Millisecond Pulsar Discovery via Gamma-Ray Pulsaons

Holger J. Pletsch (for AEI Hannover Group and LAT Collaboraon)

Max Planck Institute for Gravitational Physics (Albert Einstein Institute) © SDO/AIA (Sun), AEI

H J Pletsch Fermi Symposium 2012, Monterey, CA 1 Execuve summary

• Millisecond pulsars (MSPs): - old neutron stars, spun up by accreng maer from companion star, - reach high rotaon rates of several hundreds of Hertz. • Previously: ALL such MSPs ("recycled" pulsars) discovered by spin-modulated radio emission. DOI: 10.1126/science.1229054

• Compung-intensive blind search in Fermi-LAT data using advanced methods (with paral constraints from opcal data): ⇒ Discovery of PSR J1311-3430 - First MSP found via gamma-ray pulsaons! - Extremely compact binary: Shortest orbital period! - Clarifies nature of decade-long enigma.

© NASA (Pulsar), NASA/ESA, M.J. Jee and H. Ford (Johns Hopkins University) (Hubble Field), AEI/Milde Markeng Science Communicaon H J Pletsch 2 Background

30 years ago: First MSP detected in radio observaons. • Backer et al., Nature 1982 • Fermi LAT confirmed that many radio-detected MSPs also pulsate in gamma-rays: Abdo et al., Science 2009 → Gamma-ray pulsaons revealed

−9 only by rotaon parameters 10 Radio discovered 1E13 G 1E33 erg/s Gamma−ray discovered 1E39 erg/s 1E36 erg/s obtained from radio telescopes. −11 e.g., Smith et al., A&A 2008 10 Part of a binary system Young &

1E12 G normal pulsars )

− 1 −13 Successful blind searches for 10 1E30 erg/s • 1 kyr

10 kyr gamma-ray pulsars in LAT data: −15 Abdo et al., Science 2009 10 - 24 (with 1 year) Saz Parkinson et al. ApJ 2010 100 kyr 1 Gyr

1 Myr 10 Gyr - 10 (with 3 years) HJP et al. ApJ & ApJL 2012 −17 10 1E11 G → Large fracon of young pulsars 10 Myr 100 Gyr Period derivative (s s 100 Myr −19 → Most are radio-quiet Ray et al. 2012 10 1E10 G

−21 1E8 G MSPs 1E9 G 10 No MSP previously found in −3 −2 −1 0 1 • 10 10 10 10 10 blind search of gamma-ray data. Spin period (s) H J Pletsch 3 Computaonal Challenge

• Blind-search problem for gamma-ray pulsars: - Computaonally demanding since pulsar parameters unknown a priori → Must be explicitly searched on dense grid (for isolated pulsars: spin frequency, frequency derivave & sky posion) Brady et al., 1998 → For mulple-year observaon mes: Chandler et al., 2001 Atwood et al., 2006 number of grid points tremendously large Ziegler et al., 2008 HJP et al., 2012 • Blind searches for MSPs vastly more difficult: - Must scan up to higher spin frequencies [to and beyond 716Hz] - Plus, most MSPs are in binaries: → Addionally unknown orbital parameters → Increases computaonal complexity by orders of magnitude - Blind binary-MSP searches in LAT data were hitherto virtually unfeasible

H J Pletsch 4 Target source: 2FGL J1311.7-3429 (formerly unidenfied) First seen by EGRET. • (e.g., 2EG J1314-3430) • Most significant (43σ) unidenfied LAT source in 2FGL. • Good pulsar candidate: - low flux variability - very curved spectrum

opcal 93-minute modulaon • Crucial: In search for opcal counterparts, Romani (2012) idenfied quasi-sinusoidal opcal flux modulaon. (Romani ApJL 2012, Kataoka et al. ApJ 2012) → Conjecture: "black widow" pulsar binary - MSP irradiates companion star - Heang one side of companion, explains opcal brightness variaon

H J Pletsch 5 The search space

• "Black widow" pulsar interpretaon: Pulsar irradiang companion - Opcal variaon associated with period of circular orbit. - Confines sky posion. → These constraints made blind binary-MSP search feasible. BUT: sll enormous computaonal © AEI challenge, because uncertaines on orbital parameters . by far larger than required for pulsar detecon (and f, f unknown)

→ Pulsar search parameter space le 5-dimensional: 1. Spin frequency: 0 < f < 1400 Hz . 2. Its rate of change: -5x10-13 Hz/s < f < 0 3. Orbital period: Porb = 5626.0 ± 0.1 s 4. Time of ascending: Tasc = 56009.131 ± 0.012 MJD 5. Projected semi-major axis: 0 < x < 0.1 lt-s

H J Pletsch 6 Hierarchical search strategy

Hierarchical, 3-staged search scheme: · Goal: discarding unpromising regions in parameter space as early as possible. · Previously enabled detecon of 10 young (non-MSP) pulsars in blind searches. HJP et al. ApJ & ApJL 2012

Total data set (~4 years) Most 12-day window ... compute 1. Semi-coherent stage: cycles - Sliding coherence window, extending Atwood et al., ApJL 2006; HJP, PRD 2011 spent at summing coherent Fourier power; Zooming in this - Coarse graining: Least sensive, but most efficient to scan enre search space stage. - Incorporates photon weights (à la Kerr, ApJ 2011) - Uses heterodyning to process in bands of f using FFT

"zooming in" 2. Coherent follow-up:

Dr. Timo Mappes, www.musopn.com. - For every semi-coherent candidate compute fully coherent Fourier power over enre data set, on significantly refined grid.

3. Including higher signal harmonics (H-test): de Jager et al. 1989 - Typically pulse profile non-sinusoidal, also Fourier power at harmonics. - For every coherent candidate sum power (over enre data) from harmonically related frequencies, using a further refined grid.

H J Pletsch 7 Key novelty: metric search grids

Brady et al. PRD 1998

• Based on concepts from gravitaonal-wave searches. Metric • Define a distance metric on search parameter space: approximaon - Measure of expected fraconal loss in S/N for given signal at nearby grid point. - Local Taylor-expansion of fraconal loss around

signal locaon to 2nd order gives the metric. Fraconal loss in S/N Brady et al., PRD 1998; HJP & Allen, PRL 2009; HJP, PRD 2010 Parameter offset • Problem: Metric orbital components explicitly . depend on parameters (unlike metric in f and f ) → Simple lace would either over- or undercover • Soluon: New grid construcon algorithm to ulize metric - First place orbital grid points at random. (fast MC integraon using metric provides total number of grid points required) - Then move those that are too close or too far apart by barycentric shis towards opmal coverage. - Designed to never lose > 30% in S/N for any signal. Fehrmann & HJP, in prep H J Pletsch 8 Searching 4 years of LAT data

• Input LAT data for the search: - 1437-day me span; LAT photons within 15° around target.

• Compung done on the ATLAS cluster (6780 CPU-cores) - Analyzed full spin-frequency range in bands of 128 Hz each (via heterodyning): → accommodates memory limitaons → adapt orbital search to each band (points increase as f3) - For example, a band near 700Hz Number of grid points: . f: 108, f: 102, orb: 107 → total: 1017

ATLAS compung facility, Hannover

H J Pletsch 9 The PSR J1311-3430 system (1/2)

Photon probability weight 250 > 0.1 GeV • Following the discovery: → pulsar ming to precisely measure 200 the system parameters ( ) 150

100 Weighted counts 50 Weighted pulse profile

80 < 0.3 GeV 60 40 20 W. counts 0.3 − 1 GeV 90 60 30 W. counts 70 1 − 3 GeV 50 30

W. counts 10 3 − 7 GeV 12 9 6 3 W. counts 5 > 7 GeV 3

W. c. 1 0 0.5 1 1.5 2 Pulse phase

H J Pletsch 10 The PSR J1311-3430 system (2/2)

• Rotaonal ephemeris also constrains companion mass: mc > 0.0083 MSun (~8 MJupiter)

[for mp = 1.35 MSun, i=90°] 15 c

14 m

13 i=90° 12 1.35 Solar masses)

3 11

10

9

8 Companion mass (10 7 mp 6 0 0.5 1 1.5 2 2.5 3 • The likely most compact Pulsar mass (Solar masses) pulsar binary known: - Companion Roche lobe radius: - separaon only ~0.75 RSun ~0.63 RJupiter -3 → system easily fits into Sun - Mean density: ~45 g cm © SDO/AIA (Sun), AEI → 30 mes higher than Jupiter

H J Pletsch 11 Further studies at other wavelengths

gamma-ray • Radio follow-up observaons see Ray et al., arXiv:1210.6676 - Several targeted searches with

gamma-ray ephemeris gave no detecon radio - BUT: intermient radio detecon with GBT in one 2GHz observaon → Dispersion-measure distance: 1.4kpc c

• Further opcal observaons m see Romani et al., arXiv:1210.6884 - Radial velocity measurements of companion - Addional constraint in mass-mass diagram → Neutron star might have very large mass

mp

H J Pletsch 12 Conclusion

• With paral constraints from opcal, binary gamma-ray MSPs can be found in blind search with Fermi LAT. • PSR J1311-3430: First MSP discovered solely via gamma-ray pulsaons

- Compact "black widow" pulsar binary, with shortest Porb known. © SDO/AIA (Sun), AEI - Broader relevance: potenal key probe for binary evoluon; its high neutron star mass can constrain behavior of ultra-dense maer.

• New possibilies for future searches & studies: - More MSPs might exist among LAT unidenfied sources, which are too radio-faint or obscured for typical radio searches. - Hunt for the first radio-quiet MSP connues.

• Blind searches in LAT data for solitary MSPs also with volunteer compung system Einstein@Home hp://einstein.phys.uwm.edu/

H J Pletsch 13