SLAC’s First Step into Space: Status of the USA experiment 5-years after launch

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


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


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)


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


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


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


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


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)


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


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


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)


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


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


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


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


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!)


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


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


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


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


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


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


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


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


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


USA AGN observations

We analyzed this one so far…


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


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?


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


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)

Documents

SLAC’s First Step into Space: Status of the USA experiment 5-years after launch