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 J “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 J

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

: 250 sq. deg Main Bulge

: 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 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

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 J : 1996 (SAX), 1998(P), 2000(P), 2002(P), 2004/2005 XTE J : 1998 (ASM), 2002 (P), 2007 (P), 2005 (INTEGRAL), 2009 (P) IGR J : 2009 (P); XTE J : 2003 (P); XTE J : 2003 (PCA); SWIFT J : 2010 (Swift) SWIFT J (Swift – PCA in constraint) NGC 6440: ~periodic (PCA/Swift/Chandra) Aql X-1: ~yearly (PCA, ASM, etc.) XTE J (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 erg/s at Gal. Ctr. Flux Distributions Limiting Flux

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

Flux Distributions Limiting Flux “Atoll” (sub L Edd ) “Z” (near L Edd )

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 J hr XTE J min XTE J hr XTE J min 2002 XTE J min 2003 Ultra-Compact Systems with stripped companions (~ M  ) Less Compact Systems More Typical of Low Mass X-ray Binaries (Two are Bursters) Missing HETE J1900!! IGR J hr   ** ASM IBIS