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The BBXRT Archive
Alan P. Smale
LHEA
1 Overview
The Broad Band X-ray Telescope (BBXRT) was flown on the space shuttle Columbia
(STS-35) on 1990 December 2-December 11, as part of the ASTRO-1 payload. The
flight of BBXRT marked the first opportunity for performing X-ray observations
over a broad energy range (0.3-12 keV) with a moderate energy resolution
(typically 90 eV and 150 eV at 1 and 6 keV respectively). This energy
resolution coupled with an extremely low detector background made BBXRT a very
powerful tool for the study of continuum and line emission from cosmic sources,
and the scientific results form a tasty appetizer of what can be expected from
Astro-D. The observing program was designed to be an even mix of Galactic and
extragalactic targets, although the Galactic Center region was not available
due to the time of year BBXRT was launched.
BBXRT was designed and built at the Laboratory for High Energy Astrophysics at
NASA/GSFC, with Dr. Peter Serlemitsos as Principal Investigator. During the
mission, the instrument was controlled by the scientists and staff of the X-ray
group from an operations center at GSFC, and the group as a whole handled
mission planning and real-time data analysis.
In spite of some well-publicized technical hitches during the mission with the
instrument's pointing system, BBXRT successfully performed ~160 observations of
~80 celestial sources including clusters of galaxies, active galaxies, SNR,
X-ray binaries, cataclysmic variables, stars, and the X-ray background.
2 The BBXRT Instrument
BBXRT (Serlemitsos et al 1984, 1992) consists of two co-aligned telescopes with
focal length 3.8m and diameter 40cm. Each telescope contains a thin-foil
conical mirror assembly consisting of 118 curved, gold-plated aluminum
reflectors, and a segmented, cryogenically-cooled lithium-drifted silicon
spectrometer at the focal plane. The optical system as a whole provides a
total collecting area of 765 cm2 at 1.5 keV, and 300
cm2 at 7 keV.
Each of the detectors, 'A' and 'B', is subdivided into five pixels, each with
512 energy channels. The central pixels (A0, B0) have a field of view 4 arcmin
in diameter, and the outer pixels (A1-A4, B1-B4) extend the total field of view
to a diameter of 17.4 arcmin. The energy channel width is 16 eV for channels
0-255 and 32 eV for channels 256-512. The B detectors are rotated by 180
degrees relative to the A detectors such that e.g., A1 and B3 co-observe the
same area on the sky. Five data quality bits accompany each detected event.
Between the pixels is a mask 1.5 arcminutes across.
The optical plate holding the mirrors also contains a CID camera for real-time
aspect determination and a star tracker for updating the pointing system gyro
reference. The instrument structure for BBXRT is 4.1m long and 1m in diameter,
and weighs 840 kg. During the mission it was attached to its own dedicated
pointer in the shuttle bay, the Two-Axis Pointing System (TAPS), a generic,
two-axis gimballed pointer supplied by GSFC which was capable, in principle, of
achieving stable pointing to arc minute accuracy. The other three telescopes
making up the ASTRO-1 payload utilized a separate pointer.
3 The Mission
BBXRT was in orbit for almost exactly nine days. The instrument was powered up
three hours after launch and remained so until approximately six hours before
reentry. The detector system was powered up for nearly the entire time,
collecting source, background or calibration data. The instrument behaved
nearly flawlessly on orbit. The sole source of difficulty was due to
occasional hangups during the first few days of the processing electronics
associated with the CID aspect camera. These were eventually eliminated by
modifying the settings of a few onboard processing parameters. The cryostat
temperature remained stable within approximately two degrees over the course of
the mission, and the instrument alignment was unchanged. All other housekeeping
parameters were monitored during the mission and examined post-flight. There
were no variations outside nominal values.
Unfortunately, there were other problems which affected the total mission
efficiency. In particular, both the IPS, on which the UV instruments were
mounted, and the TAPS experienced difficulties. The primary consequence to
BBXRT of the IPS difficulties was that the entire preplanned, optimized mission
timeline was dropped and the mission was replanned on a day-by-day basis with
lower efficiency (more time spent in Earthblock, for example). The TAPS
problems reduced BBXRT's observing efficiency even further. No stable pointings
were accomplished for the first 60 hours of the mission because of an
improperly compensated gyro drift rate. Once the gyro drift was removed, the
TAPS was found to be capable of pointing quite stably, with a drift rate of
less than 0.1 arcsecond/second. It was then discovered that the accuracy to
which TAPS could acquire a source was typically 2-5 arcmin, resulting in a
several real-time tweaks of the pointing direction. Moreover, the pointer
experienced glitches during maneuvers between sources, inducing on each
occasion a several arcmin systematic error on the pointing accuracy, and
requiring each time the use of an orbit night to realign the gyros by pointing
the star tracker at bright stars. Finally, a day before reentry, an instability
in the TAPS pointer made it exceed its hardware travel limits whereupon it
became permanently engaged in its backup landing lock. This forced BBXRT to
spend the last three orbits of observations using the Orbiter jet-firings
themselves as the only source of pointing. (The sources affected are Puppis AB
and M87, both observed after MET day 8.38.)
Despite this litany of woe, BBXRT achieved a total of 185,000 seconds of
observation time on cosmic X-ray sources. An additional 100,000 seconds of the
total available observing time will be usable for studies of the diffuse X-ray
background. A total of 157 observations of 82 X-ray sources was achieved, with
typical observation times ranging from 300 to 3000 seconds (note that a single
pointing with a tweak in the middle is counted as two 'observations').
Although obviously we'd have liked more, this is still a great success
considering the unique difficulties of Shuttle-based astronomy. Table 1 lists
the sources observed by BBXRT.
Table 1 - BBXRT Source List
0114+65 A496 IC443 NGC4051
0212+735 A665 II-PEG NGC4151
0237-230 A754 LMC X-1 PERSEUS
0548-32 ALGOL LMC X-2 PUPAB
0614+09 AR-LAC M15 PUPAC
0620-003 BD61DEG M49 Q1821
0748-676 BY-DRA M81 RS-CVN
0836-42 CAPELLA M82 SCO430
0851+202 CAR+WR25 M87 SIG-GEM
1218+30 CAS-A MKN279 SN1987A
1426+428 CEN X-3 MKN3 TON490
1634+706 COMA-CL MKN335 TYCHO
1700+64 CRAB MKN876 VELA X-1
1E1048.5 CTB109 N132D VELA X
2155-304 CYG X-1 NGC1068 X-PER
24-UMA CYG X-2 NGC139 YY-GEM
3C273 CYG X-3 NGC1566 Z-CAM
44IBOOA EV-LAC NGC2110 ET-OR
A2256 EZ-CMA NGC253 ZET-PUP
A2319 GAM-CAS NGC2992
A262 H0538+61 NGC3227
4 The Detector Response
The BBXRT detector spectral response matrices have been painstakingly
constructed based on data from pre-launch, in-flight and post-flight
calibrations. Before launch, measurements of the effective area of the mirrors
and the detector efficiencies were made at GSFC. In flight, spectral
calibrations were performed by moving an internal Fe 55 source into the field
of view and observing the Mn K and
K lines. The
Crab Nebula was observed on several occasions.
More detailed modeling of partial charge in the detectors, and adjustments for
the gold M edge in the mirrors and Al and Si edges in the detectors, have been
performed since the flight of BBXRT, using further ground calibration data
obtained at GSFC and the National Institute for Standards and Technology in
1991 and 1992. Using the most recent response matrix, the systematic residuals
from a power law fit to data from the Crab Nebula show no features larger than
5% between 0.5--8 keV and 10% between 8--12 keV, and no narrow features in the
spectrum with an equivalent width >12 eV. A 10% residual uncertainty still
persists at ~0.4--0.5 keV. A full description of the BBXRT calibration process
can be found in Weaver, 1992.
5 Schedule for Data Release
The release of BBXRT data products to the community will take place in two
parts. All products will be in standard FITS formats defined by the HEASARC.
These products will be available on the HEASARC On-line Service, and by
arrangement with the National Space Science Data Center (NSSDC) at GSFC.
On 1992 SEPTEMBER 30TH the Level 2 ("Products") database was released,
including:
(a) A complete observation list for the mission;
(b) Spectra, one per pixel per observation;
(c) Background spectra, one per spectrum;
(d) Light curves, one per pixel per observation;
(e) On- and off-axis response matrices;
(f) Documentation.
(Note that calibration activities are continuing, and there may be future
refinements to the models of the response and backgrounds.)
In 1993 FEBRUARY the Level 1 ("Photons") database will be released
containing:
(a) Complete photon list files for the whole mission, including background
and calibration raw data;
(b) Housekeeping data -- temperatures, guard rate and other mechanical/technical
information from the instruments;
(c) Pointing data -- RA, Dec and roll angle of the pointing position of the
instrument throughout the mission;
(d) Mission quality data -- Sun angle, Earth angle, orbit day/night etc;
(e) A complete guide to the BBXRT mission and data analysis.
The SELECTOR software currently under development by the HEASARC will permit
FITS file manipulation and multi-mission data access and filtering. This
software will run both under IRAF, as a series of standalone packages, and as a
package under the XSELECT interface. A complete release of the SELECTOR
software to the community will coincide with the launch of Astro-D in February
1993 and the release of the full BBXRT dataset.
The BBXRT data products are available under BROWSE in the HEASARC's on-line
system, accessible on Internet at ndadsa.gsfc.nasa.gov (128.183.36.17) or on
DECnet at NDADSA (node 15761). Type XRAY at the username prompt to
enter the service. You can request documentation about the BROWSE data bases
and the on-line service by sending E-mail to request@ndadsa.gsfc.nasa.gov.
The products can also be obtained via anonymous FTP from legacy.gsfc.nasa.gov
(128.183.8.233), from the /DATA/bbxrt subdirectories.
For those who wish to obtain the observation catalogues in their entirety, they
are also available in ASCII format in the anonymous FTP account, in the
/DATA/bbxrt/doc directory.
For each scientifically useful pointing, the observation catalogs available via
BROWSE or anonymous FTP contain: an accurate start and stop time, exposure
time, a day/night flag, Medium/Low rate data flag, a sequence number and object
class, the countrate and error in each pixel, the mean aspect solution plus
errors for each pointing, when available, and the off-axis angle. The start
and stop times are based on attitude information, count rates and SAA
indicators. Four observations (Puppis AC, Cyg X-1 [MET 6.2548], Cen X-3 and
1E1048.5) are included in their entirety in the catalogue despite major SAA
contamination, and are flagged accordingly.
The legacy:/DATA/bbxrt/doc directory also contains the text of this document,
and a BBXRT publications list.
The BBXRT data processing and calibration software and the software for
conversion of BBXRT data into FITS, were written at the Laboratory for High
Energy Astrophysics, Code 666, NASA/GSFC. The SELECTOR software is being
developed by the HEASARC/Astro-D project, Code 668, NASA/GSFC.
6 BBXRT data analysis issues
In this section, I go into greater depth about critical aspects of BBXRT data
analysis. Users of the BBXRT archive should understand that calibration
activities are still continuing, and that not all issues are resolved. In the
following sections, I summarize our knowledge of various important factors in
BBXRT data analysis, and indicate where this knowledge is incomplete.
6.1 The Point Spread Function and the Mask
The distribution of photons in the focal plane can be empirically described by
a pair of error functions, one narrow to fit the central core of the beam, and
one broad to represent a halo, perhaps from geometric scattering. If
is the total power contained
within a given angular
radius (in arcminutes) and
N1
and N2 are the relative normalizations of the
core and halo components, then
For our best fitting model we find 1
= 1.8 arcmin,
= 5.8
arcmin, N1 = 0.65, N2 = 0.35. The
resulting half power radius is approximately
1.3 arcmin.
The silicon detectors consist of five separate cylindrically symmetric
detecting elements defined via grooves onto a single silicon block. In order
to prevent the detection in multiple elements of X-rays incident near the
grooves, an X-ray opaque mask was placed in front of the detector. The mask is
shaped to cover the grooves, but occults a width of 0.5mm on each side of the
grooves. Projected onto the sky, this corresponds to a width of 1.5 arcminutes.
Because of the spreading of the X-rays in the focal plane due to the mirror,
the mask reduces the effective area by 20% for an on-axis source, and as much
as 50% for a source centered on the intersection between the central circular
mask and one of the radial spurs of the mask.
Combining the effects of the PSF and the masking, for an on-axis pointing 62%
of all focussed events fall in the central detector, 20% fall on the mask and
17% are distributed among the four outer detectors.
6.2 The Detector Background
The non-X-ray background for the BBXRT detectors was very low (see Table 2).
Table 2
Central detectors Outer detectors
1 keV 2 keV 6 keV 1 keV 2 keV 6 keV
Ground cal 0.0002 0.0002 0.0002 0.0007 0.0007 0.0007
Flight data 0.003 0.003 0.002 0.02 0.01 0.005
(In units of count s^-1 keV^-1 field^-1,
i.e. the sum of data from two detector elements viewing the same part of the
sky.)
While the on-orbit background was a factor of two to three higher than we
anticipated, it should be kept in mind that the Sun was active during the
mission, with two solar flares occurring in the first three days. The
background was found to vary with time, and these variations are strongly
correlated with the guard rate (see below). For energies less than
approximately 4 keV, the diffuse X-ray background dominates the total non-X-ray
background.
At this point it may be worth mentioning some details of the anticoincidence
methods used. The photoelectrically converted signals in the five detector
elements were output to gate leads of jFETs clustered behind the detector in
the cryostat. Also in the cryostat, close to the FETs, were LEDs which were
occasionally flashed on to dissipate the accumulated charge within the gates.
Five separate anticoincidence techniques were applied: events detected in a
given element were compared against simultaneous events in that or another
element (pulse-pulse and pixel-pixel anticoincidence); against events in any
element with deposited charge larger than that expected from a 14 keV X-ray
(Very Large Event anticoincidence); against firings of the opto-feedback
circuits (LEDs); and against triggering of the guard detector. This guard
consists of a pair of photomultiplier tubes coupled to plastic scintillators,
which surround 85 percent of the solid angle around each detector. Events
failing these tests were flagged accordingly and included in the event lists
but not in the accumulations of spectra and light curves.
These anticoincidence techniques were extremely successful in eliminating
background; the best example of how effective they were was our ability to
observe the bright X-ray pulsar Cen X-3 through the South Atlantic Anomaly with
a loss of only 20 percent of the photons due to anticoincidence flagging. No
X-ray detector previously flown has ever been able to observe sources during
SAA passage.
6.3 Background Subtraction
From the point of view of background subtraction, the BBXRT observations fall
into two categories; those performed during orbit day, and those performed
during orbit night. For the orbit night observations, the background files
given in the archive are adequate and correct. To first order, the orbit night
backgrounds come from the internal background of the detectors described above,
scaled directly with the guard rate.
During orbit day, there is an additional background component caused by the
resonant scattering of solar X-rays off the Earth's atmosphere. This leads to
a substantially increased background flux at energies below 1 keV, particularly
a strong 524 eV oxygen line. Channels 20-60 suffer the most from this
geocoronal emission. The exact amount of bright Earth contamination of the
data at low energies is a steep function of Earth angle as well as guard rate,
and this requires a careful selection of background data. We are still working
on an accurate way of generalizing background subtraction under these
circumstances. Thus, the background spectra in the archive do not include a
bright Earth component, and oxygen and nitrogen lines are visible in the data
below 1 keV. Upgrades to the archive spectra can be expected when a dependable
background subtraction method is finalized.
Above 2 keV the bright Earth contribution becomes negligible and the orbit
night backgrounds work acceptably for the orbit day data.
The Mission Quality files referred to above contain six qualities derived from
the Shuttle telemetry and standard astronomy programs which can help interpret
BBXRT data:
- the Sun angle, or angle between the pointing direction of BBXRT and the Sun;
- the Moon angle (we never detected the Moon and it never interfered with
observations, but this angle is included for completeness);
- the Earth angle, or angle between the pointing direction and the center of
the Earth (note that the Earth subtends a semi-angle of 72 degrees at the
altitude of BBXRT);
- a 1/0 flag indicating whether the nearest Earth limb to the pointing
direction was bright or dark (1=bright);
- a 1/0 flag indicating whether the Shuttle was in orbit day or orbit night
(1=day);
- and the ram angle, or velocity vector angle, between the velocity vector of
the shuttle and the pointing direction.
Although these files will not be formally released to the community until 1993
February, it may be possible to make this data available to individual analysts
on request before that date.
6.4 Detector Alignment
Using the beam shape described above, we compared data from observations of
Cygnus X-1, Cyg X-2, Cyg X-3 and X0614+09. A number of observations were used
in order to ensure that a consistent result was found. Ratios of counts in the
five pixels of each detector were compared with those predicted using a series
of Monte Carlo simulations of the beam in the focal plane, for various off-axis
angles and azimuths, to determine the appropriate beam spread and off-axis
position. The bottom line is that there is a mean displacement of 0.822
arcminutes (49 arcsec) between A and B, and an azimuth of -63.24 degrees. The
azimuth of the displacement is measured from the A3/A4 boundary, and
corresponds to a rotation of 63 degrees through B4 towards B1. So, for a source
on-axis in A, the distribution of counts in the outer pixels of B should be
maximized in B4, with a slight excess in B1. Similarly, for a source on-axis in
B, the counts in the outer pixels of A should be maximized in A2, with a slight
excess in A3. The relative orientation and offset of the pixels is shown in
Figure 1.
Figure 1
6.5 The Off-axis angle; Matrices and Aspect Solutions
Observations performed off-axis suffer from a measure of vignetting by the
mirror. Because of this effect, it is important to use the response matrix
constructed for the off-axis angle relevant to the observation being studied.
Response matrices covering the full range of off-axis angles are provided in
the BBXRT archive, but it should be noted that although the energy vignetting
is handled correctly in these matrices, no absolute flux correction for losses
due to the mask have been applied.
We have two types of pointing information for the BBXRT observations. The
attitude files (part of the 1993 February release) contain pointing information
for the entire mission; a right ascension, declination and roll angle every
second, in 1950 coordinates, including slewing intervals. This pointing
information was constructed by adding the TAPS (Two-Axis Pointing System)
gimbal angles to the position of the Orbiter -Z-axis. This is only accurate to
maybe 10 or 20 arcminutes, which makes it useful as a guide to the start and
stop times of observations and as a sanity check for what area of the sky we
were looking at, but not directly useful as an absolute position reference for
the calculation of the off-axis angle. A more accurate position can be
determined from the aspect camera solutions.
We have aspect solutions for more than 60% of our observations, and the mean
aspect solution for each pointing is contained in the observation catalogues
accessible via BROWSE or anonymous FTP. For many observations, no aspect
solution exists. The principal reason for this is that the aspect camera was
turned off during orbit day, because the illumination was too great for it, and
during the SAA.
In addition, there were several problems in the first three days with the
processing electronics associated with the aspect camera. These were eventually
eliminated by modifying the settings of a few onboard processing parameters.
There were also intervals when the aspect camera was flooded with reflected
light or Earthglow; sometimes the star field observed was too sparse, and too
few stars were available for an aspect solution, and for some LMC pointings the
field was too rich. Finally, there are of course no solutions for times when we
were Earthblocked or slewing.
The aspect camera boresight was determined from observations of the Crab,
Cygnus X-1, Cyg X-2, Cyg X-3 and X0614+09. A couple of iterations were
performed before settling on the final boresight on which the aspect solutions
contained in the database are calculated. For some bright sources, the
off-axis angle of the observation can be determined using ray-tracing; these
angles are also included in the released database. Where no formal off-axis
angle can be determined, the relative count rates in each pixel can be used to
make an inspired guess. Off-axis vignetting corrections (i.e., a given
off-axis response) are not sensitive to a 1 to 3 arcminute error in the
off-axis angle, depending on source count rate.
6.6 Microphonics
From examination of data taken when the covers were closed on the BBXRT
instrument, we found evidence for bursts of spurious events that are probably
connected with microphonics. Typically such a burst lasts less than two
milliseconds, and contains up to five events from within a given detector
(usually A) but from several pixels (more often than not A4, A1 and A0). The
microphonic events form a major contribution to the low-energy background for
very-low-countrate sources. For brighter sources, the contribution from
microphonics is negligible.
A routine was written to remove such events during the accumulation of
background or low-countrate sources. For observations where the total countrate
(including events rejected by anticoincidence flags) is below 5 counts per
second, the accumulated spectra in the database were constructed using this
tool, to ensure no pollution by microphonics. The background files were also
created using this routine.
6.7 Timing information
Table 3 shows the launch and landing times for BBXRT in a range of different
systems:
Table 3
Launch Landing
UT day of 1990: 336/06:49:01 = 336.284039 345/05:54:09 = 345.245937
EST: Dec 2, 01:49:01 Dec 11, 00:54:09
JD: 244 8227.784039 244 8236.745937
MET: 0/00:00:00 8/23:05:08
There are several clocks on board the shuttle that are relevant to this
discussion. BBXRT has its own internal clock, and so do TAPS and the Orbiter.
Of the three, the Orbiter clock is the most reliable; every 60.000000 seconds,
this clock emits a so-called MIN pulse. The MET time of the last-received MIN
pulse is embedded in the BBXRT raw telemetry. Examining the MET and MIN times
throughout the mission, it was found that
1 BBXRT minute = 59.99651 0.00001 seconds.
There is no evidence for a day-to-day variation in this value. Thus, the BBXRT
clock loses 5.026 seconds per day, or about 45 seconds over the whole mission.
This estimate agrees well with a pre-launch estimate of 5.04 seconds/day
derived using a 0.73-day calibration run.
We have also determined that the TAPS clock runs 9 seconds behind the Orbiter
clock. All derived pointing information includes this correction.
As the the BBXRT observations were so short and BBXRT's strength is
spectroscopy rather than timing analysis, no barycentric corrections have been
applied to the data.
6.8 Discriminator Thresholds
For the 1992 September archival release all the channels have been written to
the spectral files, however it should be noticed that the first few channels
are below the discriminator cutoff and always have zero counts. (The number of
affected channels varies with detector and with the discriminator setting at
the time of the observation.) Also, channel 512 is the overflow channel and
should be ignored in spectral fits. Future versions of the PHA files may
include a QUALITY column to indicate the 'bad' channels.
As a rule of thumb, the central elements had an effective low energy threshold
of around 300 eV. For the outer elements, this threshold was closer to 500 eV.
The central pixels can go lower because of their intrinsically lower noise and
better shielding, surrounded as they are on all sides by active detector
elements.
6.9 Deadtime
Deadtime is negligible for all but the brightest sources we observed,
namely the Crab, Cygnus X-2 and (marginally) Cygnus X-3. It is a steep
function of countrate; for the on-axis observation of the Crab (total count
rate including flagged events approx 2000 cts/sec) the deadtime is about 35% in
total, while it is less than 10% for the on-axis Cyg X-2 observation (1150
cts/sec) and less than 5% for Cyg X-3 (278 cts/sec). All other BBXRT
observations have fewer than 200 cts/sec.
The following is a summary of our current knowledge of possible BBXRT deadtime
effects.
There are four reasons why X-rays incident on the BBXRT detectors might not
make it into the telemetry stream:
(1) The telemetry can only hold 252 events per frame for a total rate of 2016
events per second. To first order, once the telemetry buffer is full
additional events that occur before the start of the next frame will be lost.
(Actually, the data from each pixel is buffered, so that one event from each of
the ten pixels can be saved after the telemetry buffer is filled. These saved
events are then put into the telemetry buffer at the start of the next frame.)
Note that if the event rates for the pixels are different, the dead time will
be different for each pixel; pixels with large rates have larger deadtimes.)
For the Crab (start MET: 3.055), a plot of event time versus event number shows
glitches every 0.125 seconds, corresponding to the filling of the telemetry
buffer. The glitches have an average dead time of 14.5 msec or about 12% of the
frame duration.
(2) Events may be flagged with one or more of the five quality flags (LED,
guard, large event, pixel-to-pixel, or pulse pile-up). BBXRT could be commanded
not to put these events into the telemetry, although we never did this.
Spectral data are normally selected to exclude events with any of these flags
set (the archive spectra and lightcurves were accumulated using only unflagged
events).
(3) Pulse pile-up can also prevent events from getting into the telemetry. If
an event occurs within an interval of about 100 microseconds of a previous
event in the same pixel, the earlier event is flagged and the second event is
not analyzed. The interval was chosen so that unflagged events are completely
unaffected by subsequent events. Many of the flagged events will only be
minimally affected. The exact length of the interval varies by about 10% from
pixel to pixel. If a third event occurs more than 100 microsec after the first
event but less than 100 microsec after the second, the third event will be
analyzed and have the pile-up flag set.
For the Crab data referred to above, 13% of events are flagged due to pulse
pile-up, contributing to a flagging ratio of about 20% overall.
(4) Events in the same pixel that may occur within 10-20 microsec may not be
sensed as different events in the BBXRT electronics. The pulse heights will
then pile up in a nearly linear fashion. For large rates this can affect the
resulting spectrum. To be strict, the convolution of the spectrum with itself
times the fraction of such events should be subtracted from the spectrum. In
practice, considering the count rates we encountered, we consider this to be
unnecessary.
6.10 Co-adding pixels
When analyzing data for which a significant signal was detected in several
pixels (i.e. almost every observation) it may be tempting to combine data from
different pixels into a single spectrum. However, this is strongly discouraged;
the resolution of the detectors is so good and the gains are sufficiently
different that smearing of spectral features and distortion of continuum
information can result from such an addition. The best thing to do (and the
technique which is almost universally adopted in BBXRT papers originating from
the LHEA) is to fit the data from the brightest pixels simultaneously, with
chained values for continuum parameters such as photon index, temperature etc.,
and separate normalizations.
It should go without saying that for an extended source, one should be cautious
about even this approach to spectral fitting and carefully treat the pixels
separately before attempting to combine data in a joint fit.
7 Accessing BBXRT data
The BBXRT data archive is available from the HEASARC On-line Service. See
Legacy #1 or the HEASARC On-line Users Guide for more information on how
to list the catalog and display and extract the data products. The usual
command xp/spec will extract all 10 pixels for a given observation. A
new pixel option is available to allow the user to extract data products from
individual BBXRT pixels. This option takes the form
xp/spec=a0
where a0 is the pixel that is to be extracted. The command
xp/summary
lists all the available data products for BBXRT. These include, spectral,
background, response and lightcurve files for each pixel.
More information on the BBXRT archive (including on-line examples) will be
available in the new On-line Users Guide (due out in January) and in the next
issue of Legacy.
Acknowledgements
This article is based on the shared wisdom and folklore of the X-ray Group of
the Laboratory for High Energy Astrophysics, NASA/GSFC. In writing it, I've
borrowed freely from various internal memos, e-mail messages and brainstorming
sessions. Any errors and omissions, of course, remain mine alone.
References
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