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