Rossi X-ray Timing Explorer (RXTE) observations of the bright supernova remnant Cas A have revealed a hard power law component above 10 keV in addition to two thermal components inferred from ASCA measurements of the many line centroids from low-Z elements. The power law can be shown to be consistent with synchrotron emission from radio to hard x-rays by electrons of up to 4 x 10(exp 13) eV. Measurement of the 1157 keV line by CGRO (Compton Gamma Ray Observatory) from SC-44 in the chain of decay of Ti-44 predicts that the two Ti-44 lines at 68 and 78 keV should appear at the CGRO intensity. RXTE has placed upper limits on such lines that are marginally consistent with the CGRO measurement. Implications of these results on sites for cosmic ray acceleration and nucleosynthesis are discussed

Allen, G. E.

Blanco, P. R.

Gruber, D. E.

Heindl, W. A.

Higdon, J. C.

Jahoda, K.

Lingenfelter, R. E.

Lingenfelter, Richard E.

MacDonald, D.

Marsden, D. F.

Nomoto, K.

Pelling, M. R.

Swank, J. H.

Woosley, S. E.

English

NASA Technical Reports Server

FINAL REPORT
FOR
ROSSI X-RAY TIMING EXPLORER
GUEST INVESTIGATOR PROGRAM
GRANT NAG 5-6334
Jo/1/97-9/3o/98
FOR:
RXTE SEARCH FOR 44TI DECAY LINES FROM THE CAS A SUPERNOVA REMNANT
BY
RICHARD E. LINGENFELTER
PRINCIPAL INVESTIGATOR
CENTER FOR ASTROPHYSICS & SPACE SCIENCES
UNIVERSITY OF CALIFORNIA
9500 GILMAN DRIVE
LA JOLLA, CA 92093-0424
rlingenfelter@ ucsd.edu
The results of this investigation are published in the attached paper:
RXTE Observations of Cas A. Nuclear Physics B (Proc. Suppl.), 69, 1998. pp. 68-73. by R.
E. Rothschild, R. E. Lingenfelter, P. R. Blanco, D. E. Gruber, W. A. Heindl, D. MacDonald, D.
F. Marsden, M. R. Pelling, K. Jahoda, G. E. Allen, J. H. Swank, S. E. Woosley, K. Nomoto and
J. C. Higdon.
https://ntrs.nasa.gov/search.jsp?R=19990014469 2020-06-15T22:39:35+00:00Z
RXTE Observations of Cas A
R. E. Rothschild, R. E. Lingenfelter, P. R. Blanco, D. E. Gruber, W. A. Heindl, D. MacDonald, D. F.
Marsden, M. R. Pelling, a • K. Jahoda, G. E. Allen, J. H. Swank b S. E. Woosley c K. Nomoto d and J.
C. Higdon "
aCenter for Astrophysics and Space Sciences, UCSD
9500 Gilman Dr., La Jolla, CA 92093-0424, USA
bCode 666, Goddard Space Flight Center
Greenbelt, MD 20771 USA
¢Board Study Astronomy/Astrophysics, UCSC
Santa Cruz, CA 95064 USA
dUniversity of Tokyo, School of Science, Dept. of Astronomy
Bunkyo-ku, 113 Tokyo Japan
eThe Claremont Colleges, Joint Science Center
11th St & Dartmouth Ave., Claremont, CA 91711 USA
RossiX-rayTiming Explorer(RXTE) observationsofthebrightsupernox-aremnant Cas A haverevealeda hard
power law component above 10 keV inadditionto two thermalcomponents inferredfrom ASCA measurements
ofthe many linecentroidsfrom low-Zelements.The power law can be shown to be consistentwith synchrotron
emissionfrom radioto hard x-raysby electronsofup to4xi02s eV. Measurement ofthe 1157 keV lineby CGRO
from 44Scinthe chainofdecay of_4Tipredictsthatthe two 44Tilinesat68 and 78 keV shouldappear at the
CGRO intensity.RXTE has placedupper limitson such linesthat are marginallyconsistentwith the CGRO
measurement. Implicationsoftheseresultson sitesforcosmicray accelerationand nucleosynthesisarediscussed.
1. INTRODUCTION
Two ofthe major questionsinastrophysicsdeal
with the accelerationofcosmic raysand the origin
and evolutionofthe chemicalelements.While the
basicconcepts,such as shock acceleration,Fermi
mechanisms, the r-and s-processesof nucleosyn-
thesis,are wellaccepted, detailedverificationhas
been difficult.Both of these questionsare inti-
mately relatedto supernovae and theirremnants.
Thus, by studying brightsupernova remnants, we
hope to be able to shed some lighton these im-
portant aspectsofwhat happens to matter inthe
Universe.
"The HEXTE team acknowledges the support of NASA
Contract NAS5-30720
1.1. Cosmic Ray Acceleration
The observed spectrum of cosmic rays is an
E -2"7 power law up to -_10 is eV, where it steep-
ens to an E -s° power law which extends to more
than 10 is eV. Prevailing wisdom is that galactic
cosmic rays are accelerated to ~1014 eV by shocks
in supernova remnants, but this has been diffi-
cult to confirm observationally. Radio measure-
ments imply acceleration to ---10 _° eV, and x-ray
measurements of non-thermal power law emis-
sion from supernova remnants, such as SN1006
and IC443, have been interpreted as a continua-
tion of the synchrotron emission seen in the radio.
These measurements imply the existence of elec-
trons with energies approaching 10 z4 eV, and it
is believed that nucleons will also be accelerated
to these energies in these environments.
Onecanusethesynchrotronrelation
E_ ... 0.6B_GE_4keV (1)
to estimate the maximum electron energies in
units of 1014 eV, E_4, from the maximum detected
photon energy in keV, E_, and the assumed mag-
netic field in microGauss, B_G. Extending the
detection of non-thermal emission to higher en-
ergies and to more remnants, will aid in our un-
derstanding of the fundamental questions of the
acceleration sites in the galaxy.
1.2. Explosive Nucleosynthesis
As we learned from Burbidge, Burbidge,
Fowler, and Hoyle [1], and from many others,
the lighter elements are formed during the nu-
clear burning process in stars, while the heav-
ier elements are created in supernova explosions.
Detection of short-lived isotopes can aid in the
understanding of supernovae and refining nucle-
osynthesis calculations. The details of supernovae
light curves and isotopic abundances in gains are
affected by these isotopes•
44Ti is produced by explosive Si burning and
the freeze out from nuclear statistical equilibrium
in supernovae, and is believed to be the primary
source of 44Ca [2]. 44Ti decays to 4_Sc produc-
ing two nuclear lines at 67.9 and 78•4 keV of es-
sentially equal intensity•The 44Sc decays to 44Ca
producing a 1.157 MeV photon. All three nuclear
fines are expected to have the same line strength.
The 44Ti mean life is about 85 years and that for
44Sc is 5.7 years. These three lines can then be
used as a signature of young supernovae was well
as an indicator of nudeosynthesis.
1.3. Cas A
Cas A is the remnant of the explosion of a mas-
sive star about 300 years ago at a distance of ~3
kpc. If indeed Flamsteed observed it in 1680, its
age is 317 years and it was markedly dim for a
massive star. Optical observations suggest the
progenitor was greater than 25 M o -- most prob-
ably a Wolf-Rayet star, and that the supernova
was a type Ib or type II. X-ray imaging of Cas A
has revealed an outer, weak ring of emission that
is assumed to be the expanding shock wave inter-
acting with the interstellar medium or the wind
from the progenitor• There is also a brighter in-
ner ring representing the reverse shock impinging
on the ejecta.
Measurements by ASCA [3] have shown that
thermal equilibrium has not been reached and
that simple one- or two-temperature thermal
models are insufficient to model the data• Mea-
surements above 10 keV by HEAO-1, CGRO, and
other high energy missions require a non-thermal
component must also be present. The presence
of 13 line features in the ASCA spectrum tells us
that at least two temperatures are present -- one
for the non-Fe elements and one for Fe itself.
The COMPTEL on CGRO has discovered and
mapped the 1.157 MeV nuclear line from 44Sc
decay coming from Cas A [4].The COMPTEL
map clearly shows the line radiation coincident
with the position of Cas A. The spectrum shows
the significant measurement of the line and the
nearby energy bins• According to COMPTEL
calculations, about 1 × 10 -4 M O of 44Ti is inferred
to have been produced. This large amount of 44Ti
is consistent with some models by Woosley and
co-workers, Nomoto, and Thielemann, but it pre-
dicts a much brighter supernova than apparently
occured.
The OSSE instrument on CGRO has also made
observations of Cas A in the 40 keV to few MeV
range, and does not detect either the 1.157 MeV
line or the two hard x-ray lines in the 70-80 keV
range [5]. The OSSE 99% upper limit of 5.7x
10 -s "y/cm2s is still consistent with the COMP-
TEL value of (4.2=i:0.9)x 10 -5 -y/cm 2 s. OSSE
observed the continuum to ~100 keV to be a
power law with photon index 3 06 +°'41
• --0.44"
2. RXTE OBSERVATIONS
Cas A has been viewed 4 times during the
RXTE mission: 2 long 200 ks observations in
AO-1 and AO-2, once in IOC, and once as a cal-
ibration observation. Results from the two long
observations will be presented.
2.1. Instrumentation
The observations were made with both the Pro-
portional Counter Array (PCA) [6] and the High
Energy X-ray Timing Experiment (HEXTE) [7].
Theseinstrumentsarecoalignedwiththesame1°
FWHM field of view and together they cover the
2-250 keV range. The line search and extension
of the non-thermal flux maximum energy to high
energies is mostly in the perview of the HEXTE
instrument, so I shall describe it in more detail.
The HEXTE is comprised of 8 NaI/CsI
phoswich scintillation detectors of 200 cm 2 each
and grouped 4 to a cluster. Each detector's
field of view is limited by a lead collimator to
1 ° FWHM, and the gain of each detector sys-
tem is actively controlled. The 241Am calibra-
tion and gain control centroid is stable to 0.02
PHA channels on a one-day timescale. One PHA
channel is approximately 1 keV in width. The
entire detector-collimator-anticoincidence shield
assembly is rocked on- and off-source with 16
s dwell times at each position. The rotation
axes of the two clusters are orthogonal to each
other, enabling 4 background regions to be sam-
pled around a given source.
2.2. HEXTE Background
Of primary importance to thisinve_-tigationis
the background subtractionaffordedby the rock-
ing ofthe HEXTE dusters. HEXTE background
subtractionperformance isclearin the following
examples. Firstof all,during In-Orbit Checkout
(IOC), RXTE viewed 19 blank fieldsallover the
sky for 10 minutes each. Figure 1 show'sthe net
counting rate (i.e.,on-sourceminus the sum ofthe
two off-sourceobservations)infour energy bands
forthe 19 observations.The energy bands are 16-
30, 31-60,61-100,and 101-240 keV. The crosses
are duster A and the boxes are clusterB. The
dotted linesrepresent zero net flux,and as you
can see,these short observationsare consistent
with no blank sky net flux.
Secondly,we have the net spectrum ofthe dif-
ferenceof the two off-sourcepositionsfor a 200
ks obserx_tion of MCG 8-11-11 during the IOC
period (Figure 2). The data are binned every
5 PHA channels ('_5keV) over the entire15-250
ke¥ HEXTE energy range. The twin dashed lines
represent +1% of background and the ratesare
expressed as percent ofbackground. Clearly,any
systematicsin background subtractionare of or-
der lessthan I% of background.
I .....I!iI
i
)i!Ii i! ..........ii.....:I iI1 1
m
Figure 1. Background subtracted counting rates
for 19 ten-minute observations of blank fields on
15 January 1996. The four sets of rates cover
the energy range 16-30, 31-60, 61-100, and 101-
240 keV. The crosses (x) represent Cluster A and
boxes (O) Cluster B. The dotted line indicates
zero net flux.
Finally, as part of an investigation of the spec-
trum of the diffuse flux fluctuations, we were
awarded the off-source data for all observations
greater than 50 ks. We have separately accumu-
lated the two off-source spectral histograms for
observations with a range of orbital and environ-
mental parameters. Figure 3 displays the net off-
source spectrum along with lines to denote =1:0.5%
of background for 5 × 106 s. Again, the systemat-
ics axe less than 0.5% of background.
3. RESULTS
We presentresultson the two long 200 ks ob-
servationsmade in AO-I and AO-2. All spectral
analyses were made using XSPEC 10.0,afterex-
tractingthe PCA data with the Guest Observer
j:i _. 4
o _ lo_ 11¢ _oo
Figure 2. The net counting rate histogram of the
difference of the two off-saurce positions for each
cluster and their sum during the MCG 8-11-11
observation. One percent of the total off-source
flux is shown by pairs of horizontal dashed lines
for comparison. "Left" and "Right" correspond
to the plus and minus directions of the off-source
viewing.
Facility supplied ftoois and the HEXTE data with
UCSD specific IDL software.
3.1. Continumn
R.XTE clearlyseesthe E -3 power law contin-
uum, as seen by ASCA and othermissions,above
I0 keV. Below this energy,the continuum isa
combination of the power law and the thermal
bremsstrahlung associatedwith the lineemission.
The ironK lineisclearlyseen,also.This work has
been reported by Allen et al. [8].One can esti-
mate the temperatures ofthe variousthermal dis-
tributionsnecessarytoproduce the observed lines
in the ASCA data. Two are required:I) kT-0.7
keV will reproduce the lightelements (Ne, Mg,
Si,S, Ar, and Ca), and 2)kT=2.8 keV forthe Fe
o,oo_
_o_
o©_©_©
___1 "+_'__"__/_i- " _ + ' : '
e
©'cI_ I
o_
io-.
-o,©le
c++itl, B Bock_,o_ l;is;o.o_
Figure 3. The net off-source counting rate his-
togram versus pulse height channel for a 5x 106 s
accumulation of observations with greater than 50
ks duration for the two HEXTE dusters. The de-
tector gain is approximately one keV per channel,
and the energy range covered is 15-240 keV. The
dotted lines represent ±0.5% of the background
per channel.
line. As seen before, a power law dominates these
thermals above 10 keV with an index of about 3.
Such a power law cannot continue well below 10
keY, or it will be in conflict with the thermal com-
ponents. Indeed, fitting the ASCA data with just
a power law for the continuum gives an accept-
able fit [3]. This indicates the high energy power
law must flatten at these energies.
We fit the ASCA GIS and the RXTE
PCA/HEXTE data simultaneously with a model
containing2 Raymond-Smith thermals, a broken
power law, and a ganssian at6.4 keV to represent
ironon grainsin Cas A. The best fitfor the bro-
ken power law had a low energy index of 1.8 that
broke at 16 keV to 3. Ifwe represented the high
energy fluxwith another thermal, a significantly
worse fitwas achieved forkT=24 keV.
Various possibleoriginsof the power law flux
have been discussed[8],with the conclusionthat,
since pulsed flux is not detected in the range
of 1 ms to 10 s, synchrotron emission ismost
likely. Assuming this origin and assuming a 1
mG equipartition magnetic field, we find the max-
imum electron energy to be at least 4 x 1013 eV. If
accelerated electrons exist at these energies, pro-
tons are also expected to experience the same ac-
celeration for energies much larger than the rest
mass of the proton [9]. Thus, if Cas A is typical,
supernova remnants can provide the acceleration
for cosmic rays at least to these energies.
3.2. 44Ti Lines
o
.=
i'
Nf_TF J_-1 _sl_t_o_ Of COs A
Figure 4. HEXTE counts histogram plus model
fit for the AO-1 observation of Cas A. The lower
panel displays the residuals to the model fit.
We searched the two long observations of Cas
A for the "Ti lines at 68 and 78 keV, and
the two observing periods gave very different re-
sults. As I have demonstrated, the HEXTE baz.k-
ground subtraction has systematic uncertainties
at less than 0.5% of background. Yet, in AO-1,
there is evidence of an oversubtraction of back-
ground at the 1-2% level. Since the lead colli-
mator produces fluorescence lines at 74 and 85
keV, and an iodine activation line is present at
66 keY, this oversubtraction greatly affects the
search for the 44Ti lines. Figure 4 shows the
best fit of the AO-1 HEXTE data to a power law
plus two gaussians model. The line energies were
:_ "z.
i
M[X_[ _)-2 _St_Ot;On O_ COS A
1"
20 50 100 200
cao_el *r_rSly (a*_
Figure 5. HEXTE counts histogram plus model
fit for the AO-2 observation of Cas A. The lower
panel displays the residuals to the model fit.
fixed at the known 44Ti _-alues and the widths
of the lines were broadened by 2.5% to account
for the expansion velocity of Cas A, assuming
a nominal value of 7500 lan/s [10]. The value
for the flux of both lines is (-1.4±1.7)x10 -5
-y/cm _ s for both dusters fit simultaneously. The
power law is 3.09±0.10 with a 20-50 keV flux of
(3.2±0.2)x10 -11 ergs/cm _ s..
The other long observation,fitsimilarly,yields
evidence for excess emission at 68 and 78 keV,
as shown in Figure 5. The best fitline flux
was (4.3d:l.8)x10-5 q'/cm2 s,which isin excel-
lentagreement with the COMPTEL result.Un-
til,however, the discrepancy between the two
observationsisresolved,I cannot claim a true
detection. If we combine the two data sets
and fit them, the result for the line flux is
(1.3:1:I.2)xi0-s -y/cm2 s,or a 90% upper limit
of 3.3x10 -s 7/cm 2 s.
4. CONCLUSIONS
The PCA/HEXTE instruments on RXTE have
detected x-ray emission from Cas A in the range
2-100 keV. The continuum above 10 keV is a
power law with photon index _,3, which we
have interpreted as due to synchrotron processes
withintheremnant.Nopulsationsaredetected
in the1msto 10s range.Assuminganequipar-
tition magnetic field of 1 raG, we infer that elec-
trons are accelerated up to at least 4×102s eV.
Previous theoretical work has demonstrated that
nucleons will also be accelerated to the same ener-
gies, and thus, supernox_ remnants are capable of
providing the acceleration mechanism for cosmic
rays in the galaxy, at least to _1014 eV.
The search for hard x-rays from 44Ti decay at
68 and 78 keV had mixed resu}ts. One observa-
tion agreed with the COMPTEL measurement of
the 1.157 MeV line and one did not detect any
line flux at all. This latter measurement is sus-
pect due to the clear indication of an oversub-
traction of background. Until this discrepanQ" is
resolved, no definitive statement can be made.
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2. F.X. Timmes, S.E. Woosley, D.H. Hartmann,
& R.D. Hoff_nan 1996, ApJ, 464, 332.
3. S.S. Holt, E.V. Gotthelf, H. Tsunemi, & H.
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4. A.F. Iyudin et al. 1994, A&A, 284, L1.
5. L.-S. The et al., A&ASS 1996, 120, 357.
6. K. Jahoda et al. 1996, in EUV, X-ray and
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