Newsletter No. 65 November 1994
IN THIS ISSUE:
- Notes from the Editor - K. Hurley
- HEAD Newsletter Available on Mosaic - K. Hurley
- News Items
- Meetings
- Elections - Voting Instructions
- Candidates' Statements
- Ballot
- BACODINE - The BATSE Coordinates Distribution Network
- S. Barthelmy et al.
- The ALEXIS Mission - History and Current Status - J.
Bloch
- News and Notes from Headquarters - A. Bunner
- 75th Anniversary Debate - R. Nemiroff
- XTE - J. Swank
In the last newsletter, I included a ballot which allowed members to vote for an e-mail
newsletter, and reconstructing the HEADmail list. I also invited comments on other
possibilities for the HEAD newsletter. There were 63 replies. 78% percent of you voted to
drop HEADmail, with 22% voting to reconstruct it. 80% voted for an e-mail newsletter, 7%
for a hardcopy version only, and 13% for both. Therefore, I will drop HEADmail, and
distribute an e-mail newsletter to those who can and will read it, but distribute a
hardcopy by mail to those who want one. I think that I know who you are, but if you find
that you are not getting the regular mail version, please let me know.
A number of interesting suggestions were made for the newsletter, but one which came up
several times was to make it available on the World Wide Web. I am happy to report that,
as of now, the newsletter will be available on the AAS Mosaic site. See the following
article for details. Another was to keep the newsletter brief; the last issue was almost
1200 lines long, and some complained that it was difficult to read; this issue is 30%
shorter.
I continue to welcome suggestions for the newsletter, as well as news items you wish to
have published. Please send them to me at khurley@sunspot.ssl.berkeley.edu, or fax them to
(510) 643-8302.
The AAS has a Mosaic site (http://blackhole.aas.org/AAS-homepage.html) which they have
kindly agreed to let us use for our newsletter. We appear to be the first division to use
the site for this purpose, but we probably will not be the last. The first electronic
version of our newsletter will soon be placed on their site, as well as this newsletter,
with figures and hypertext references; you will also be able to find the membership list
there. I will send instructions for accessing the HEAD home page in a week or so. For the
time being, the AAS site has unrestricted access, which means that people who are not HEAD
members can read the newsletter. However, I am told that the AAS is considering
restricting access in the future. My plan is to continue to issue the e-mail and regular
mail versions as long as they are considered useful, but of course they will not contain
the figures that the Mosaic version will.
On March 24, 1994, Donald D. Clayton was presented with the Governor's Award for
Excellence in Science by Governor Carroll Campbell of South Carolina.
The AAS has sent out the 1995 invoices. If you have not received yours, please contact
Sharon Savoy immediately (ssavoy@blackhole.aas.org, or fax at (202) 588-1351)
Flares and Flashes, A Multi-Wavelength Approach to Short-Term Phenomena, IAU Colloquium
151, 5-9 December, 1994, Sonneberg, Germany. Contact: W. Duerbeck, fax 49 251 833669,
hilmar@cygnus.uni-muenster.de
17th Texas Symposium on Relativistic Astrophysics, Munich, Germany 12-17 December,
1994. Contact: G. Morfill, MPE Garching, fax 89 3299 3235
185th AAS, Tucson, AZ, January 8-12, 1995. HEAD Sessions: 11 January
Second Waterville Valley Workshop on High Energy Solar Phenomena: The Lessons of Cycle
22 and the Promise of Cycle 23, Waterville Valley, New Hampshire, March 6-10, 1995.
Contact: preble@unhesp.unh.edu
Workshop on High Velocity Neutron Stars and Gamma-Ray Bursts, March 15-17, University
of California, San Diego. Contact: workshop@mamacass.ucsd.edu
Astrophysics in the Extreme Ultraviolet (IAU Colloquium 152), Berkeley, California,
March 27-30. Contact: iau152@cea.berkeley.edu
Towards the Source of Gamma-Ray Bursts, ESTEC, Noordwijk, The Netherlands, 25-27 April,
1995. Contact: eslab29@astro.estec.esa.nl
The Distance Scale to Gamma-Ray Bursts (debate), Washington, D.C., April 1995. Contact:
75th Anniversary Astronomical Debate, CSI Institute, George Mason University, Fairfax, VA
22030 or http://enemy.gsfc.nasa.gov/htmltest/gifcity/debate.html
3rd Compton Symposium, Munich, Germany, 12-14 June 1995. Contact:
cgro95@mpe-garching.mpg.de
24th International Cosmic Ray Conference, Rome, Italy, August 28 - September 8, 1995.
Contact: icrc95@roma1.infn.it or VAXROM::ICRC95
Eleventh Colloquium on UV and X-Ray Spectroscopy of Astrophysical and Laboratory
Plasmas, May 29 - June 2, 1995, Nagoya, Japan. Contact: watanabe@uvlab.mtk.nao.ac.jp or
JAOUVL(41881)::WATANABE
Aspen Center for Physics. Programs on Physics of Dense Stellar Systems, Big Bang
Nucleosynthesis, Inflation: from Theory to Observation and Back, Elementary Processes in
Astrophysical Dense Matter, and others. Contact: http://andy.bu.edu/aspen, or Sally
Mencimer, Aspen Center for Physics, 600 W. Gillespie, Aspen, CO 81611, (303) 925-2585
Interpretation of Gamma-Ray Sources and Related High-Energy Sources, Institute for
Theoretical Physics, University of California, Santa Barbara, mid-August - December 1995;
the workshop will start with a conference on Nonthermal Gamma-Ray Sources, August 21-25.
Contact: langer@sbitp.ucsb.edu
Three HEAD executive committee members (Hakki Ogelman, Rob Petre, and Belinda Wilkes)
are coming to the ends of their terms, and it is time to elect new members; they will
serve for two years. You will find the candidates' statements below, but first I would
like to explain the election process as it now stands. The HEAD by-laws at present do not
specifically allow for e-mail ballots. I am working to change the by-laws to allow e-mail
voting, but this cannot be accomplished rapidly. However, for this election, I will give
the membership the option of returning the ballot by e-mail or regular mail. To maintain
secrecy, e-mail ballots should be returned to headmtg@sunspot.ssl.berkeley.edu, where they
will be stripped of their originating headers by my secretary before they are turned over
to me. Alternatively, those of you who are receiving the e-mail version of this newsletter
may print the ballot out, and return it in a signed envelope to me by mail. In either
case, the ballots must be received by December 31.
Lynn Cominsky, Sonoma State University:
This fall's HEAD meeting marks the first time in many years that the division has tried
to develop a ``Multi- mission Perspective''. In the recent past, many topical conferences
were held - often one for each active mission - that diluted the scientific impact while
increasing the amount of travel funds expended by the community. As someone who has worked
with data from both X-ray and gamma-ray missions, I would encourage HEAD to continue to
sponsor such multi-wavelength meetings, as it is clear that the broadest base possible is
needed to truly understand the richness of the scientific phenomena that exist in the high
energy domain.
Additionally, I would use my position as a HEAD executive committee member to encourage
both NASA and the AAS to expand the opportunities available for undergraduate
participation in NASA funded research activities. Currently, the AAS has a limited special
program to support this type of work (which does not provide enough funds to employ a
student for more than one semester), while NASA has programs that support both K-12
outreach and research by graduate students, but skips over the training of college
undergrads. (NSF sponsors an undergraduate research program, but is not very interested in
supporting space-based data analysis activities.) As a faculty member at a small
undergraduate university, I have enjoyed doing research with undergraduates, some of whom
have gone on to either jobs as scientific assistants or to graduate school. While a glut
of physics graduate students interested in high energy astrophysics research is not
sustainable under present funding scenarios, all of my students have acquired useful
skills and training in scientific methods that have given them a competitive advantage in
today's increasingly technical society, as well as a deep appreciation for the importance
of the continued funding of astrophysics research. Augmented support for the scientific
training of undergrads is something that HEAD can and should encourage.
Chuck Dermer, Naval Research Laboratory:
Although this is a great time for science, it is not such a great time for all
scientists. It is distressing to see colleagues about to enter mid-career who lose their
funding and are forced out of the field. To improve the visibility of more junior people,
we should consider taking a few of the selections for invited speakers out of the HEAD
committee and let people canvass individually to HEAD members to support their invitation.
A plan to improve public outreach must be formulated. As a start, I plan to propose an
initiative to NASA whereby there is a travel fund available to people on soft money,
provided they give a public lecture en route (for example, to local amateur astronomy
clubs or civic groups). I only started to realize the depth of the public's interest in
astronomy after speaking to the Houston Astronomical Society. Someone may emerge who has a
gift for conveying the excitement of high-energy astrophysics to the public. Related to
this, a useful function that HEAD could provide is to have a library of images from
high-energy astronomy available for all who are interested. A yearly HEAD meeting is a
significant advance, and recognizes the integral nature of X-ray and gamma-ray astronomy.
But we live in a nature without borders, and these meetings should include experts from
lower energy wavelength ranges to ensure cross-fertilization. I have worked at
universities (UCSD, UC Berkeley SSL, Rice U., UMd), NASA (GSFC) and government labs (LLNL,
NRL), and think this background gives me some awareness of the problems faced in our
field. I would like your ideas too-- dermer@osse.nrl.navy.mil. Thank you for your support.
Chryssa Kouveliotou, NASA Marshall Space Flight Center:
In the current situation, where all space missions are fighting for survival,
regardless of whether they are successfully flying or scheduled to fly in the future, the
high-energy astrophysics community needs strong (internal and external) communication
links, for timely information dispersion and, when needed, short response times. We need
public support, raised through an increased public awareness of the role of high-energy
astrophysics in the understanding of the Universe; to achieve this a continuing dedicated
educational effort is required.
If elected, as a member of the HEAD executive committee, I will do my utmost to serve
the interest of our community.
Dan McCammon, University of Wisconsin:
The best assurance for future missions and opportunities in high energy astrophysics is
a widespread appreciation of the excitement of the results and their bearing on the most
fundamental astronomical problems. I think that the executive committee can best further
HEAD goals by continuing to develop opportunities for disseminating the exciting results
in high energy astrophysics: among ourselves, to practicing astronomers, and to the
public. An outstanding example of the first is the very stimulating program scheduled for
the Napa meeting. Given the particularly close relation of high energy astrophysics to
some traditional branches of physics, I would also like to explore possibilities for
closer association with the Astrophysics division of the American Physical Society.
Richard Mushotsky, NASA Goddard Space Flight Center:
I have been involved in high energy astrophysics since my Ph.D work on X-ray emission
on active galaxies with OSO-7. I have been a permanent member of the GSFC staff since
1981. I have worked on the data from virtually all the missions launched in the last 15
years, have been the author of numerous papers and am a member of the ASCA, AXAF and XMM
science working groups. I find that our field is in a peculiar state right now. We have
several new missions flying (Rosat, GRO, ASCA), several preparing for flight soon (XTE,
SAX, Spectrum X) and 3 very powerful missions at the end of the decade (AXAF, XMM and
Astro-E). In many ways this is the golden era of high energy astrophysics; new, very high
quality data is arriving at a unprecedented rate and a large number of new discoveries are
being made. Yet many members of our field are very unhappy- there are no planned future
hardware missions and it is perceived that jobs are scarce.
I believe that a major cause of the perception of job scarcity in our field is the
relative lack of university professorships. While it is unclear how to "solve"
this problem, I propose that the HEAD council take a serious look at ways to encourage
universities to hire promising researchers in high energy astrophysics. This is not
unreasonable, since more than 50 professorships were advertised in the last year, yet,
very few (if any!) high energy observational astrophysicists were hired. As members of a
vigorous and intellectually exciting field there must be some way to convince American
universities that this field should be represented in their faculties.
Mark Leising, Clemson University:
The future of high-energy astrophysics as an experimental science is not assured. Like
it or not, we must sell other astronomers, public officials, and the general public on its
importance. In order to promote high-energy astrophysics within the AAS, it is important
to maximize our participation in regular AAS meetings and to minimize talking to ourselves
at specialist meetings. We must insist that AAS meeting organizers place our talks in
subject-oriented rather than energy-segregated sessions. We must also question the
usefulness of separate HEAD meetings if they decrease our communication with other
astronomers. IF we continue to organize separate HEAD meetings, they should be topical and
should include a large number of invitees from outside the HEAD. We should also
participate fully in and contribute to AAS lobbying and education efforts, but when
necessary do both as a Division rather than through the Society.
Vote for no more than three candidates. Return by e-mail to
headmtg@sunspot.ssl.berkeley.edu or by regular mail in a signed envelope to Kevin Hurley,
University of California, Space Sciences Laboratory, Berkeley, CA 94720-7450
Lynn Cominsky ___
Chuck Dermer ___
Chryssa Kouveliotou ___
Dan McCammon ___
Richard Mushotsky ___
Mark Leising ____
by Scott Barthelmy (scott@xtelab.gsfc.nasa.gov), Neil Gehrels, Thomas Cline
(NASA/GSFC); Jerry Fishman, Chip Meegan (NASA/MSFC), Chryssa Kouveliotou (USRA, NASA/MSFC)
The Gamma Ray Burst (GRB) puzzle remains one of astrophysics' great problems 25 years
after its discovery. Follow-up multi-wavelength observations of GRB locations are being
made with ever decreasing time delays, but still no source objects for GRBs have been
identified. The BATSE instrument on GRO detects roughly one GRB per day. All the data are
telemetered to the ground in real time with no on-board storage. The purpose of BACODINE
is to process the data as quickly as possible and deliver the coordinates for a GRB to
ground-based instruments to make follow-up observations while the burst is still occuring.
BACODINE is completely automated and runs 24 hours a day. It receives and monitors the
real-time telemetry data from GRO-BATSE and calculates a set of rough coordinates for a
GRB quickly -- the total time between when the first gamma rays from a burst hit the BATSE
detectors to when the coordinates are available on the ground is 3.5 to 5.5 seconds. These
coordinates are then distributed to many sites around the world so that multi-band,
follow-up observations can be made. The goal, of course, is to "see" some well
localized emission and identify the counterpart to the GRB.
To handle a wide variety of situations and to maximize the number of sites and
instruments making follow-up observations, the BACODINE system distributes the BATSE GRB
coordinates via dedicated phone lines, Internet socket connections, e-mail, and paging
beepers. While the first is the fastest distribution method (0.3 sec maximum distribution
time), Internet sockets (0.5 to 2 seconds) and e-mail (5 to 30 seconds) are still very
fast, cheaper and offer the convenience of BACODINE-computer-to-site-computer linkage. For
sites that do not have Internet access, the alpha-numeric pagers (1 to 2 minutes) offer a
convenient method of getting the RA,Dec coordinates of the GRBs.
Follow-up observations of BACODINE coordinate locations require instruments with a wide
field of view (FOV) or an ability to tile a large region of the sky in a relatively short
time period. The accuracy of the coordinates depends on the burst intensity -- the
brighter the burst the smaller the error circle. For example, folding in the sky viewing
efficiencies of a ground-based optical instrument, a BACODINE location with an error
circle of 12 degrees in radius can be observed once every 3 weeks. For very bright bursts,
the observation rate is once every 12 weeks and the error circle is 6 degrees.
In addition to the distribution of BATSE GRB coordinates, the BACODINE system captures
the COMPTEL instrument's data when the GRB is determined to be in their FOV and
automatically transfers it to their computers. This automation cuts the time to yield a
COMPTEL GRB position. BACODINE can also determine if a GRB is in the FOV of other
spacecraft, e.g. ALEXIS, and notify appropriate people for a faster response. And in the
future, BACODINE will redistribute GRB locations from the HETE spacecraft.
Currently, there are 19 sites around the world receiving BACODINE GRB notices. The
optical, radio, and gamma-ray band passes are covered. Three of the sites are fully
automated (no humans in the loop) with fast slewing instruments and therefore have the
shortest time delay between the onset of the GRB and the first follow-up observation. Both
professional and amateurs can make meaningful observations. New collaborations are
encouraged by the BACODINE project. Please contact us at the above e-mail address or call
301-286-3106 to discuss becoming part of the BACODINE network.
By Jeffrey Bloch, LANL, jbloch@lanl.gov
Each of
the images above shows the detected EUV/ultrasoft x-ray emission from the white dwarf HZ43
in each of the six ALEXIS telescopes. Each image is a raw count map constructed from
roughly 0.5 days of data. The maps represent only a small section of the sky scanned by
each telescope for the associated time period. (Each telescope has a 33 degree FOV, and is
continuously scanning the sky around the spacecraft spin axis.) The dotted lines on each
map are 5 degrees apart in RA and Dec. The annotation on each image indicates the date
that the data were collected, and the center energy of the telescope's bandpass. The pixel
sizes in the images used to bin the counts are 0.25 degrees in diameter. The crosses
indicate the actual position of HZ43. These maps were constructed with the latest attitude
reconstruction software (October 1994) using the preflight alignment information only. The
deviations of the images from the crosses show the magnitude of the boresight and attitude
bias corrections that need to be applied to these data. The width of the HZ43 images
indicate that the current attitude solution algorithm is achieving sub-1 degree precision
performance. (The maps for telescopes 1A and 2A have been boxcar smoothed to qualitatively
bring out the point source, the other maps are binned raw counts with no additional
processing.)
Los Alamos National Lab's ALEXIS (Array of Low-Energy X-Ray Imaging Sensors) satellite
contains an ultrasoft X-ray or EUV monitor experiment that consists of six compact
normal-incidence telescopes operating in narrow bands centered on 66, 71, and 95 eV. The
satellite also contains a VHF broadband ionospheric survey experiment called BLACKBEARD.
The scientific objectives for the telescope experiment include mapping the diffuse EUV
background in these narrow energy bands, searching for bright EUV transients, and
constructing a point source catalog. The ALEXIS satellite and experiments are funded by
the Department Of Energy (DOE), and have been built as a collaboration between Los Alamos
and Sandia National Laboratories (LANL, SNL), and Space Sciences Laboratory/UC Berkeley
(SSL/UCB). The spacecraft bus, built by AeroAstro, Inc., is a custom, low-cost, miniature
satellite, made to be compatible with several expendable launch vehicles. The Air Force
Space Test Program provided the launch for ALEXIS.
The six EUV telescopes are arranged in three co-aligned pairs and cover three
overlapping 33 degree fields-of-view. During each rotation of the satellite, ALEXIS was
designed to monitor the entire anti-solar hemisphere. Each telescope consists of a
spherical mirror with a Mo-Si layered synthetic microstructure (LSM) coating, a curved
profile microchannel plate detector located at the telescope's prime focus, a UV
background-rejecting filter, electron rejecting magnets at the telescope aperture, and
image processing readout electronics. The geometric collecting area of each telescope is
about 25 cm2, with spherical aberration limiting resolution to about 0.25 degrees.
Analysis of the pre-flight X-ray throughput calibration data indicates that the peak
on-axis effective collecting area for each telescope's response function ranges from 0.25
to 0.05 cm2. The peak area-solid angle product response function of each telescope ranges
from 0.04 to 0.015 cm2-sr.
The spacing of the molybdenum and silicon layers on each telescope's mirror is the
primary determinant of the telescope's photon energy response function. The ALEXIS
multilayer mirrors also employ a "wavetrap" feature to significantly reduce the
mirror's reflectance for He II 304 Angstrom geocoronal radiation which can be a
significant background source for space borne EUV telescopes. These mirrors, produced by
Ovonyx, Inc., are highly curved yet have very uniform EUV reflecting properties over their
entire surfaces.
ALEXIS was launched by a Pegasus air launched booster on April 25, 1993 into a 844 x
749 km orbit of 70 degree inclination. The Pegasus dropped from the wing of a B-52 bomber
at 13:56 UT. Initial reports from the launch control center indicated a perfect, nominal
launch.
Initial attempts to contact the ALEXIS satellite after the April 25 launch were
unsuccessful. The first passes over the Los Alamos ground station were on the evening of
the 25th. Video telemetered to the ground from the second stage of the Pegasus booster
became available before these passes. The video showed the ALEXIS +Y solar paddle to have
prematurely unstowed. The paddle pivoted into the video field during a booster attitude
maneuver. The back side of the paddle showed, proving that its lower hinge attachment to
the bus had failed. The upper latch mechanism securing the paddle to the payload was
designed to actuate via a hot wax plunger. This mechanism had either prematurely deployed
or broken. The cause for the premature release, and the damage it caused, could not be
deduced from the video, and is the subject of an anomaly investigation, whose report was
released in early January, 1994. (The root cause of the anomaly was found to be a
too-flexible bracket that held the solar paddle hinge assembly. The bracket on the paddle
that failed was much more flexible than the other three brackets. Another contributing
cause was some modifications that were made to the hot wax actuator mating plates which
put additional stress on the tab that the actuators mated to.)
During the weeks after launch, data gathered from external assets indicated that all
four solar paddles had deployed. This implied that ALEXIS was not dead on arrival in
orbit, but turned on and stayed on long enough to deploy the three undamaged solar
paddles. This was our greatest cause for hope during the weeks that followed.
On 2 June, ALEXIS spoke via a 15-second transmission. The ALEXIS transmitter turns on
as soon as ALEXIS receives an uplink signal, but housekeeping data is transmitted only
after a full acknowledgment between uplink and downlink. Until this acknowledgment, ALEXIS
repeats a time packet word based on the spacecraft clock. The transmission consisted only
of time packets, but was enough to prove that a) the transmission was, without a doubt,
from ALEXIS, b) the processor, receiver, and transmitter were functional, and c) the
batteries were functional, because the clock indicated that ALEXIS had run through the
last orbital night. Hopes buoyed, but ALEXIS did not respond again for four more weeks.
The team occupied itself with various false alarms from the Los Alamos and Vandenberg
Tracking Sites.
On June 30, ALEXIS transmitted a strong signal and stayed in contact for 4 minutes. The
housekeeping data were complete and revealed the status of the satellite. ALEXIS was
spinning about an axis nearly 90 degrees from the Sun. All systems appeared functional
except the magnetometer, which was flatlined. The transmission ceased after 4 minutes when
the battery power was drained. A brief contact on July 1 returned a smaller set of
housekeeping data. Because ALEXIS's solar arrays were approximately edge-on to the sun,
they generated little power. Both the 30 June and 1 July contacts ended when the power
code set the spacecraft into a power conservation mode ('nap'), which turned off the
transmitter and receiver. Analysis of the housekeeping data showed that the spacecraft bus
was drawing ~425 ma (~13 W), while the power system generated an orbit average of only 300
ma (~9 W). Aggravating the power situation was the fact that, with the magnetometer out,
the attitude control code set the X and Y torque coils to maximum settings. As 225 ma of
the bus draw was due to the torque coils, and nap mode turned off the torque coils, it
seemed that ALEXIS would cycle in and out of nap mode. Contact would only be possible when
the spacecraft was not in nap mode. By July 5, commands had been successfully sent to
ALEXIS to conserve power usage and the batteries became fully charged.
The information gleaned from the first contacts with the spacecraft provided the basis
for a scenario of what happened after launch. When the spacecraft was deployed from
Pegasus, the lack of valid magnetometer readings prevented the autonomous attitude control
system from working. With the magnetometer readings at a constant DC level, the control
system would actuate the two radial coils (X and Y) at a maximum level in a vain attempt
to spin the spacecraft up. In fact, the spacecraft was already spinning at 4 rpm about the
Z axis from the impetus of the launch vehicle. The spacecraft was programmed to attempt to
attain proper attitude within 30 hours. If the proper attitude had not been attained by
this time, the solar arrays would be deployed and the spacecraft would attempt to make
contact with the ground station. During the interval between separation and automatic
paddle deployment the spacecraft entered a "flat spin" about a transverse axis.
When the paddles deployed there was roughly an equal chance that the positive or negative
Z axis would align with the spin vector. In yet another demonstration of Murphy's law, the
spacecraft -Z axis ended up aligned with the spin vector and the spacecraft was facing
almost exactly away from the Sun. Shortly thereafter the batteries discharged and the
spacecraft became inactive.
Once regular contact with ALEXIS had been established, we devised a manual method of
controlling the magnetic torque coils that did not depend on the broken magnetometer to
control the orientation of the spin axis for the satellite. By the end of July, the solar
panels were finally facing the Sun. Telescope doors were opened, and scientific operations
with the telescopes began.
The remaining outstanding issue for the ALEXIS experiment is the completion of a new
attitude determination algorithm that takes into account the modified mass properties of
the spinning satellite, the missing magnetometer, and the possible motion of the broken
solar paddle. Three separate efforts are underway to devise and improve this algorithm.
Progress so far has given us methods that recover the attitude with an accuracy of better
than 1 degree. We are very optimistic that we can improve these methods to recover the
original 0.25 degree accuracy specification (0.25 degrees is the FWHM of the telescopes'
spatial response). The modified mass properties of the satellite have also changed the way
that the telescopes scan the sky.
In the meantime, we have since July of 1993 been collecting time tagged photon data
with all six telescopes. As each improvement to the attitude determination software comes
on line, we can back-process all of the archived data to produce better sky maps.
Prior to launch, the experiment team had expected that the ALEXIS telescopes would be
highly robust to a variety of on-orbit non-cosmic backgrounds that have been problematic
for other x-ray and EUV telescope systems. The fact that the design uses normal incidence,
rather than grazing incidence mirrors means that any charged or neutral particles have to
bounce at a large angle off of the mirror surface (thereby losing a lot of energy) in
order to travel towards the detector. After that the particle would still have to travel
through the filter material before reaching the microchannel plate. High energy particles
which might fluoresce the mirror and generate a photon background for the detector were to
be rejected by an octopole magnet assembly at the aperture of the telescope. These magnets
were designed to reject electrons of up to 0.5 MeV. The filters were carefully optimized
to reject FUV radiation present on orbit. Count rates outside of the auroral zones and
South Atlantic Anomaly were expected to range from 10 to 50 counts per second over the
entire telescope field-of-view depending on the telescope and the assumptions about the
spectrum of the soft x-ray background. He II 304 Angstrom radiation from the geocorona
would cause ~1 count/second/Rayleigh in the telescopes with Aluminum/Carbon filters
(typical intensity values for He II 304 Angstrom range from 0 to 12 Rayleighs), and not
effect the telescopes with Lexan/Boron/Titanium filters at all. In the calibration
laboratory, all detectors had background rates between 5 and 8 counts/second due to
residual radioactivity in the microchannel plate glass. Penetrating radiation on orbit was
expected to add an additional ~5 counts/second of background over the entire
field-of-view.
First light for the ALEXIS experiment occurred on July 27, 1993 at 5:45 UT when a set
of real-time commands turned on high voltage to the microchannel plate detector on
telescope 3A during a nightime pass over the Los Alamos ground station. For safety, the
voltage was set so that the detector operated at a very low gain level. Count rates were
monitored in real time and the high voltage was shut down prior to losing contact with the
satellite. Repeated operations of this type showed that count rates were fluctuating up
and down at the spin period of the satellite, and at times were significantly higher than
expected prior to flight. With no attitude information available at the time, a precise
understanding of the character and mechanism producing the higher than expected
backgrounds was elusive.
From August through November of 1993, the ALEXIS experiment team tried to characterize
empirically the on-orbit backgrounds and devised ways to optimize data collection
procedures. One strategy was to run telescopes at very low gain settings for several
orbits to survey the places and times when low and high count rates occurred. In order to
implement new schemes for safe, autonomous high voltage operations with the count rate
time profiles that were actually seen on-orbit, the team also uploaded several new
versions of the Data Processing Units (DPUs) software. This software controls the high
voltage supplies for the telescopes. By late 1993, all six telescopes were operating at
nearly full gain.
The large oscillations in count rate were very puzzling. The duty cycle of low count
rates to high count rates is about 50%, as if half the sky were very bright, and half the
sky very dark. The low count rate values for each telescope are consistent with what we
reasonably expected prior to flight. The double horned appearance of the count rate time
profiles in telescopes 1A, 1B, 2A, and 2B suggested that perhaps they were scanning over
an airglow layer beneath the satellite. The "horns" would occur when the
telescope was looking tangentially to the layer. Also, telescope pair 3, which looks
closest to the spin axis and looks farthest away from the earth during orbit night tended
to not display the oscillatory behavior as often, which might indicate that the background
was earth related.
By early 1994, some preliminary software routines became available for attitude
reconstruction. When this attitude information was combined with the count rate data, it
became apparent that the high count rates were not associated with the Earth. Zenith angle
of the telescope look direction was not a good predictor of high or low counting rates. In
fact, examination of telescopes 1A and 1B's zenith angle plots show that both the earth
and sky were quite dark at times.
Another possibility to explain the highly oscillatory signals would be a correlation
with angle between the telescope look direction and the local direction of the magnetic
field. However plots of count rate versus telescope look direction angle to the local
magnetic field produced plots showing no striking relationship. If the telescopes were
responding to trapped electrons spiraling around the field lines, there should be a sharp
peak at 90 degrees on these plots. Plots of count rate vs. Declination and Right Ascension
also do not show any sensible correlation, as would be the case if the background source
were fixed in inertial space.
The most intriguing results to date arise from plots of count rate vs. angle to the
velocity or ram direction. It appears that the low count rates all occur for ram angles
higher than 90 to 100 degrees on all six telescopes, i.e. when the ram flow can no longer
get into the telescope aperture. This suggestion is still highly speculative, and needs
further investigation.
The white dwarf HZ 43 is the brightest soft x-ray source in the sky. ALEXIS was
positioned favorably to view HZ 43 with telescope pair 3 during the period between April
25 and April 27 1994. A special effort was made to generate attitude determination files
for that period. The Moon had been detected with ALEXIS several times prior to April, but
HZ 43 was the first stellar EUV source that was unambiguously identified in the data.
Images of HZ43 have now been found in data from all six ALEXIS telescopes and are being
used to further evaluate the attitude determination algorithms. As of October 1994, the
attitude reconstruction software appears to be producing solutions with precisions
significantly less than 1 degree. (See the accompanying figure in the Mosaic version
displaying raw count images from each of the six ALEXIS telescopes of the source HZ43 at
different times of the year).
HZ 43 has a well known spectrum and is therefore well suited to use as a standard
candle for measuring the EUV response of a telescope. The telescopes appear to have
roughly the same performance after 9 months of being exposed to the space environment as
when they were calibrated prior to flight. Needless to say this is very good news.
As of October 1994, the ALEXIS spacecraft and experiments continue to function
nominally. No degradation of any system on the spacecraft (except for the initial launch
damage) has occurred since launch. All six ALEXIS telescopes continue to produce data.
Over 60 CDROMs (650 MB) each of spacecraft and payload data have been archived to date,
about half of which are telescope data.
For more information about the ALEXIS mission, click
here for the homepage menu, then select "NIS projects", then select
"ALEXIS".
By Alan Bunner, abunner@gm.ossa.hq.nasa.gov
Formation of Gamma-Ray Astronomy Program Working Group:
At the last meeting of the High Energy Astrophysics Management Operations Working Group
(HEAMOWG, June 9, 1994), we discussed the need to form a new Program Working Group for the
discipline of gamma-ray astronomy, to provide new discipline priorities for the future of
space gamma-ray astrophysics.
A report entitled, "Gamma Ray Astrophysics to the Year 2000," was compiled by
a similar committee during 1987-88 and published in October 1988. This report noted the
difficulty of making specific mission recommendations for the post-Gamma Ray Observatory
era before the launch of GRO. Now that the discoveries of Compton Observatory and GRANAT
are well-known, some of the promising directions for future research are better
understood. Furthermore, the community of scientists interested in gamma-ray astrophysics
has grown substantially since the advent of Compton Observatory. Finally, the plan to
provide a major U.S. high-resolution spectroscopy instrument for ESA's INTEGRAL mission
has fallen victim to the new tight budget environment at NASA.
In view of these developments, the HEAMOWG concluded that it is appropriate to
reappraise the situation and develop a strategy for NASA's future gamma-ray astronomy
program. Therefore, we are constituting a new committee to perform this assessment and to
identify needs and priorities for this field for the 1996-2010 period. It is expected that
the recommendations of this committee will provide ideas for the next decadal astronomy
and astrophysics survey in the area of gamma-ray astronomy.
The purpose of this note is simply to bring to your attention the formation of this
working group. Further news will be forthcoming.
Graduate Student Research Program Announcement:
This is to inform you of a call for proposals for NASA's Graduate Student Research
Program (GSRP). The solicitation for this program is issued annually, and is directed
towards qualified graduate students desiring NASA support of their research leading to
award of the Ph.D. Fellowships of up to $22,000 are awarded for one year and are
renewable, based upon satisfactory progress, for a total of three years. To obtain a copy
of the brochure describing this opportunity, please contact Ms. Jane Davis at (202)
358-0370. For questions concerning the guidelines for participation in this program, you
may contact Ms. Dolores Holland at (202) 358-0734.
By Robert Nemiroff, nemiroff@grossc.gsfc.nasa.gov
In April 1920, Harlow Shapley and Heber Curtis debated "The Scale of the
Universe" at the National Academy of Sciences meeting in Washington, DC. In April
1995, 75 years after the "Great Debate", Donald Lamb (Chicago) and Bohdan
Paczynski (Princeton) will debate "The Distance Scale to Gamma-Ray Bursts."
Martin Rees (Cambridge) has agreed to moderate.
The distance scale to gamma-ray bursts (GRBs) is as uncertain today as the distance
scale to spiral nebulae was in 1920. Evidence appears to be mounting that GRBs occur in
our Galaxy, but conflicting evidence also appears to be mounting that GRBs occur at
cosmological distances. Therefore, at this debate, Lamb and Paczynski will publicly
disagree, and each display evidence and reasoning on why one distance scale should be
preferred over the other.
Details, including an introductory program on the original Curtis - Shapley debate, are
still being finalized. Currently the debate program is scheduled to take place in April
1995 in Washington, DC. A limited number of tickets for this event will be available
starting January 1995. These tickets must be requested in writing, but will be free.
Ticket requests may be directed to:
75th Anniversary Astronomical Debate
CSI Institute
George Mason University
Fairfax, VA 22030
More information will appear on the "75th Anniversary Astronomical Debate Home
Page" which has an URL of http://enemy.gsfc.nasa.gov/htmltest/gifcity/debate.html
To view this Home Page, click here: 75th
Anniversary Astronomical Debate
by J. Swank, XTE Project Scientist, NASA/GSFC (swank@pcasun1.gsfc.nasa.gov)
The X-Ray Timing Explorer is scheduled to be launched Aug. 31, 1995, and begin science
observations in October, 1995, after a brief in-orbit checkout. The spacecraft is
currently following its schedule of testing with instruments on it. The observing program
will be selected from proposals in response to the NASA Research Announcement to be
released in December, with proposals due around Mar 15.
The appendices for the NRA will contain detailed descriptions of the instruments and
operation modes and the software tools available for preparing proposals. They will be
available by anonymous FTP from legacy.gsfc.nasa.gov:/xte/nra_info, or by mail through
requests by e-mail to xtenra@athena.gsfc.nasa.gov or by mail to
Dr. Alan Smale
XTE Guest Observer Facility
Code 668
NASA/GSFC
Greenbelt, MD 20771, USA
What does XTE offer?
- If you want to explore fast and ultra-fast X-ray variability in relatively bright
sources, XTE should be unparalleled. The area will give about 15,000 counts/s from the
Crab nebula in the energy range 2-200 keV. Events are tagged with microsecond accuracy.
Data modes can be chosen to optimize the selection of data transmitted and achieve high
rates of information retrieval.
- If you want simultaneous information across the 2-200 keV band, XTE offers coaligned
2-60 keV and 20-200 keV detectors which all have a 1 degree peak to zero response. The
energy resolution is 18% at 6 keV, 10% at 20 keV, and 17 % at 60 keV.
- If you want to look at AGN of various types (Seyferts, BL Lacs, Quasars), at energies
above 10 keV, XTE offers detectors with background low enough to detect 0.2 milliCrab
sources.
- If you want X-ray data simultaneous or quasi- simultaneous with radio, optical, UV,
other X-ray, or gamma-ray observations, XTE can point anywhere outside of 30 degrees to
the sun and can be scheduled to support other observations which may not be scheduled
until shortly before the observation.
- Targets that would be "targets of opportunity", that is, not predictable, can
be proposed. The criteria to be used in identifying whether the event has occurred must be
clear.
The XTE orbit will be at an altitude of about 600 km with an inclination of about 23
degrees and a period of about 100 minutes. For many targets there may be 15-30 minute gaps
in coverage due to earth occultation and the satellite's passage of the South Atlantic
Anomaly regions of high charged particle density. The gaps will be minimized by scheduling
when longer uninterrupted data trains are important.
The three instruments on XTE are the Proportional Counter Array, the High Energy X-Ray
Timing Experiment, and the All Sky Monitor. HEXTE has half its area copointed with the
PCA, with the other half accumulating background from nearby positions. Independently, the
ASM steps around the sky scanning 70% of it each satellite orbit, each of 3 detectors
viewing (through coded masks) a 10 degree by 90 degree swath for 100 seconds at a time.
XTE's monitor, obtaining 2-10 keV fluxes, is sensitive to many kinds of transient X-ray
sources, which can be proposed as targets of opportunity.
The XTE Science Operations Center Guest Observer Facility at GSFC will be distributing
the proposal information, answering questions of prospective proposers and maintaining
software useful in evaluation of feasibility of observations.
HEADNEWS, the electronic newsletter of the High Energy Astrophysics Division of the
American Astronomical Society, is issued by the Secretary-Treasurer, at the University of
California Space Sciences Laboratory, Berkeley, CA 94720-7450. The HEAD Executive
Committee Members are:
Martin Elvis, Chair (elvis@cfa.harvard.edu)
Neil Gehrels, Vice-Chair (gehrels@lheavx.gsfc.nasa.gov)
Kevin Hurley, Secretary-Treasurer (khurley@sunspot.ssl.berkeley.edu)
Rob Petre, Member (petre@lheavx.gsfc.nasa.gov)
Belinda Wilkes, Member (belinda@cfa.harvard.edu)
Hakki Ogelman, Member (ogelman@astrog.physics.wisc.edu)
Rick Rothschild, Member (rrothschild@ucsd.edu)
Mel Ulmer, Member (ulmer@ossenu.astro.nwu.edu)
Diana Worrall, Member (dmw@cfa.harvard.edu)
Virginia Trimble, Member and Past Chair (vtrimble@astro.umd.edu)
Please send newsletter correspondence to khurley@sunspot.ssl.berkeley.edu
