The USA Experiment and Elliott Bloom

Kent S. Wood with thanks to Michael Lovellette

and Michael Kowalski

March 18, 2016

SLAC’s Role in a Space Experiment led by the Naval Research Lab

Introduction

• Asked to review the USA Experiment and Elliott Bloom’s role

• This marked a transition for Elliott, into space research — DoE research interests pursued with space experiments

• How USA started • How it became a joint NRL and Stanford project • Entry of SLAC and Elliott’s group

• Goals of the USA Collaboration • How they were realized • Where they led afterward

• Unlike some of the other talks that are from either a Stanford or HEP perspective, this one is neither. It concerns astrophysics and involves Elliott viewed via a long-distance collaboration stretching over more than three decades.

How it started — a compressed history

For myself and for NRL, USA was part of a Stanford collaboration that began in 1983

Interest in high-energy astrophysics took me to NRL (1973) NRL heritage was X-ray surveys, X-ray timing End phase of NRL’s HEAO-1 led into collaboration with Stanford

Phases of that collaboration (1983-present): initially, quest for CFS instability seen in X-rays and GW development of XLA concept, tool for the CFS search evolution of that into USA, more modest but diversified USA realization as collaborative project

SLAC (Elliott) as main partner, Stanford lead on project USA era (1992 to circa 2003) GLAST/Fermi (1992-present) (note overlap)

Origins of USA and NRL-Stanford Collaborations

• X-ray source classes were surveyed through 1970s, 80s. • After HEAO-1 Survey (shown below), emphasis at NRL switched

to timing of compact objects, neutron stars and black holes. • This was also returning to lab’s pre-HEAO specialties, X-ray

pulsars (Crab) and black holes . • Key development: link from NS astrophysics to gravitational

radiation, the CFS (Chandrasekhar-Friedman-Schutz) instability — developed in a paper by R. Wagoner Ap.J.278, 345 (1984).

Wood et al Ap J.Suppl. 56, 507 (1984)

Gravitational Waves and CFS• GW interest now focuses on bursts (coalescence)

• CFS is a way to generate gravitational waves without coalescence: continuous-waves gravitational radiation signal source caused by gravitational radiation reaction instability in a rapidly-rotating neutron star.

• Wagoner (1984) calculated observing requirements under a novel accretion scenario. It would give a sinusoidal signature in GW and XR at the same frequency, but it would not be the NS rotation frequency. • In Wagoner model, sinusoidal h(t) ~ [NS mass accretion rate](1/2) • X-ray modulation (sinusoidal) proportional to same mass accretion rate • Periodic X-ray component of X-ray flux is proportion to h(t)2

• X-ray modulation is low amplitude but if found would lead to GW signal, i.e., X-ray signal would reveal frequency, phase of GW. • Low amplitude X-ray signal detection is impeded by its being in a

binary system — a problem now familiar in Fermi work on binary pulsars.

•Bottom line: need for large exposure in X-rays

XLA Concept leading to USA Mission

• One idea from the 1980s was a 100 m2 array called the X-ray Large Array (XLA), which had a NASA study but never flew. It was similar in emphasis to LOFT, recently proposed to ESA, and similar future concepts.

• Stanford (Physics Dept) participated in XLA from the conception. (R. Hofstadter and W. Fairbank provided encouragement and advice.)

• XLA’s immediate legacy: large exposure (area x time product) on sources. This led to the Unconventional Stellar Aspect Experiment on the ARGOS satellite.

• USA was done within the Defense Department’s Space Test Program (STP) which must be explained to understand how SLAC became involved.

Space Test Program and USA

STP is an alternative path to space access for US research. Launch is not via NASA, though NASA may be involved in any specific payload. STP can support bus, launch, mission ops, data collection. Selection process follows a DoD model, dating from the 1960s. Major feature of STP: proposer must produce the payload — the payload is never produced on STP program resources. It must be done on either proposer’s resources or the proposer’s sponsor’s resources. NRL had to find suitable sponsors and partners.

• During this phase, USA evolved from an initial modest concept into a much more ambitious instrument for ARGOS mission. By 1991, ARGOS looked real. USA could fly if (and only if) it could solve the resource problem

• Peter Michelson and Richard Taylor suggested working with Elliott and SLAC. Through meetings starting 1992, SLAC became the primary partner to NRL in producing the payload

SLAC, Elliott Bloom and USA Mission

Elliott was seeking paths to space-based payloads for DoE research goals.

It was a good fit; Elliott could access resources at SLAC. It closed the deal, made the mission possible.

USA had four distinct aspects, one of which was physics in the strong-gravity regime… … but the other aspects were applied objectives

Elliott was strongly supportive of the entire program, which was greatly appreciated — and it eventually brought benefits back to basic research

USA at SLAC: Concept, Design, Build, Flight

• From SLAC standpoint, USA had to stress fundamental physics. Stanford group helped develop science focus but also worked on payload — thermal analysis, structure, collimator, test setups; then it contributed to post-launch ops/analysis

• Elliott was the focal point for the Stanford portion — he held frequent meetings of all SU participants around the table in his office and coordinated with essentially everyone, as well as contributing personally in many areas • members from Elliott’s SLAC group (Broeder, Godfrey, others) • Other team members from Bay area (Michelson, Scargle, Cominsky;

sometimes Aero/Astro Engr. and Electrical Engineering (Center for Reliable Computing)

• students (graduate students, postdocs) • Build phase: two crucial Stanford contributions

• collimator —essential to definition of FOV and PSF • dip brazing construction of instrument

Collimator

• In X-ray telescopes without X-ray optics, the collimator is the element that defines the instrument response. Must deliver uniform, calibrated PSF.

• Also functions as critical structural element, supporting thin membrane gas barrier window under pressure. Must survive stresses at launch and in the space environment.

• Further requirements, e.g., fluorescence management • Elliott’s group designed a new collimator, customized for

USA; then fabricated it and tested it. It was essentially hand-built (by J. Broeder).

• The design had later use as well: was carried over into a later NRL UV experiment, J-PEX

SLAC Collimator photos, partly culled from from archives of the flight of NRL’s J-PEX* (EUV spectrometer)

*BTW, the later history of J-PEX illustrates importance of finding colleagues like Elliott to help build / sustain programs — NRL could not find SU partner for this one. (R.I.P.)

Dip-Brazed Structure; Test Phase and Other Activities

• To achieve strong, light-weight structure Stanford undertook dip-brazing the pylon assembly from pre-shaped metal pieces.

• Elliott’s group designed test setups for the finished USA instrument and analyzed the test data

• Worked within Space Test Program procedures and policies, reconciling with applicable Stanford policies. Attended many meetings at DoD venues in support of the mission

USA Experiment — (Unconventional Stellar Aspect)

• Design – Large proportional counter (0.2 m^2) – Mounted in 2-axis gimbal – Could track X-ray sources independently

of spacecraft, with gimbal for offset • 4 research areas:

• X-ray timing, compact objects, strong gravity

• X-ray navigation (1st flight) • X-ray aeronomy (1st flight) • computing in space (unique

testbed)

• This meant we published in four distinct communities

pylon / gimbal

collimator is behind heat shields, here

Launch and Post-Launch

• Elliott’s team participated fully in USA through launch and activation

• Participated also in planning of observations (USA was 100% pointed, so targets were constantly being selected and programmed)

• Did analysis of particular astrophysics targets, sometimes leading and sometimes supporting.

• SLAC staff: Bloom, Godfrey, Broeder • Students: Engovatov, Reilly, Saz-Parkinson, Shabad,

Tournear, Wen; Giebels; also Hanson, Oh, Sirvani

USA / ARGOS Stacked for Launch

ARGOS• Launch 23 February 1999 — after a series of

launch attempts that began 21 January 1999

What Mattered Most?

• USA was a test program experiment. Its legacy comes largely from ideas pioneered in the test flight: • Applied programs (XR Nav, computing,

atmospheric diagnostics) • Long term astrophysics studies • Experience heritage — SLAC entry into

space-based research

Astrophysics

• X-ray timing, compact objects

• XTE J1118+480 • QPO studies — same QPO tracked for many days as

it evolved in frequency • Other Black hole transients: QPOs, aperiodic analysis

• XTE J1550-564 • XTE J1859+336 • GRS 1915+105

• Other QPOs in black hole candidates • Other variable sources / transients • Exo 0748-676 eclipse studies • plus: Ph.D. theses on astrophysics topics — all of them

supervised by Elliott

USA’s Applied Program, and Legacies

• X-ray navigation • post-USA involved corporations, DARPA, NASA • led to NICER/SEXTANT — flying 2017

• will do onboard demonstrations of X-ray navigation • will do pulsar studies that should feed back into Fermi

• one of Elliott’s students (D. Tournear) was involved in X-ray navigation program management, after finishing degree

• J. Hanson (thesis, Aero/Astro) — he has maintained X-ray navigation as a long-term career interest

• Reliable Computing in space • two more Ph.D. theses (Electrical Engineering) • built up NRL and SLAC experience in this area and led into

GLAST DAQ concept

• X-ray aeronomy probe • aeronomy tool, available for future

USA’s X-ray Navigation

• X-ray navigation was the primary applied goal • For this purpose, USA was essentially an avionics

package, using X-ray sensors viewing celestial sources

Position, time-determination, and attitude were all deemed worthy goals, as was time transfer

(This is a broad, inclusive interpretation of X-ray navigation)

• USA was used as an X-ray star-tracker to diagnose and correct a problem that developed with the ARGOS spacecraft attitude control — first use of an X-ray sensor for feedback to control of a satellite in orbit

• Wood et al “The Unconventional Stellar Aspect (USA) Experiment on ARGOS,” AIAA Space 2001 Conference and Exposition, paper AIAA 2001-4664 (2001)”

Computer Testbed (ASCAT) was built into USA Central Electronics. Compared performance of fully rad-hard and rad-tolerant COTS

Showed latter was sufficient for many purposes.

Legacy: The testbed brought space based computing experience to NRL and SLAC. The approach was used in Fermi. NRL and SLAC collaborated on Fermi DAQ and FSW

Processors:Rad-hard: Harris RH3000COTS: IDT 3081

Computer Testbed

.

Probing Atmosphere with Limb-grazing X-raysImportance has grown with concern over climate change

Occultations can provide atmospheric diagnostics for remote sensing

A study of Earth-occultations of the Crab Nebula using USA and RXTE was used to derive atmospheric density profiles (Determan et al, JGR, 112, AO6323 (2007))

When X-ray Earth occultations are used to probe the atmosphere they are most sensitive in the altitude range from 80 to 150 km.

This is the range where models predict effects associated with global climate change

Roble, R.,”Major Greenhouse Cooling (yes cooling): The upper atmosphere response to increased CO2,” Rev Geophys Suppl., pp 539-546 (1995)

1. Ray, P., Wood, K.S., Fritz, G., Hertz, P., Kowalski, M.P., Johnson, W.N., Lovellette, M.N., Wolff, M.T., Yentis, D., Bandyopadhyay, R.M.,

2. Bloom, E., Giebels, B., Godfrey, G., Reilly, K., Saz-Parkinson, P., Shabad, G., Michelson, P.,

3. Roberts, M., Leahy, D., Cominsky, L., Scargle, J., Beall, J., Chakrabarty, D., and Kim, Y.

USA team evolved into Fermi LAT team and other missions. Following is the author list for one post-launch paper on the USA instrument, grouped as NRL(1), Stanford(2), and other(3) institutions.

From USA team onward to Fermi LAT

Others not listed above: NRL: D. Wood, T. Crandall, J. Ampe SU: H. Wen, D. Engovatov, D. DeBra, J. Hanson, E. McCluskey, N. Oh, P. Shirvani

Summary

• Stanford collaboration was important to all phases of USA • USA became four experiments in one • Elliott’s SLAC group was the center of Stanford activity:

• developing scientific focus • design, build, and testing • financing, and keeping program on track • personnel, recruitment, organization, management • post launch operations • scientific analysis and publications • grad students, theses, in high-energy astrophysics and other

topics • evolution into subsequent GLAST work

• Elliot succeeded at using USA to effect SLAC’s entry into space-based research on DoE objectives

• Many legacies. USA team merged into GLAST / Fermi team