RXTE and Observations of GC Transients

1. RXTE and Observations of GC Transients C. B. Markwardt (NASA/GSFC)

2. Outline • RXTE PCA scans (2-10 keV) • Motivation • Technique • Sample of Results • X-ray Transient Population Studies

3. 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

4. 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

5. Millisec X-ray Pulsars: Difficult to Discover SAX J1808.4-3658 • 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

6. 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

7. 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 *

9. Example scan • Galactic diffuse emission

10. 1999-2004: 250 sq. deg Main Bulge

11. 2004-2008: 500 sq. deg Bulge “wings”

12. 2008-present: 625 sq. deg Aquila Region

15. 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/

16. Results: Millisecond Pulsars • Bulge scans trigger follow-up confirming observations • Long term monitoring (durations wks. - mos.) • Recurrences: extra observation opportunities; accretion torques; orbital evolution

17. Recurrences XTE J1751-305 (P=PCA) • 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.)

18. Distributions of Spin and Orbital Periods U.L. calculated in 2003 by Chakrabarty

19. 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

20. Wide Distribution of Position & Flux 180 sources Size α flux

21. Flux Distributions Max Flux Median Limiting Flux Min Flux

22. Flux Distributions Max Flux ~few 1038 erg/s at Gal. Ctr. Median Limiting Flux Min Flux

23. Flux Distributions (faint & transient) (bright &persistent) Limiting Flux

24. Flux Distributions “Atoll”(sub LEdd) Limiting Flux “Z”(near LEdd)

25. On/Off Duty Cycle

26. Distribution of Duty Cycles Quiescent Persistent

27. Distribution of Duty Cycles Quiescent Persistent 15-20% of sources are persistent

28. Distribution of Duty Cycles Quiescent Persistent ~40% of sources are have >50% duty cycle

29. Distribution of Duty Cycles Quiescent Persistent 50% of sources are “on” less than 1/4 of the time!

30. Outburst Duration (defined as: contiguous 2 weeks >10s or 4 weeks >5s)

31. “Outburst” Duration

32. Quiescence Duration Upper Limits(≤1 outburst) Measured(2 outbursts or more)

33. “New Detection” Rate (Date of occurrence of first outburst detection)

34. “New Detection” Rate “Wings” Aquila Region Main Bulge Steady increase of 4-5 per year (Date of occurrence of first outburst detection)

35. 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

36. 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

37. 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…

38. 2002 2002 2003 41 min 42 min 43 min XTE J1807-294 XTE J0929-314 2002 XTE J1751-305 2003 1998 2004 2.5 hr 2.1 hr 4.3 hr XTE J1814-338 SAX J1808.4-3658 IGR J00291+5934 Rogue's Gallery   ASM Ultra-Compact Systems with stripped companions (~0.01 - 0.02 M) *  IBIS Less Compact Systems More Typical of Low Mass X-ray Binaries (Two are Bursters) Missing HETE J1900!!