When a standard candle flickers crab nebula variations in hard x rays
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When a Standard Candle Flickers: Crab Nebula Variations in Hard X-rays. Colleen A. Wilson-Hodge (NASA/MSFC). Collaborators.

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When a standard candle flickers crab nebula variations in hard x rays

When a Standard Candle Flickers: Crab Nebula Variations in Hard X-rays

Colleen A. Wilson-Hodge (NASA/MSFC)


Collaborators Hard X-rays

G.L. Case, M.L. Cherry (LSU), W.H. Baumgartner (NASA/CRESST), E. Beklen (METU/SDU), P.N Bhat, M.S. Briggs (UAH), A. Camero-Arranz (NSSTC), G.L. Case (LSU), V. Chaplin, V. Connaughton (UAH), M.H. Finger (USRA), N. Gehrels (NASA/GSFC), J. Greiner (MPE), K. Jahoda (NASA/GSFC), P. Jenke (NPP/NASA/MSFC), R.M. Kippen (LANL), C. Kouveliotou (NASA/MSFC), H. A. Krimm (CRESST/NASA/GSFC/USRA), E. Kuulkers (ESA/ESAC), C.A. Meegan (USRA), L. Natalucci (INAF-IASF), W.S. Paciesas (USRA), R. Preece (UAH), J.C. Rodi (LSU), N. Shaposhnikov, G.K. Skinner (UMD/CRESST/NASA/GSFC), D. Swartz (USRA), A. von Kienlin, R.Diehl, X. Zhang (MPE)

MAXI data from http://maxi.riken.jp

The crab nebula 1999 2011
The Crab Nebula 1999-2011 Hard X-rays



Wilson-Hodge et al. 2011, ApJ, 727, L40;

Wilson-Hodge et al. 2011, PoS(HTRS 2011), 043

Light curves for each instrument are normalized to its average rate from MJD 54690-54790.

RXTE/PCU2 - Black

BAT - Red


JEM X2 - orange

SPI - Light blue

GBM - Blue squares

Instruments on four separate spacecraft show ~7% decline in Crab (nebula+pulsar) flux from summer 2008 to summer 2010.

Crab pcu2 spectra

Each Individual observation fitted Hard X-rays

PCU2 layer 2&3 data

Absorbed Power-law

Nh fixed 0.971022 cm-2

Increase in spectral index

Crab PCU2 Spectra

Rxte pca spectra

Averaged fit results in 100-day bins Hard X-rays

Colors denote “rising”, “declining” and “flat” intervals.

Photon index softens from 2.15 to 2.17 during 2008-2010 flux decline

8-sigma effect

RXTE PCA Spectra

Comparing rising declining and flat flux intervals

Time Hard X-rays


Comparing “rising”, “declining”, and “flat” flux intervals

  • Photon index softened from 2.14 to 2.17

  • Softening occurring in declining phases

  • Hardening during initial rise

  • Similar results in PCU 3 & 4

Evidence for softening in swift bat
Evidence for Softening in Swift/BAT Hard X-rays

  • Color scheme matches RXTE Softening during 2008-2010 decline

  • Earlier intervals consistent with constant hardness

  • Hardness ratios 14-50 keV/50-100 keV BAT 58-month survey data

  • 50-day averages

Spectral softening in gbm

55297-55343 Hard X-rays


Spectral Softening in GBM

  • GBM 8-1000 keV Earth occultation measurements

  • Beginning and end of decline interval

  • Spectral index increases from 2.11(1) to 2.17(2)

Rxte mission long light curves

Very active period 2001-2010 Hard X-rays

December 2011 flux is at or below level before 2001.

Larger variations in 15-50 keV band

3 PCUs, layers 2&3

Count rates corrected for dead time and response

RXTE Mission-long Light Curves


15-50 keV

Recent data 2008 2012
Recent Data 2008-2012 Hard X-rays

<15 keV

15-50 keV

20 year crab nebula light curve
20-year Crab Nebula Light Curve Hard X-rays

15-50 keV

50-100 keV

Summary conclusions
Summary & Conclusions Hard X-rays

  • The Crab Nebula was surprisingly variable from 2001-2010, with less variability before 2001 and since mid-2010.

  • We presented evidence for spectral softening from RXTE, Swift/BAT, and Fermi GBM during the mid-2008-2010 flux decline.

  • We will miss RXTE, but will continue our monitoring program using Fermi/GBM, MAXI, and Swift/BAT.

  • Thank you RXTE team, especially Jean, Tod, Keith, Nikolai, Evan, & Divya!