The Crab is the only bright steady source in the X-ray sky. The Crab consists of a pulsar wind nebula, a synchrotron nebula, and a cloud of expanding ejecta. On small scales, the Crab is extremely complex and turbulent. X-ray astronomers have often used the Crab as a standard candle to calibrate instruments, assuming its spectrum and overall flux remains constant over time. Four instruments (Fermi/GBM, RXTE/PCA, Swift/BAT, INTEGRAL/ISGRI) show a approx.5% (50 m Crab) decline in the Crab from 2008-2010. This decline appears to be larger with increasing energy and is not present in the pulsed flux, implying changes in the shock acceleration, electron population or magnetic field in the nebula. The Crab is known to be dynamic on small scales, so it is not too surprising that its total flux varies as well. Caution should be taken when using the Crab for in-orbit calibrations

Baumgartner, Wayne H.

Beklen, Elif

Bhat, P. Narayana

Briggs, Michael S.

Camero-Arranz, Ascension

Case, Gary L.

Chaplin, Vandiver

Cherry, Michael L.

Connaughton, Valerie

Diehl, Roland

Finger, Mark H.

Gehrels, Neil

Greiner, Jochen

Jahoda, Keith

Jenke, Peter

Kippen, R. Marc

Kouveliotou, Chryssa

Krimm, Hans A.

Kuulkers, Erik

Lund, Niels

Meegan, Charles A.

Natalucci, Lorenzo

Paciesas, William S.

Preece, Robert

Rodi, James C.

Shaposhnikov, Nikolai

Skinner, Gerald K.

Swartz, Doug

Von Kienlin, Andreas

Wilson-Hodge, Colleen A.

Zhang, Xiao Ling

arXiv

Colleen A. Wilson-Hodge

Michael L. Cherry

Gary L. Case

Wayne H. Baumgartner

Elif Beklen

P. Narayana Bhat

Michael S. Briggs

Ascension Camero-Arranz

Vandiver Chaplin

Valerie Connaughton

Mark H. Finger

Neil Gehrels

Jochen Greiner

Keith Jahoda

Peter Jenke

R. Marc Kippen

Chryssa Kouveliotou

Hans A. Krimm

Erik Kuulkers

Niels Lund

Charles A. Meegan

Lorenzo Natalucci

William S. Paciesas

Robert Preece

James C. Rodi

Nikolai Shaposhnikov

Gerald K. Skinner

Doug Swartz

Andreas von Kienlin

Roland Diehl

Xiao-Ling Zhang

Crossref

The Astrophysical Journal

WHEN A STANDARD CANDLE FLICKERS

Wilson-Hodge, C.

Cherry, M.

Case, G.

Baumgartner, W.

Beklen, E.

Bhat, P.

Briggs, M.

Camero-Arranz, A.

Chaplin, V.

Connaughton, V.

Finger, M.

Gehrels, N.

Greiner, J.

Jahoda, K.

Jenke, P.

Kippen, R.

Kouveliotou, C.

Krimm, H.

Kuulkers, E.

Lund, N.

Meegan, C.

Natalucci, L.

Paciesas, W.

Preece, R.

Rodi, J.

Shaposhnikov, N.

Skinner, G.

Swartz, D.

Kienlin, A.

Diehl, R.

Zhang, X.

MPG.PuRe

When a standard candle flickers

The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma-ray telescopes. Although small-scale variations in the nebula are well known, since the start of science operations of the Fe r m i Gamma-ray Burst Monitor (GBM) in 2008 August, a ∼7% (70 mCrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ∼15-50 keV band with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the Imager on-Board the INTEGRAL Satellite (IBIS). A similar decline is also observed in the ∼3-15 keV data from the RXTE/PCA and in the 50-100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ∼3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in 2007 April. As of 2010 August, the current flux has declined below the 2007 minimum. © 2011. The American Astronomical Society

Louisiana State University

Colleen Wilson-Hodge

Wayne Baumgartner

P. Naryana Bhat

Ascension CAMERO-ARRANZ

G. K. Skinner

When a Standard Candle Flickers

The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma ray telescopes. Although small-scale variations in the nebula are well-known, since the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in August 2008 a 7% (70 mcrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the 15-50 keV band with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the INTErnational Gamma-Ray Astrophysics Laboratory Imager on Board INTEGRAL (IBIS). A similar decline is also observed in the 3 - 15 keV data from the RXTE/PCA and in the 50 - 100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The change in the pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a 3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in April 2007. As of August 2010, the current flux has declined below the 2007 minimum

Beklen Elif

NASA Technical Reports Server

When A Standard Candle Flickers
Colleen A. Wilson-Hodge ∗1 , Michael L. Cherry2 , Gary L. Case2 , Wayne H.
Baumgartner3 , Elif Beklen4,5 , P. Narayana Bhat 6 , Michael S. Briggs6 , Ascension
Camero-Arranz7 , Vandiver Chaplin 6 , Valerie Connaughton 6, Mark H. Finger 8 , Neil
Gehrels9 , Jochen Greiner 10, Keith Jahoda9 , Peter Jenke 1,11 , R. Marc Kippen 12 ,
Chryssa Kouveliotou 1 , Hans A. Krimm 3,13 Erik Kuulkers 14, Niels Lund 15 , Charles A.
Meegan 8 , Lorenzo Natalucci 16 , William S. Paciesas6, Robert Preece 6 , James C.
Rod i 2 , Nikolai Shaposhnikov3,17, Gerald K. Skinner3,17 , Doug Swartz 8 , Andreas von
Kienlin 10 , Roland Diehl 10 , Xiao-Ling Zhang 10
1 VP 62 Space Science Office, NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
2Department ofPhysics & Astronomy, Louisiana State University, Baton Rouge, LA, 70803, USA
3 CRESST/NASA GSFC, Astrophysics Science Division, Greenbelt MD 20771, USA
4Physics Department, Middle East Technical University, 06531 Ankara, Turkey
5Physics Department, Suleyman Demirel University, 32260 Isparta, Turkey
6 University ofAlabama in Huntsville, Huntsville, AL 35899, USA
7National Space Science and Technology Center, Huntsville, AL 35805, USA
8 Universities Space Research Association, Huntsville, AL 35805, USA
9NASA Goddard Space Flight Center (GSFC), Greenbelt, MD 20771, USA
10Max-Planck Institutfür Extraterrestische Physik, 85748, Garching, Germany
11NASA Postdoctoral Program Fellow
12Los Alamos National Laboratory, Los Alamos, NM 87545
13 Universities Space Research Association, Columbia, MD 21044, USA
14ISOC, ESA, European Space Astronomy Centre (ESAC), 28691 Villanueva de la Cañada
(Madrid), Spain
15Danish National Space Center, Technical University ofDenmark, 2100 Copenhagen, Denmark
16INAF-IASFRoma, via Fosso del Cavaliere 100, 00133, Roma, Italy
17 University ofMaryland, Astronomy Department, College Park, MD 20742, USA
E-mail: colleen. wilson@nasa.gov , cherry@phunds.phys.lsu.edu,
case@phunds.phys.lsu.edu
Q Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. 	 http://pos.sissa.it/
https://ntrs.nasa.gov/search.jsp?R=20110014630 2019-08-30T16:58:34+00:00Z
The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady
enough to be easily used as a standard candle. As a result, it has been used as a normalization
standard by most X-ray/gamma ray telescopes. Although small-scale variations in the nebula
are well-known, since the start of science operations of the Fermi Gamma-ray Burst Monitor
(GBM) in August 2008 a — 7% (70 mcrab) decline has been observed in the overall Crab Neb-
ula flux in the 15–50 keV band, measured with the Earth occultation technique. This decline
is independently confirmed in the — 15 — 50 keV band with three other instruments: the Swift
Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array
(RXTE/PCA), and the INTErnational Gamma-Ray Astrophysics Laboratory Imager on Board IN-
TEGRAL (IBIS). A similar decline is also observed in the — 3 - 15 keV data from the RXTE/PCA
and in the 50 - 100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The change in the
pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indi-
cating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a
— 3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008,
with a flux minimum in April 2007. As of August 2010, the current flux has declined below the
2007 minimum.
8th INTEGRAL Workshop
The Restless Gamma-ray Universe
Integral2010,
September 27-30, 2010
Dublin Ireland
*Speaker.
Q Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.
When a Standard Candle Flickers
	
Colleen A. Wilson-Hodge
1. Introduction
X-ray and gamma-ray astronomers frequently consider the Crab supernova remnant to be a
steady standard candle suitable as a calibration source (e.g.,[1 –4]). Jourdain & Roques (2009)
presented over 5 years of INTEGRAL (SPI, 20 keV - 8 MeV) observations, with fitted flux nor-
malizations at 100 keV consistent with being constant to within the _ 3% quoted errors. On the
basis of data from XMM-Newton, INTEGRAL, Swift, Chandra, RXTE, and several earlier mis-
sions, Kirsch et al. [1] concluded that the Crab flux can be described at least up to 30 keV
by the same spectrum proposed by Toor & Seward [5] three decades earlier: dN/dE = (9 . 7 f
1 .0 )E -(2.1f0.03)photons cm-2 s- 1 keV-1 , and they describe the Crab as a standard candle.
Driven by the central pulsar’s spin-down luminosity, the surrounding remnant consists of a
cloud of expanding thermal ejecta and a synchrotron nebula [6] with an integrated luminosity
_ 1038 ergs- 1 . The pulsar provides a shocked wind that accelerates electrons and positrons to
energies _ 107 GeV and a source of kinetic energy driving turbulent motion of a ring of wisps
nearly surrounding the synchrotron nebula. High resolution observations reveal wisps and knots
moving at velocities up to 0.7 c from radio to X-ray energies (e.g., [7–11]). A central torus and jet
structure extending out from the pulsar were observed in X-rays by Chandra [12], aligned closely
with the pulsar’s proper motion [13]. The nebular emission is considered to be a combination of
synchrotron radiation up to _ 100 MeV and a harder inverse Compton spectrum extending up to
TeV energies [14].
Variations in the observed Crab nebular flux have been previously reported at various wave-
lengths, for example, in radio [15], optical [16], and in soft gamma rays [17]. At higher energies (1-
150 MeV), large flux changes have been reported [18, 14]. Recently, flares in the > 100 MeV flux
were reported by Fermi/LAT [19] and AGILE [20]. Conversely, at the highest energies, HEGRA
[21], H.E.S.S. [22], MAGIC [23], and VERITAS [24] report no evidence for time variability.
2. Observations & Results
2.1 Fermi GBM
The GBM instrument (8 keV to 40 MeV) [25] provides nearly continuous full-sky coverage via
the Earth occultation technique, adapted from BATSE [26] for GBM [27, 28]. The GBM detector
response [29, 30], gain, and energy resolution are stable over time with background lines typically
within 1% of their expected position.
The Crab rate measured in the 12-50, 50-100, 100-300, 300-500 keV energy bands with GBM
from August 12, 2008 through July 13, 2010 (MJD 54690-55390) decreased steadily by more than
5%: The decrease is 5 .4 f 0 .4%, 6 .6 f 1 .0%, 12 f 2%, and 39 f 13% in the four bands, respectively,
relative to MJD 54690. Inclusion of a linear decline in the 12-50 keV band improves reduced x2
to 605.8/130=4.66 from 956.3/131=7.30 for a constant Crab [31].
2.2 RXTE PCA
Frequent observations with the RXTE PCA were made to monitor the radio-X-ray phase of the
Crab pulsed emission [32] and for calibration purposes [33, 34]. In the PCA, the Crab is bright ( _
2500 counts s -1 detector-1 ). Unrejected background from all sources amounts to about 1 mCrab.
2
When a Standard Candle Flickers	 Colleen A. Wilson-Hodge
The PCA is a relatively simple instrument, with commanded changes in operating conditions lim-
ited to the high voltage. Data since the last high voltage change in 1999 for PCU 2,3, and 4 are
used in this paper.
The PCA response has two small time-dependent effects, both accounted for in the response
matrices: the changing opacity of the front veto layer (negligible in the 15-50 keV band) and a
small energy drift in the pulse height channel boundaries. Our observed changes in the Crab rate
(see Figure 1 and [31]) are more than 5 times larger than these effects combined.
From MJD 54690-55435 the Crab rate in PCU 2 declined by 5 . 1 f 0 . 2% and 6 . 8 f 0 . 3% in
the 2-15 and 15-50 keV bands, respectively, relative to MJD 54690 [31]. Similar results, variations
of 2-7%, are seen if the bands are further subdivided. In spectral fits to individual PCA observa-
tions, the power law index softens and the normalization and absorption column gradually increase
with time, with no clear correlation with flux. These light curves were produced using RXTE/PCA
standard 2 data (129 energy channel, 16-second) that were extracted, background subtracted, dead-
time corrected using standard RXTE recipes 1 and corrected for the known time dependence of the
response.
2.3 INTEGRAL IBIS and JEM-X
Here we present results from JEM-X (3 - 35 keV,[35]) and the IBIS/ISGRI (15 keV - 10 MeV,
[36]) on-board INTEGRAL [37]. The Crab has been observed every spring and fall with INTEGRAL
since 2002, mainly for calibration purposes. To reduce systematic effects as much as possible, we
selected JEM-X and IBIS/ISGRI observations within ( < 0 . 25 ° ) and 10° of on-axis. We include
only JEM-X data using the latest on-board software (since MJD 53068).
Figure 1 includes ISGRI and JEM-X2 count rates from individual pointings averaged over the
3-day INTEGRAL orbit with rms errors. The ISGRI data were analyzed with the Off-line Analysis
(OSA) package [38] version 9 with the settings used for the INTEGRAL Galactic Bulge monitoring
program2
 light curves [39]. Using images from individual pointings, the point-spread function
of ISGRI is fitted. These images are integrated in a given energy band after gain, offset, and
charge-loss corrections are performed for each event. A time-dependent effective area correction,
usually performed by assuming that the Crab flux is constant, has been excluded from these data,
meaning that not all systematic effects are taken into account. Known effects include residuals in
gain and charge loss corrections, present with an amplitude of — 1-2%, varying on month-years
timescales. Similarly, for JEM-X, the ad-hoc piecewise linear correction (added to OSA to reduce
time trends in the Crab flux) was excluded from the standard OSA analysis. JEM-X consists of two
identical units, JEM-X1 and JEM-X2. During the period of interest, JEM-X2 has mostly been in
standby-mode and JEM-X1 the active unit. A gradual decrease has been observed in the sensitivity
of JEM-X1, so only JEM-X2 is shown. The scatter in the JEM-X data is large compared to the
observed Crab variations, especially below 10 keV. From MJD 54690-55390, the ISGRI 15-50, 50-
100, and 100-300 keV flux decreases by 8 .2 f 1 . 1, 8 .3 f 1. 1, and 5 .7 f 1 . 0%, respectively, relative
to MJD 54690 [31].
1 http://heasarc.gsfc.nasa.gov/docs/xte/recipes/
2 http://integral.esac.esa.int/BULGE/
When a Standard Candle Flickers
	
Colleen A. Wilson-Hodge
2.4 Swift BAT
Swift/BAT is a coded aperture telescope operating in the 14 - 150 keV range [40]. The
Swift/BAT 14-50 and 50-100 keV light curves (see Figure 1) are based on publicly available 58-
month light curves3 from the Swift/BAT all-sky hard X-ray survey [41, 42] extended to May 30,
2010 by the BAT team. We binned data from individual Swift pointings in 50-day intervals, elim-
inating pointings of less than 200 seconds duration and those in which less than 15% of the BAT
detectors were illuminated by the Crab. The statistical errors on each data point are small (0.1%)
and are dominated by systematic errors. We estimate the systematic errors to be — 0 . 75% by as-
suming that the long term variations in the lightcurve are due to real variations in the Crab, and
that the shorter term variations around that trend are representative of the systematic error. The
BAT data show variations in the Crab flux at the level of — 3% yr − 1 . From MJD 54690-55340,
BAT observes a decrease of 6 . 7 f 0 .7 and 10 . 4 f 0 . 8% [31], in the 15-50 and 50-100 keV bands,
respectively, relative to the rate on MJD 54690, similar to the decrease seen by GBM in the same
energy range.
3. Discussion and Summary
Figure 1 shows composite light curves combining the overlapping results from RXTE, INTE-
GRAL, Swift, and Fermi/GBM. All instruments agree well from 2008 to 2010, with all instruments
registering a decline in the Crab 15-50 keV flux of —7% (70 mcrab) over the two years starting at
MJD 54690, with a similar decline in the 50-100 keV band. PCA and BAT continue to agree back
to the start of the Swift mission. For RXTE, Swift, and INTEGRAL/ISGRI the latest measurements
shown are significantly below the previous minimum. INTEGRAL/ISGRI shows evidence for the
dip near MJD 54100-54200 and the increase before —MJD 53700, with similar but less significant
variations seen in JEM-X2. Prior to this time, the PCA measurements show continued variations
extending back to —MJD 52000, which are not seen with ISGRI in the 20-50 keV band. Known
systematic errors in ISGRI energy reconstruction are expected to account for — 1-2% deviations.
Beginning at —MJD 54000, there is a strong correlation among the results from the four indepen-
dent instruments with very different signal to noise characteristics and observing techniques: Earth
occultation, coded-mask imaging, and collimated detectors. The range of techniques strengthens
the case that the variation is intrinsic to the Crab. We found no apparent correlations between
these variations and variations in the INTEGRAL/SPI anticoincidence detector count rates or GBM
count rates, evidence against local background condition changes as a possible origin, and further
supporting a Crab origin. The pulsed flux stability [31] suggests that the observed variations are
nebular.
In summary, the widely-held assumption that the Crab is a standard candle, suitable for nor-
malizing instrument response functions and for calibrating X-ray instruments, is now in question.
Although obtaining absolute calibrations and instrument normalizations at — 1% is difficult, the
results presented here from four independent spacecraft demonstrate that in fact the nebular X-
ray/gamma ray emission from the Crab varies at a level of — 3 . 5% yr− 1 . The variation is seen
in the nebular emission, and so apparently results from changes in the shock acceleration or the
3http://swift.gsfc.nasa.gov/docs/swift/results/
4
When a Standard Candle Flickers 	 Colleen A. Wilson-Hodge
Figure 1: Composite Crab light curves for RXTE/PCA (15-50 keV - black diamonds), Swift/BAT (Top:
14-50 keV, Bottom: 50-100 keV - red filled circles), Fermi/GBM (Top: 15-50 keV, Bottom: 50-100 keV
- open blue squares), INTEGRAL/ISGRI (Top: 20-50 keV, Bottom: 50-100 keV - green triangles), and
INTEGRAL/JEM-X2 (10-25 keV). Each data set has been normalized to its mean rate in the time interval
MJD 54690-54790. All error bars (except Swift/BAT) include only statistical errors.
nebular magnetic field. We cannot predict if the present decline will continue or if the — 3 year
pattern [31] will persist. Longer baselines and multi-wavelength observations are needed to answer
these questions.
Acknowledgments
This work is supported by the NASA Fermi Guest Investigator program, NASA/Louisiana
Board of Regents Cooperative Agreement NNX07AT62A (LSU), the Louisiana Board of Re-
gents Graduate Fellowship Program (J. Rodi), and the Spanish Ministerio de Ciencia e Innovación
through the 2008 postdoctoral program MICINN/Fulbright under grant 2008-0116 (A. Camero-
Arranz). This research has made use of data obtained through the High Energy Astrophysics
Science Archive Research Center Online Service provided by the NASA/Goddard Space Flight
Center, public Swift/BAT results made available by the Swift/BAT team, and observations with IN-
TEGRAL, an ESA project funded by ESA member states (especially the PI countries: Denmark,
France, Germany, Italy, Switzerland, Spain), Poland and with the participation of Russia and the
USA.
5
When a Standard Candle Flickers 	 Colleen A. Wilson-Hodge
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6


When A Standard Candle Flickers

The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma ray telescopes. Although small-scale variations in the nebula are well-known, since the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in August 2008 a ∼ 7% (70 mcrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ∼ 15 - 50 keV band with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the INTErnational Gamma-Ray Astrophysics Laboratory Imager on Board INTEGRAL (IBIS). A similar decline is also observed in the ∼ 3 - 15 keV data from the RXTE/PCA and in the 50 - 100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The change in the pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ∼ 3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in April 2007. As of August 2010, the current flux has declined below the 2007 minimum. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence

Kippen, R. M.

Finger, M. H.

Case, G. L.

Rodi, J. C.

von Kienlin, A.

Paciesas, W. S.

Cherry, M. L.

Bhat, P. N.

Briggs, M. S.

Meegan, C. A.

When a Standard Candle Flickers
Colleen A. Wilson-Hodge (NASA/MSFC), M.L. Cherry (LSU), 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), J. Greiner (MPE), K. 
Jahoda (NASA/GSFC), P. Jenke (NPP/NASA/MSFC), R.M. Kippen (LANL), C. 
Kouveliotou (NASA/MSFC), E. Kuulkers (ISOC), C.A. Meegan (USRA), L. Natalucci 
(INAF-IASF), W.S. Paciesas, R. Preece (UAH), J.C. Rodi (LSU), N. Shaposhnikov 
(CRESST/NASA/GSFC), D. Swartz (USRA), A. von Kienlin (MPE)
https://ntrs.nasa.gov/search.jsp?R=20100042150 2019-08-30T13:17:50+00:00Z
Introduction
• The Crab is the only bright steady source in 
the X-ray sky.
• The Crab consists of  a pulsar wind nebula, a 
synchrotron nebula, and a cloud of expanding 
ejecta. 
• On small scales, the Crab is extremely 
complex and turbulent.
• X-ray astronomers have often used the Crab 
as a standard candle to calibrate instruments, 
assuming its spectrum and overall flux 
remains constant over time.
GBM
Bismuth 
Germanate
(BGO)
Detector
GBM 
Sodium Iodide
(NaI)
Detector
LAT
• GBM 
•12 NaI detectors
• 8keV - 1 MeV
• 2 BGO detectors
•150 keV - 40 MeV
• CTIME data
• 8 channel, 0.256 
s 
• CSPEC data 
• 128 channel, 
4.096s
Fermi Gamma Ray Burst Monitor 
(GBM)
GBM Earth Occultation Technique
• Current catalog includes 83 
sources, primarily recently 
active X-ray binaries, the Crab, 
4 AGN, 2 SGRs, and the Sun
• Fluxes for cataloged sources 
measured by fitting the change 
in count rate due to Earth 
occultation
• Source model: assumed 
spectrum convolved with 
changing detector response 
and atmospheric transmission
• 8 energy bands in NaI or BGO 
detectors
• 6 persistent and 2 transient 
sources detected above 100 
keV; 50+ sources detected 
<100 keV.
• Over 85% of sky viewed every orbit
• Entire sky viewed every ~26 days
• Sensitivity exceeds CGRO/BATSE 
below 25 keV and above ~1 MeV
• No solar constraints
GBM Observations of the Crab 
Nebula
• 25-day averages
• Normalized to long-
term average in each 
band
• Decline in Crab flux:
– 5.4 ± 0.4% 12-50 keV
– 6.6 ± 1.0% 50-100 keV
– 12 ± 2% 100-300 keV
– 39 ± 12% 300-500 keV
• Decline becomes larger 
as energy increases
• No changes in GBM 
response or calibration
12-50 keV
50-100 keV
100-300 keV
300-500 keV
Swift BAT Transient Monitor
• Swift Burst Alert 
Telescope (BAT)
• 2 steradian field of view
• Scaled maps in 15-50 
keV band
• Maps on timescales 
>64s
• Corrections for 
geometry, varying 
numbers of detectors, 
material in the field of 
view, etc.
http://swift.gsfc.nasa.gov/docs/swift/results/transients/index.html
Swift BAT Transient Monitor:
Crab Light Curve
• Points shown are ~50 
day averages
• Constructed from orbital 
light curves
• Selected good data (data 
quality flag =0 )
• Restricted partial coding 
fractions to >80% 
• Excluded bright flaring 
intervals from A0535+26
• Flux decline of ~4.5% 
observed during overlap 
with GBM 
GBM
INTEGRAL ISGRI and JEM-X Crab 
Light Curves
• Publically available 
Crab observations
• Produced using 
OSA 9.0
• Offset <10° (ISGRI); 
< 3°(JEM-X)
• Corrections based 
upon constant Crab 
are omitted.
• During the overlap 
with GBM, a ~5% 
decline is seen in 
the 18-40 and 40-
100 keV bands GBM
JEM-X 3-10 keV
ISGRI  18-40 keV
ISGRI 40-100 keV
http://integral.esac.esa.int/BULGE/SOURCES/Crab/Crab.html
Rossi X-ray Timing Explorer (RXTE)
• Proportional Counter Array 
(2-60 keV)
• 1 deg FWHM field-of-view
• Last gain change for the 
PCA in March 1999
• More than 400 observations 
with same channel-to-
energy conversion
• Response Time 
Dependence
– Gradual change in energy 
edges with time
– Xe Leakage into the 
Propane Layers
PCA
RXTE PCA Crab Light Curve
• Extracted light curves 
using standard 2 data
• Observations shorter than 
300 s were excluded
• Background subtracted 
and deadtime corrected
• Corrected for response 
time dependence using 
response predicted count 
rate and by selecting 
layers 2+3
• Variations of ~5% visible 
in the GBM era in all 3 
PCUs
2-15 keV
15-50 keV
GBM
RXTE Crab Pulsed Flux
• Event mode data (250μs, 
129 channel)
• 3.2-35 keV, all PCU2 
layers
• Pulsed flux shows stead 
decrease at 0.2% per 
year – consistent with 
pulsar spin down.
• The larger ~5% per year 
variation not seen in 
pulsed emission
• Likely has nebular origin
Pulsed Flux
RMS
Total Flux
RXTE PCA – Search for Periodicity
• PCA light curve has 3 
peaks. Is there a 
periodicity?
• Power spectrum from 
evenly binned 15-50 
keV PCU 2 data (3 bins 
per year). Power law 
index 2.1±0.4
• Frequency search –
fitted quadratic + 
sinusoid. 
• Highest peak 1176±96 
days, only 2σ 
Comparing Instruments
• Light curves for each 
instrument are normalized 
to its average rate from 
MJD 54690-54790.
• RXTE/PCU2 (15-50 keV)
– Black Diamonds
• BAT (15-50 keV) 
– Red Circles
• ISGRI (18-40 keV)
– Green triangles
• ISGRI (40-100 keV)
– Pink asterisks
• GBM (15-50 keV)
– Blue squares Instruments on four separate 
spacecraft show ~5% yr-1 decline 
in Crab flux
Summary & Conclusions
• Four instruments (Fermi/GBM, RXTE/PCA, 
Swift/BAT, INTEGRAL/ISGRI) show a ~5% (50 
mCrab) decline in the Crab from 2008-2010. 
• This decline appears to be larger with increasing 
energy and is not present in the pulsed flux, 
implying changes in the shock acceleration, 
electron population or magnetic field in the nebula.
• The Crab is known to be dynamic on small scales, 
so it is not too surprising that its total flux varies as 
well. 
• Caution should be taken when using the Crab for 
in-orbit calibrations.


When a Standard Candle Flickers

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