SLAC’s First Step into Space: Status of the USA experiment
5-years after launchLarry Wai
SLAC / Group K
The beginning of particle astrophysics at
SLACExperimental struggles
and the rewards of science
Larry Wai, SLAC seminar 3
Outline of talk
1. The life and times of the USA experiment• The people who made it work• On-orbit adventures• The detector and its calibration
2. Science from the USA experiment• Tests of general relativity in black hole
systems The high frequency power spectrum of Cygnus X-1 A search for x-ray bursts from ~10 solar mass
compact objects
• Physics of “jets” in black hole systems Flares In BL LAC object 1ES1959+65
3. Summary and plans
Larry Wai, SLAC seminar 4
Part 1: The life and times of the USA detector
• 1991-1998. Design, manufacturing, integration, testing, calibration, storage (satellite late)
• T0 = February 23, 1999; Delta-II launch from Vandenberg AFB, CA
• End of USA mission at T0+21months (3 months shy of design lifetime of 24 months)
The people who made it work
How SLAC students and staff got their hands into a
space based experiment and made it fly
Larry Wai, SLAC seminar 6
USA Collaboration
NRL: R. Bandyopadhyay, G. Fritz, P. Hertz, M. Kowalski, M. Lovellette (P.S.), P. Ray, L. Titarchuk (& GMU), M. Wolff, K. Wood (P.I.), D. Yentis, W.N. Johnson
SLAC/Stanford: E. Bloom (S.U. Lead Co-I), W. Focke, B. Giebels, G. Godfrey, P. Michelson, K. Reilly, M. Roberts, P. Saz Parkinson, J. Scargle ( & NASA Ames), G. Shabad, D. Tournear
USAUSA X-ray Telescope(1-16keV)
Larry Wai, SLAC seminar 7
The USA project - pushing the limits
~1/5 the manpower of GLAST in ~½ the time!
SLAC contributions:
• SLAC mechanical and thermal design, validation - John Hanson (Ph.D. Aero-Astro), Alex Leubke (M.S./Engineering Aero-Astro)
• SLAC manufacturing of mechanical framework, collimators – John Hanson, John Broeder
• Flight software – SLAC contributions
•Detector calibration - Gary Godfrey, Ganya Shabad (Ph.D. Physics), Pablo Saz-Parkinson (Ph.D. Physics) , Berrie Giebels•Ground software - Kaice Reilly (Ph.D. App.Physics), Derek Tournear (Ph.D. Physics), Warren Focke•Science operations - Student & post-doc involvement on a weekly basis deciding what sources should be observed (many students defined the subject of their Ph.D. science in this way); heavy involvement in all the publications
On-orbit adventures
How USA did it the hard way and made it work
Larry Wai, SLAC seminar 9
The pointing challengeThe challenge:• Spacecraft (ARGOS) axis was continuously oriented normal
to earth’s surface throughout the orbit – need to keep USA pointed on a celestial point source to ~0.05 degrees
Yaw
Pitch
The USA innovation: •Use mechanical rotation system to point detector at celestial sources by setting yaw (X-axis rotation), following source in pitch (Y-axis rotation)
Earth’s center
Celestial source
ARGOSUSA
Earth’s center
Celestial source
ARGOS
USA
Earth’s center
Celestial source
ARGOS
USA
Larry Wai, SLAC seminar 10
10,000 orbits
USA reached 87% of design lifetime on-orbitDuring lifetime of detector on-orbit•10% time used for satellite calibration•14% time used solving pointing problems (due to satellite misinformation on orientation; USA diagnosed the problem)•76% time had good pointing
The polar orbit challenge•Unusual USA characteristic - equatorial orbits are preferable for astronomy (backgrounds are better)•Orbit was divided into segments by passage through earth’s radiation belts and the South Atlantic Anomaly•Two 20min (equatorial) and two 10min (polar) observations segments per orbit
Larry Wai, SLAC seminar 11
Celestial source observations
•Strategy: accumulate long object observing times ~fraction of a month•About 90 Sources Observed by USA•Top sources in observing time had 0.1-0.5 months each!
Source Name ksec commentsCrab_Pulsar 1220. pulsarX1630-472 716.6 black holeCyg_X-1 706.0 black holeCyg_X-2 626.6 neutron starXTE_J1118+480 601.1 black holeSMC_X-1 415.4 pulsarE_2259+586_SNR 397.4 pulsarCas_A 370.0 supernova remnantCircinus X-1 345.3 neutron starMkn_421 278.2 active galactic nucleusCen_X-3 265.7 pulsarX0614+091 249.0 neutron starGRS_1915+105 243.6 black holeGX_349+2 232.9 neutron starX0142+614 224.9 pulsar
The detector and its calibration
A story of careful work on the ground, an on-orbit surprise, and a lesson
learned
Larry Wai, SLAC seminar 13
Detector design goals
•Low energy threshold (~1 keV)
•Large collecting area (~2000 sq.cm)
•High time resolution (~1 microsecond)
•Sustained high data rates (40-128kbps)
Larry Wai, SLAC seminar 14
X-ray detection technique: collimator + standard multiwire proportional chamber
X-ray photon
Initial Ionization
~105 electrons/event
Collimator ~ 1.3o FWHM
2 2 sec time resolutionsec time resolution(typical ~ 32 (typical ~ 32 secsec res)E/E ~ 17% at 6 keV17% at 6 keV
50 m wire anode
Gas Volume (P10) 90%Argon10%Methane
~10% of 1KeV photons pass through pressure window
40-600 e-ion pairs
Larry Wai, SLAC seminar 15
USA detector details
12.5"
29"
4.5
"
Kapton(2.5 um) + Aluminum(.1 um)Sun Shield
Collimator
Proportional Chamber
Support Mesh (85% Transmission)Mylar (5.0 um) + Nichrome (.01 um)Pressure Window
1.3 DegFWHM
Copper
Strongback
USA X-ray Detector (1 of 2 Identical Modules)
~1/8" Diam(point to point)
Strongback
12"
4.5
"
3"
.062"
.00011" Deflection
1 atm pressure
USA Proportional Chamber
Thin Window
Top View End View
12.5
"
29"
2.8
cm
2.8 cmPeriphery anode wires (2) for charged veto
2 interleaved wires running serpentine through each layer x 2 layers + 1 wire around the outside for anticoincidence
Larry Wai, SLAC seminar 16
Effective area determinationAbsorption edge of Argon
Transmission of 5.0 m Mylar + 2.5 m Kapton
•Energy dependence of effective area derived from Livermore x-ray cross-section formulae for various detector materials •Superior effective area below 4 keV as compared to PCA (proportional counter array aboard RXTE – NASA mission up since Dec. 1995)
• ~200’ long tube w/ 55Fe source at one end, collimator on rotation fixture at other; ~1Hz count rate through collimator
• Measure acceptance vs angle of incidence (point spread function, collimator effective area ~1000 sq. cm)
Larry Wai, SLAC seminar 17
T-Vac high rate tests
• 55Fe source fastened to yoke, scan in yaw
• Ganya fitted histograms of time difference between events with event time domain model including deadtime and other electronics effects
• Extraction of deadtime as a function of rate
55Fe source mounted to yoke
16.5 s
Detector 11035 cts/sec2 / dof = 0.979DOF = 493
Larry Wai, SLAC seminar 18
Power spectrum tests
•Power spectrum: convert the time series of counts into the frequency domain
•Basic idea for the test: check for subtle systematic effects in the calibration data
General procedure for power spectrum:• Break down all data into equal length time segments
(T), each with N equal length bins• For each segment calculate the “Leahy normalized”
power spectrum Pj=2|Xj|2/Ncounts where Xj is the amplitude of the discrete Fourier transform:
xk (k=0,1,…,N-1) is the number of counts in the kth time bin
• Average segments to get mean and RMS
2Purely Poisson process
0
Leahy normalized power spectrum
power
Frequency05200 Hz
Good agreement betweendeadtime model and datausing all energy channels!
Rate = 4075 cts/secPj=P1+P2cos(2j/N)P1=1.763, P2=-0.02452 / dof = 1.08DOF = 2046
Good agreement between calibration data and Ganya’s deadtime model – when using all energy channels combined
USA calibration data:deadtime introduces correlations between photon times
Larry Wai, SLAC seminar 19
An on-orbit surprise
• Pablo notices recurring patterns of distortions in the power spectrum of celestial sources when selecting energy bins
Energy channel 1Energy channel 2
Larry Wai, SLAC seminar 20
Going back to the calibration data
• Ganya goes back to calibration data and confirms an energy dependent instrumental effect (a.k.a. EDIE) on power spectrum
• Positive vs negative spectral slope (as measured in detector) inverts shape in frequency domain
• Effect cancels out when all energy channels are combined; that’s why it was missed during T-Vac testing
• Working hypothesis is pulse tail oscillations; phenomenological corrections used at present
Larry Wai, SLAC seminar 21
A lesson for space-based detectors
• More manpower and time in analysis of data during detector testing on the ground could have unearthed EDIE before launch and allowed us to characterize the effect more carefully than was possible on-orbit
• The lesson learned: let’s check the data carefully during testing of the GLAST large area telescope at SLAC! (2004-2005!)
Larry Wai, SLAC seminar 22
Part 2: Science from the USA experiment
Selected Astrophysical Journal papers:• USA and RXTE Observations of a Variable Low-Frequency QPO
in XTE J1118+480, K. S. Wood et.al. , ApJ (2000)• Disk Diffusion Propagation Model for the Outburst of XTE
J1118+480, Kent S. Wood et al., ApJ (2001)• USA Observation of Spectral and Timing Evolution During the
2000 Outburst of XTE J1550-564, K. T. Reilly et.al., ApJ (2001)• Eclipse Timing of the Low Mass X-ray Binary EXO0748-676 III.
An apparent Orbital Period Glitch Observed with USA and RXTE, M. T. Wolff et.al., ApJ (2002)
• Observation of X-ray variability in the BL Lac object 1ES1959+65, Berrie Giebels et.al., ApJ (2002)
• X-ray Bursts in Neutron Star and Black Hole Binaries from USA and RXTE, D. Tournear et.al., ApJ (2003)
• High frequency power spectrum of Cygnus X-1 from the USA experiment, W. Focke and L. Wai et.al., (in progress)
High frequency power spectrum of Cygnus X-1
Testing a prediction of General Relativity: the
innermost stable circular orbit
Larry Wai, SLAC seminar 24
An innermost stable orbit
• Innermost stable circular orbit (ISCO) at slightly more than x (M/Msolar)3km, 2<<4.5
1.875Sch
L
mcR
Distance from center of black hole
Relativistic Effective Potential
Stable circular orbit
Larry Wai, SLAC seminar 25
Looking for the innermost stable orbit in Cygnus X-1
• Cygnus X-1: a ~10 solar mass black hole candidate with a companion star donating matter to an accretion disk around the black hole
• X-ray luminosity from Cygnus X-1 originates in ~10KeV plasma upscattering “seed photons” from orbiting matter in the disk
Mass-Donor Companion Star
Accretion Disk
Black hole
~103 km
~106 km
•Non-uniform orbiting matter in the disk will produce variations in observed flux at orbital frequency•Models, e.g. Bao and Ostgaard (1995), predict power spectrum P~f-1 up to the frequency of the innermost stable orbit fISCO= (Msolar/M)2.2kHz
•Signature of the innermost stable circular orbit is a sharp drop-off in the power spectrum at ~220Hz
Larry Wai, SLAC seminar 26
Extracting the power spectrum
• For each time segment (~1sec) calculate the power spectrum and subtract the noise including deadtime distortion (from Ganya’s results)
• Average all the resulting power spectra over all segments (~400k)
• Fit in region above 2kHz to correct for residual noise/deadtime
• Fit in region above 300Hz to correct for residual EDIE
Larry Wai, SLAC seminar 27
Cygnus X-1 Power Spectrum•Model the “drop-off” as a broken power law•Best fit broken power law has 2=1457 for 1437 DOF•Best fit single power law has 2=1465 for 1439 DOF2=8 with 2 additional degrees of freedom•2.5 sigma effect – marginal evidence for a dropoff
Residual deadtime
Residual EDIE
f-1.6
Focke, Wai, Bloom, et.al.
A search for x-ray bursts from 10 solar
mass compact objectsTesting another prediction of General Relativity: the
event horizon
Larry Wai, SLAC seminar 29
The measured masses of compact objects
• In a binary system, need orbital period, velocity, partner mass, and angle of inclination to estimate the mass of the compact object
• Two populations emerge, one around ~1.4 solar masses, and ~10 solar masses
XTE J1118+480GRS 1915+105XTE J1859+226XTE J1550-564Cyg X-3
Cyg X-2Sco X-1
Black Hole CandidatesNeutron Stars
Neutron star mass limit
Miller (1998)+Tournear (2003)
•Maximum neutron star mass is 3.2 solar masses•Sample of observed ~10 solar mass objects are widely believed to be black holes - with an event horizon at (M/Msolar)3km
Larry Wai, SLAC seminar 30
Using bursts as an event horizon litmus test
• Observation of thermonuclear burning on the surface of the black hole candidate would reject the event horizon hypothesis
• The signature: type 1 x-ray bursts• These bursts are due to unstable
thermonuclear burning on the surface of neutron stars (cooling blackbody temperature, radiating area corresponding to 10-15km radius sphere, and linear correlation between burst flux and time delay)
• Narayan-Heyl (2002) prediction for bursting luminosity region for 1.5 and 10 Msolar compact object w/ baryonic surface
Larry Wai, SLAC seminar 31
Black hole candidate burst rate limit
• Result: BHC burst rate is less than 5% of that for neutron stars (at 95% C.L.)
• Black hole candidates quantitatively don’t have baryonic surfaces!
Tournear, Bloom, et. al. (2003)
Flares In BL LAC object 1ES1959+65
Testing a prediction about how an AGN jet works
Larry Wai, SLAC seminar 33
Large!
~10 solar mass black
hole
~106-9 solar mass black
hole
Black holes, small and large
Active galactic nuclei (AGN)
• ~106-9 solar mass black hole
• ~109 km disk• Jets of
electrons!• E.g.
1ES1959+65
Galactic black hole candidate
• ~10 solar mass black hole
• ~103 km disk• Jets!
Small!
•GLAST bread and butter
Microquasar
•E.g. GRS 1915+105
•E.g. Cygnus X-1
Larry Wai, SLAC seminar 34
USA AGN observations
We analyzed this one so far…
Larry Wai, SLAC seminar 35
Flaring in BL Lac object 1ES1959+65
• 2000 Sept-Nov. observation of variability by USA led to search in TeV
• 2002 May-July observations by Whipple of clear TeV gamma ray flaring
Giebels, Bloom, et.al. (2002)Daily x-ray flux
Hardness ratio
Holder, et.al. (2003)daily TeV gamma flux
Larry Wai, SLAC seminar 36
Confirmation of a prediction
• 1ES1959+65 was predicted to be the 3rd brightest extra-galactic TeV source by Stecker et.al. (1996) based upon data from the two known extragalactic TeV sources Mrk 421 and 501
• Prediction based upon “synchrotron self compton” scattering in AGN jets as the mechanism for TeV emission
• x-rays come from synchrotron radiation of jet electrons, and TeV gammas are the x-rays Compton upscattered by the same jet electrons
• Example of a multiwavelength campaign (which we will need in GLAST to study jet physics)
Summary and plans
How did we do, and what is left?
Larry Wai, SLAC seminar 38
How did we do?
SLAC’s first step into spaceSLAC people contributed to the design,
made flight hardware/software, tested and calibrated the detector, helped define the observation schedule, took good data, and published science results
Cranked out 7 Stanford Ph.D.’sEstablished an experimental
astrophysics presence at SLAC
Larry Wai, SLAC seminar 39
What’s left for USA?
More papers:• High frequency
power spectrum of Cygnus X-1
• High frequency QPO searches (Circinus X-1, XTE J1859+226)
• AGN studies
Another Ph.D. • Han Wen (Physics Ph.D.)• Andrew Lee (Physics
Ph.D.)• John Hanson (Aero-Astro
Ph.D.)• Alex Leubke (Aero-Astro
M.S/Engineering)• Ganya Shabad (Physics
Ph.D.)• Kaice Reilly (App. Physics
Ph.D.)• Derek Tournear (Physics
Ph.D.)• Pablo Saz-Parkinson
(Physics Ph.D.)• Daniel Engovatov (Physics
Ph.D., in progress)