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RXTE and Observations of GC Transients. C. B. Markwardt (NASA/GSFC). Outline. RXTE PCA scans (2-10 keV) Motivation Technique Sample of Results X-ray Transient Population Studies. Introduction: PCA Scans. - PowerPoint PPT Presentation
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RXTE and Observations of GC Transients
C. B. Markwardt (NASA/GSFC)
Outline
• RXTE PCA scans (2-10 keV)
• Motivation
• Technique
• Sample of Results
• X-ray Transient Population Studies
Introduction: PCA Scans
• RXTE PCA scanning observations of the galactic center region and galactic ridge, twice per week
• Designed to detect faint transients, especially in a confusing region for the RXTE All-Sky Monitor
• Results: discoveries of many new millisecond X-ray pulsars, weak recurrent transients
Motivations
• Discovery in 1998 of millisecond pulsations from SAX J1808.4-3658
• “Accidentally” discovered in outburst by PCA scan between targets over the region
• 1996 data from BeppoSAX and ASM suggested previous weak outbursts
• Other weak transient activity from known and new sources
• King (1999) had predicted a population compact binary systems, which would have faint, low duty-cycle transients
Millisec X-ray Pulsars: Difficult to Discover
• Outburst recurrences of ~years• Low duty cycles (few weeks)• Low flux (peak fluxes, ~60 mCrab)• Compare to XTE ASM sensitivity of 30 mCrab / day unconfused• Concentrated in galactic center where high density of sources confuses
the ASM
Wijnands 2005
SAX J1808.4-3658
Specialized Detection Methods
Rossi X-ray Timing Explorer - PCA detector High collecting area, high time resolution Poor spatial resolution (1 full-width half max) Used in scanning mode to cover large solid angle
PCA Scans Begun Feb 1999• Twice weekly• 250 sq. deg (initially)• Two scans over a given
position• Scan rate 6 deg/min;
collimator FOV of 1 deg (FWHM)
• 20 sec of on-source exp.• Not so sensitive to X-ray
bursts*
Example scan
Galactic diffuse emission
1999-2004: 250 sq. degMain Bulge
2004-2008: 500 sq. deg
Bulge “wings”
2008-present: 625 sq. deg
Aquila Region
Summary of Results
• Over 625 sq. deg covered
• Time coverage from 1999 (250 sq. deg), 2004 (500 sq. deg), 2008 (625 sq. deg)
• Approx. 1800 dedicated scan observations
• More than 41,000 distinct scan segments
• 190 sky positions monitored
• 115,400 flux samples measured
• Approximately 50 new sources detected during the program lifetime
• http://lheawww.gsfc.nasa.gov/users/craigm/galscan/
Results: Millisecond Pulsars
• Bulge scans trigger follow-up confirming observations
• Long term monitoring (durations wks. - mos.)
• Recurrences: extra observation opportunities; accretion torques; orbital evolution
Recurrences
• SAX J1808.4-3658: 1996 (SAX), 1998(P), 2000(P), 2002(P), 2004/2005
•XTE J1751-305: 1998 (ASM), 2002 (P), 2007 (P), 2005 (INTEGRAL), 2009 (P)
• IGR J17511-3057: 2009 (P); XTE J1814-338: 2003 (P); XTE J1807-294: 2003 (PCA); SWIFT J1749.4-2807: 2010 (Swift)
• SWIFT J1756.9-2508 (Swift – PCA in constraint)
• NGC 6440: ~periodic (PCA/Swift/Chandra)
• Aql X-1: ~yearly (PCA, ASM, etc.)
XTE J1751-305
(P=PCA)
Distributions of Spin and Orbital Periods
U.L. calculated in 2003 by Chakrabarty
Population Studies of Transients
• Flux distribution (log N vs. log S curve)
• How to account for variability?(min/max/median flux distributions)
• Estimated population duty cycles
• Outburst and quiescent durations
• “New discovery” rate
Wide Distribution of Position & Flux
180 sources Size α flux
Min Flux
Max Flux
Median
Flux Distributions
Limiting Flux
Min Flux
Max Flux
Median
~few 1038 erg/s at Gal. Ctr.
Flux Distributions
Limiting Flux
Flux Distributions
Limiting Flux
(faint & transient)
(bright &persistent)
Flux Distributions
Limiting Flux
“Atoll”(sub LEdd)
“Z”(near LEdd)
On/Off Duty Cycle
Distribution of Duty Cycles
PersistentQuiescent
Distribution of Duty Cycles
15-20% of sources are persistent
PersistentQuiescent
Distribution of Duty Cycles
~40% of sources are have >50% duty cycle
PersistentQuiescent
Distribution of Duty Cycles
50% of sources are “on” less than 1/4 of the time!
PersistentQuiescent
Outburst Duration
(defined as: contiguous 2 weeks >10 or 4 weeks >5)
“Outburst” Duration
Quiescence Duration
Measured(2 outbursts or more)
Upper Limits
(≤1 outburst)
“New Detection” Rate
(Date of occurrence of first outburst detection)
“New Detection” Rate
(Date of occurrence of first outburst detection)
Main Bulge
“Wings”
Aquila Region
Steady increase of 4-5 per year
Prospects for MAXI
• There are a large number of low duty cycle sources
• Many sources also have long recurrence times
• These sources will benefit from long-term dedicated monitoring of MAXI
Summary• RXTE PCA scans show that using new,
dedicated, monitoring techniques can detect new transient behaviors
• Long term monitoring will uncover the most interesting, faintest transients
Future of RXTE
• Call for proposals this Summer 2009 (Cycle 15)
• Review occurred in November
• Results available on RXTE website
• Funded through February 2011
• Beyond that…
Rogue's Gallery
SAX J1808.4-3658
19982.1 hr
XTE J0929-314
200243 min
XTE J1814-338
20034.3 hr
XTE J1751-305
42 min2002
2002XTE J1807-294
41 min 2003
Ultra-Compact Systems with stripped companions (~0.01 - 0.02 M)
Less Compact Systems More Typical of Low Mass X-ray Binaries (Two are Bursters)
Missing HETE J1900!!
IGR J00291+5934
20042.5 hr
*
ASM
IBIS