectral fitting with a broken power-law model. The difference between the two photon indexes of the broken power-law model in harder and softer energy bands is in the range of < 2.54. Taking into account a flux variation of 6.3% and a spectral variation time-scale of a few days, multi components of the broken power-law-shaped synchrotron emission with different cooling times are suggested.

[1]  J. Wheaton.  GAMMA-RAY SPECTRA AND VARIABILITY OF THE CRAB NEBULA EMISSION OBSERVED BY BATSE , 2003, astro-ph/0307508.

[2]  Kazuhiro Nakazawa,et al.  On-orbit calibration status of the hard x-ray detector (HXD) onboard Suzaku , 2010, Astronomical Telescopes + Instrumentation.

[3]  Arnold H. Rots,et al.  Absolute Timing of the Crab Pulsar with the Rossi X-Ray Timing Explorer , 2004 .

[4]  K. Makishima,et al.  Improvements in Calibration of GSO Scintillators in the Suzaku Hard X-Ray Detector , 2011, 1107.4857.

[5]  I. Grenier,et al.  Polarization of the Crab Pulsar and Nebula as Observed by the INTEGRAL/IBIS Telescope , 2008, 0809.1292.

[6]  R. S. Wolff,et al.  Measurement of the X-ray polarization of the Crab Nebula , 1976 .

[7]  W. Hermsen,et al.  Absolute timing with IBIS, SPI and JEM-X aboard INTEGRAL. Crab main-pulse arrival times in radio, X- , 2003, astro-ph/0309178.

[8]  B. Aschenbach,et al.  Temporal Variability of the X-Ray Emission of the Crab Nebula Torus , 1998, astro-ph/9808208.

[9]  D. Palmer,et al.  BATSE observations of gamma-ray burst spectra. I: Spectral diversity , 1993 .

[10]  P. Giommi,et al.  Log-parabolic spectra and particle acceleration in the BL Lac object Mkn 421: Spectral analysis of the complete BeppoSAX wide band X-ray data set , 2003, astro-ph/0312260.

[11]  Yoshitaka Ishisaki,et al.  In-Orbit Timing Calibration of the Hard X-Ray Detector on Board Suzaku , 2008 .

[12]  J. Duyvendak,et al.  FURTHER DATA BEARING ON THE IDENTIFICATION OF THE CRAB NEBULA WITH THE SUPERNOVA OF 1054 A.D. PART I. THE ANCIENT ORIENTAL CHRONICLES , 1942 .

[13]  Tuneyoshi Kamae,et al.  Spatially dependent response of thick and large area p-i-n diode for ASTRO-E hard X-ray detector , 2001 .

[14]  Laurence E. Peterson,et al.  A scanning modulation collimator observation of the high-energy X-ray source in the Crab Nebula , 1987 .

[15]  F. Camilo,et al.  Young Neutron Stars and Their Environments , 2004 .

[16]  Roland Diehl,et al.  WHEN A STANDARD CANDLE FLICKERS , 2010, 1010.2679.

[17]  Naoki Isobe,et al.  The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images , 2009, 0906.0631.

[18]  Koji Mori,et al.  Hubble Space Telescope and Chandra Monitoring of the Crab Synchrotron Nebula , 2002 .

[19]  D. Yonetoku,et al.  Development of a Monte Carlo Simulator for the Astro-E2 hard X-ray detector (HXD-II) , 2005, IEEE Transactions on Nuclear Science.

[20]  J. Roques,et al.  THE HIGH-ENERGY EMISSION OF THE CRAB NEBULA FROM 20 keV TO 6 MeV WITH INTEGRAL SPI , 2009, 0909.3437.

[21]  T. Tanaka,et al.  In-Orbit Performance of the Hard X-Ray Detector on Board Suzaku , 2006, astro-ph/0611233.

[22]  D. N. Burrows,et al.  Spatial Variation of the X-Ray Spectrum of the Crab Nebula , 2004 .

[23]  T. Piran,et al.  Spectra and Light Curves of Gamma-Ray Burst Afterglows , 1997, astro-ph/9712005.