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

Abstract

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, the 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 in 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. The spectral changes are 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 flux variation of 6.3% and spectral variation time-scale of a few days, multi components of the broken power-law-shaped synchrotron emission with different cooling times are suggested.

Alternative Explanations for Extreme Supersolar Iron Abundances Inferred from the Energy Spectrum of Cygnus X-1

“…For PIN, we further subtracted the cosmic X-ray background (CXB) based on previous observations (Gruber et al 1999). We used the PIN spectrum from 15-55 keV except for the 40-45 keV range due to the Gd K line structure (see Kouzu et al 2013). For GSO, we used data in the 82-192 keV band, with the upper end of the range being set by the statistical quality of the spectrum.…”

Pulsar-Wind Nebulae

“…Consequently, the lack of information about the site of the flare in the PWN has led to a variety of models being suggested (see § 3). Given the lack of contemporaneous variability at lower frequencies, it might be possible that some of the energy released during the process associated with the γ-ray flares will manifest itself as more gradual flux changes at lower frequencies 4 occurring on much longer timescales (e.g., hard X-ray variability reported by Kouzu et al 2013;Wilson-Hodge et al 2011).…”

Crab Flares Due to Turbulent Dissipation of the Pulsar Striped Wind

“…Discovery of γ-ray flares from the Crab Nebula is among the foremost contributions to recent high-energy astrophysics. While it has now been five years since their announcement in 2011 (Tavani et al 2011;Abdo et al 2011), no longer wavelength counterparts have been established (Bietenholz et al 2014;Kouzu et al 2013;Bietenholz et al 2014;Rudy et al 2015;Madsen et al 2015), and so the active region within the nebula remains unlocalized. The flares also present a formidable theoretical challenge because their power and duration cannot be accounted for within conventional theories of charged particle acceleration (e.g.…”