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

Wilson-Hodge, Colleen A.

NASA Technical Reports Server

When a Standard Candle Flickers: 
Crab Nebula Variations in Hard X-rays
Colleen A. Wilson-Hodge 
(NASA/MSFC)
https://ntrs.nasa.gov/search.jsp?R=20120014490 2019-08-30T22:07:06+00:00Z
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)
Collaborators
MAXI data from http://maxi.riken.jp
• Light curves for each 
instrument are 
normalized to its 
average rate from MJD 
54690-54790.
• RXTE/PCU2 - Black
• BAT - Red
• IBIS/ISGRI - Green
• 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.
The Crab Nebula 1999-2011
Wilson-Hodge et al. 2011, ApJ, 727, L40;
Wilson-Hodge et al. 2011, PoS(HTRS 2011), 043
1999 2011
RXTE PCA Spectra
• Colors denote “rising”, 
“declining” and “flat” 
intervals.
• Photon index softens 
from 2.15 to 2.17 during 
2008-2010 flux decline
• Individual observations 
fitted, results averaged
• PCU2 layer 2&3 data 
• Absorbed Power-law
• Nh fixed 0.971022 cm-2
Comparing “rising”, “declining”, and 
“flat” 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
• Color scheme 
matches RXTE 
Softening during 
2008-2010 decline
• Earlier intervals 
consistent with 
constant hardness
• Harness ratios 14-
50 keV/50-100 keV 
BAT 58-month 
survey data
• 50-day averages
54690‐54763
55297‐55343
Spectral Softening in GBM
• GBM 8-1000 keV 
Earth occultation 
measurements
• Beginning and end 
of decline interval
• Spectral index 
increases from 2.11 
to 2.17
RXTE Mission-long Light Curves
• Very active period 
2001-2010
• 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
<15keV
15-50 keV
Recent Data 2008-2012 
<15 keV
15-50 keV
20-year Crab Nebula Light Curve
15-50 keV
50-100 keV
Summary & Conclusions
• 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.


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

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