A rip in the MLI at the exit aperture of OMA, the NuSTAR optic aligned with detector focal plane module FPMA, has resulted in an increased photon flux through OMA that has manifested itself as a low energy excess. Overall, the MLI coverage has decreased by 10%, but there is an additional time-varying component, which occasionally causes the opening to increase by up to 20%. We address the problem with a calibration update, and in this paper, we describe the attributes of the problem, the implications it has on data analysis, and the solution

Brightman, Murray

Forster, Karl

Grefenstette, Brian W.

Harrison, Fiona A.

Madsen, Kristin K.

Miyasaka, Hiromasa

Pike, Sean

English

Caltech Authors - Main

Draft version May 5, 2020
Preprint typeset using LATEX style emulateapj v. 12/16/11
NUSTAR LOW ENERGY EFFECTIVE AREA CORRECTION DUE TO THERMAL BLANKET TEAR
Kristin K. Madsen1, Brian W. Grefenstette1, Sean Pike1, Hiromasa Miyasaka1, Murray Brightman1, Karl
Forster1, Fiona A. Harrison1
1 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA
Draft version May 5, 2020
ABSTRACT
A rip in the MLI at the exit aperture of OMA, the NuSTAR optic aligned with detector focal plane
module FPMA, has resulted in an increased photon flux through OMA that has manifested itself as
a low energy excess. Overall, the MLI coverage has decreased by 10%, but there is an additional time
varying component, which occasionally causes the opening to increase by up to 20%. We address the
problem with a calibration update, and in this paper we describe the attributes of the problem, the
implications it has on data analysis, and the solution.
Subject headings: space vehicles: instruments
1. INTRODUCTION
Throughout 2019, the NuSTAR science operations cen-
ter (SOC) were with increasing regularity receiving re-
ports of large differences between modules FPMA and
FPMB. These differences take the form as shown in Fig-
ure 1 (left panel), where an excess of photons is seen in
FPMA at low energies with respect to FPMB. Investi-
gations into the reported cases has confirm the excess,
but when compared to other sources with a similar in-
strument configuration, or even with the source itself at
a different epoch, the excess is not always present.
Of immediate concern, was a time dependent change
of the detector and instrument response. A change in
the effective area can be achieved by an unknown tilt of
the optic, which would move the optical axis to a differ-
ent location than presumed by the original calibration.
We investigated this possibility and rejected it on the
grounds that only the low energy spectrum shows devi-
ations, while the high energy spectrum between FPMA
and FPMB hasn’t changed, as would have been expected,
due to the mirror response being more sensitive with
off-axis angle at increasing energies. We also consid-
ered the possibility that one of the detector responses
had changed. Fortuitously, however, we observed the ex-
cess during a Crab calibration observation in which we
also obtained a stray light observation (see Madsen et al.
(2017) and Madsen et al. (2017) for details on the stray
light), and were able to confirm that in the stray light
observation, which excludes the optics, there were no dif-
ferences in the FPMA and FPMB spectra.
This narrowed down the source of the excess to be
associated with one of the static absorption elements en-
countered by the photon on its way from the optic to the
detector. The apparent variations as a function of time
could be a geometric effect as in the way the source illu-
minates the detector array and entrance apertures. We
investigated the known variations in the detector absorp-
tion dead layer (see details on dead layer in Madsen et al.
(2015)), and while significant, it could be ruled out since
sources illuminating the same patch on the detector were
observed to have a different excess.
This left two possibilities: 1) the Multi Layer Insula-
FPMA
FPMB
MAXI	J1348-630
R
at
io
	(D
at
a/
M
od
el
)
0.95
0.975
1
1.05
1.1
Energy	(keV)
105
MLI	fraction	=	64%		±	1%
FPMA
FPMB
MAXI	J1348-630
R
at
io
	(D
at
a/
M
od
el
)
0.95
0.975
1
1.1
Energy	(keV)
105
Fig. 1.— Left Panel: Spectra of MAXI J1348-630 (Obsid:
80402315010) fitted with the same model exhibiting a low energy
excess for FPMA. Right Panel: Same spectra including a model
for the MLI covering fraction where the FPMB covering fraction
≡ 1.0 and the FPMA covering faction = 0.64 ±0.01.
tion (MLI), which encloses the optics, or 2) the Be win-
dow located at the entrance of the focal module housing.
A hole in the Be window would have caused noise in
the detectors, and since this has not been seen, the MLI
remains the only plausible source.
2. BEHAVIOR OF THE MLI RIP
The conjecture that the MLI is responsible for the in-
creased flux in FPMA is supported by the fact that Op-
tics Module A (OMA matched to FPMA) has for years
been suffering from a rip in the MLI. In August 2017 we
began observing increased temperature swings in OMA,
which indicated that part of the backside of the optic
(facing the detector) was being exposed to space. We
interpreted this as a rip in the MLI cover. In January
2019 we began to see the effect of the rip also in an-
other temperature sensor on the backside of the optics
and indicated that the rip had enlarged. Combining this
evidence with the low energy excess, we concluded the
the most likely explanation for the excess was that the
rip was large enough to allowing more photons through
to the focal plane.
To test this hypothesis, we constructed an XSPEC
mtable to fit the fraction, F , of the MLI covering the
optic. At the affected energies, it is fair to assume (con-
firmed by raytracing) that the number of photons exiting
the optics is proportional to the area and not strongly
ar
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2affected by vignetting, so a F = 1 means the MLI fully
covers the opening, and F = 0.9 that only 90% of the
opening is covered by the MLI. Figure 1 (right panel)
shows MAXI J1348-630 (ObsID 80402315010) without
and with the corrected MLI, yielding a 65% covered frac-
tion of the aperture on the backside of the optic.
The next step was to investigate why only some obser-
vations showed the excess. To research the dependency
on time, we constructed a simple method to quantify
the excess. We multiplied the effective area (Aeff ) with
the detector efficiency (Deteff ) and divided the source
counts with this simplified response to remove the in-
strument effect, then divided the two modules with one
another and scaled by the exposure time, which may dif-
fer between modules:
FPMA/FPMB =
countsA
countsB
× AEff (FPMB) ◦Deteff (FPMB)
AEff (FPMA) ◦Deteff (FPMA) ×
expB
expA
.
(1)
At energies below 10 keV the detector response is
largely diagonal, and the ratio mostly removes the im-
pact of source spectrum, which for the purpose of mea-
suring the MLI covering fraction is accurate enough. We
can therefore fit the ratio directly with the MLI fraction
without having to deal with the source spectrum, and
performed this operation on the entire NuSTAR data li-
brary for all sources above 3 count second−1 and within
2.5 arcminutes of the optical axis, with the additional
restriction that all counts in the extraction region had
to be located on Detector 0. We then correlated this
data with the average temperature of OMA for each ob-
servation, and Figure 4 shows as a function of time and
temperature the evolution of the rip. The plot illustrates
several points:
1. the accuracy of the MLI fraction is on the order
of ±5%, due to other effects between FPMA and
FPMB such as different cross-calibration attributes
of the effective area,
2. the FPMA/FPMB difference has since launch been
at about 0.96, indicating that we did not have the
pre-launch thickness of the FPMA (or FPMB) cor-
rect,
3. although the temperature sensors only began to de-
tect the rip in mid 2017, it was already present in
early 2017,
4. the propagation of the rip appears to have stopped
in 2018,
5. however, since 2019 we occasionally see very large
differences in FPMA/FPMB correlated with a very
cold temperature of the optic.
The data show that under some circumstances the rip
is apparently able to increase in size, and that when this
happens the sensor measures a low temperature, pre-
sumably because the insulation properties of the MLI
are compromised. We verified this with a data set of
MAXI J1348-630 taken during a period when the tem-
perature changed in the middle of the observation, and
shown in Figure 3 is the temperature in the lower panel
10 12.5 15 17.5 20
Optics	Temperature
M
LI
	fr
ac
tio
n
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
Year
2013 2014 2015 2016 2017 2018 2019 2020
Fig. 2.— The covering fraction of MLI as a function of time color
coded with the temperature of OMA sensor OPT0 5TEMP. The
observations are used are from the entire NuSTAR flight library
fulfilling the following requirements: average count rate greater
then 3 counts s−1, average off-axis angle < 2.5′ , and extraction
region entirely on Det 0. Solid lines show mark the average level
of the MLI fraction for the given ranges.
T	<	9	deg/C,	F=0.73
T	>	15	deg/C,	F=0.82
FP
M
A
/F
PM
B
0.95
0.975
1
1.025
1.05
1.075
1.1
Energy	(keV) 10
Low	temperatureHigh	temperature
Te
m
pe
ra
tu
re
	(d
eg
/C
)
8
10
12
14
16
18
Time	(s)2.8756×10
8 2.8757×108 2.8759×108 2.876×108
Fig. 3.— Two epochs of the MAXI J1348-630 (ObsID:
80402315008) FPMA/FPMB ratios. During this observation the
temperature changed drastically, indicating the MLI ‘opened up’.
The high temperature spectrum spectrum (red) had temperature
above 15 deg/C and the low temperature spectrum (blue) had a
temperature below 9 deg/C. The difference in the MLI covering
fraction was 0.82 for high and 0.73 for low.
and associated FPMA/FPMB ratios for the two epochs
in the upper. This confirms that the fraction of MLI in
the photon path is correlated with temperature, and in-
dicates that the rip in the MLI is able to enlarge under
certain circumstances, resulting in a decreased tempera-
ture and increased photon flux.
Mechanically, it is difficult to explain how the rip in
the MLI can expand and contract, however, the change
in the temperature is confirmed through multiple OMA
temperature sensors, ruling out the possibility that the
low temperature readings are due to a single faulty ther-
mistor.
3. THE SOLUTION
We can correct the issue by decreasing the fraction of
MLI covering the backside of the optics in the instrument
3TABLE 1
Time dependent responses
Date range MLI fraction
date < 2017 0.96
2017 ≥ date < 2018 0.91
date ≥ 2018 0.86
TABLE 2
Temperature dependent responses
Temperature range MLI fraction
12 < T ≤ 15 0.8
10 < T ≤ 12 0.75
8 < T ≤ 10 0.7
response files. We can track the decrease of the MLI as
a function of time by assuming the rip occurred in 2017
and that it stabilized in 2018. Taking the mean value of
the data set described above for observations before 2017,
we find an average MLI coverage of 0.96. Because this
happened before the rip, we interpret this to mean that
we overestimated the pre-launch thickness of the MLI
by 4% on OMA, not to be confused with the covering
fraction after the rip. For the observations after 2018
we find an average MLI fraction of 0.86, showing that
the tear has reduced the MLI coverage by ∼10%. To
correct for the difference, we have adjusted the on-axis
ARF of FPMA by decreasing the thickness of the MLI to
F=0.96 before 2017, and changing the covering fraction
by F=0.91 in 2017 and F=0.86 after 2018. See Figure 2
and Table 1 for the valid ranges.
We applied these new ARFs to the NuSTAR data and
recalculated the MLI fractions as shown in Figure 4. The
result has been to re-normalized the A/B distribution at
1.
The additional time varying component, caused by the
expansion of the rip, can be correlated with temperature
as shown in Figure 5. The plot shows in black circles the
average temperature of all observations (above 3 counts
a second, 2.5′ on-axis, and on Detector 0) together with a
line fitted through the points; in triangles MAXI J1348-
630 (obsID 80402315008) divided into temperature bins
and fitted with the MLI fraction; in squares the Crab
(obsID 10502001006) also divided into temperature bins
and fitted with the MLI fraction. The data shows there
is a spread when using the average temperature during
an observations, but that when the measurement of the
MLI fraction is used with more precise temperature data
then the points fall close to the line fitted through the
average temperature data. Based on this we picked three
temperature bands and assigned the constant MLI frac-
tions as shown in Table 2 to each band.
These corrections only apply if the temperature is be-
low 15 deg/C. Otherwise the global corrections discussed
above apply.
When applying the corrections from Table 1 and 2 to
the NuSTAR library we see in Figure 6 that the cold
outliers are now brought into family.
The corrections are not perfect and there may be
individual cases for which one may still measure an
FPMA/FPMB difference. For those cases we supply the
XSPEC mtable, however, care must be taken when using
it. The exponential shape of the MLI correction is un-
Optics	Temperature
10 12.5 15 17.5 20
M
LI
	fr
ac
tio
n
0.7
0.8
0.9
1
1.1
1.2
1.3
Year
2013 2014 2015 2016 2017 2018 2019 2020
Fig. 4.— The covering fraction of MLI as a function of time color
coded with the temperature of OMA sensor OPT0 5TEMP. This
has been adjusted with the average MLI covering fraction from
Table 1.
80402315008
10502001006
M
LI
	fr
ac
tio
n
0.65
0.7
0.8
0.9
0.95
Temperature	(deg/C)
8 10 12 14 16
Fig. 5.— MLI covering fraction as a function of temperature.
Black dots are observations with mean OPT0 5TEMP ≤ 15 deg/C
and solid line is the linear fit to the points. Red squares: spectra
of ObsID 80402315008 filtered into temperature bins and fitted for
the MLI covering fraction (see Figure 3). Cyan triangles: spectra
of ObsID 10502001006 filtered into temperature bins and fitted for
the MLI covering fraction. Colored horizontal lines show the MLI
covering fraction assigned to the temperature range.
fortunately degenerate with many astrophysical models,
and we strongly advise to follow these steps to minimize
cross-talk with other continuum models:
1. First ignore FPMA between 3 – 7 keV, when doing
your original fits. Make sure the model includes
a constant between FPMA and FPMB. Find the
value of the constant and freeze it.
2. Notice FPMA 3 – 7 keV and add the mtable to the
model. Set the fraction value of the MLI to 1.0 for
FPMB and freeze it. Allow only the MLI fraction
of FPMA to fit.
3. Please note that the range of MLI fraction should
now lie between 0.9 and 1.0, and if the range is
outside that, something else may be wrong.
If these steps are not followed, and the mtable param-
eters and the constants between A and B are allowed to
410 12.5 15 17.5 20
Optics	Temperature
M
LI
	fr
ac
tio
n
0.8
0.9
1
1.1
1.2
1.3
Year
2013 2014 2015 2016 2017 2018 2019 2020
Fig. 6.— The covering fraction of MLI as a function of time color
coded with the temperature of OMA sensor OPT0 5TEMP. This
has been adjusted with the average MLI covering fraction from
Table 1 and 2.
fit freely, incorrect estimates of the continuum and line
shapes may occur.
4. MTABLE AND CALDB RELEASES
The XSPEC mtable is available at the NuSTAR SOC
webpages 1. It should only be used with consideration
to the caveats discussed above. It should not be used
to force a model into agreement, but only if significant
differences are observed between FPMA and FPMB.
For expediency, the CALDB will be released in two
parts:
1. Time dependent release containing the corrections
from Table 1. This will add three new arfs to the
CALDB, version 20200429.
2. Temperature dependent release containing the cor-
rections from Table 2. The temperature release
requires an update to the numkarf NUSTARDAS
module and will be released with the upcoming
heasoft update (to be determined).
This note will be updated with the second release of
the CALDB discussed above and is currently only valid
for CALDB version 20200429.
5. CONCLUSION
We have presented in this paper an important change
and analysis update to NuSTAR that must be considered
for all observations. The issue concerns an excess in low
the energy photons in FPMA with respect to FPMB, and
stems from a rip in the MLI cover of FPMA. We will re-
lease two CALBD updates to adjust the FPMA response
to account for the reduces amount of MLI absorption.
The first is the time dependent release, CALDB version
20200429, which only replaces the on-axis ARF. The sec-
ond it the temperature dependent release, which requires
an update to NUSTARDAS, and will be released with
the upcoming heasoft update. These updates will correct
most data. However, where may be a subset of observa-
tions, particularly bright ones, which may not entirely be
1 www.srl.caltech.edu/NuSTAR Public/NuSTAROperationSite/mli.php
corrected by this, and we have in addition provided an
mtable to be used for those observations. Care must be
taken in applying this mtable as it may potentially skew
the results if not used properly.
This work was supported under NASA Contract No.
NNG08FD60C, and made use of data from the NuSTAR
mission, a project led by the California Institute of Tech-
nology, managed by the Jet Propulsion Laboratory, and
funded by the National Aeronautics and Space Admin-
istration. We thank the NuSTAR Operations, Software
and Calibration teams for support with the execution
and analysis of these observations. This research has
made use of the NuSTAR Data Analysis Software (NuS-
TARDAS) jointly developed by the ASI Science Data
Center (ASDC, Italy) and the California Institute of
Technology (USA).
Facility: NuSTAR
REFERENCES
Madsen, K. K., Christensen, F. E., Craig, W. W., Forster, K. W.,
Grefenstette, B. W., Harrison, F. A., Miyasaka, H., & Rana,
V., 2017, Journal of Astronomical Telescopes, Instruments, and
Systems, 3, 3
Madsen, K. K., Forster, K., Grefenstette, B. W., Harrison, F. A.,
& Stern, D., 2017, ApJ, 841, 56
Madsen, K. K., et al., 2015, ApJS, 220, 8


NuSTAR low energy effective area correction due to thermal blanket tear

https://authors.library.caltech.edu/103715/1/2005.00569.pdf

NuSTAR low energy effective area correction due to thermal blanket tear

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