Anomalous X-ray Pulsars

Transcription

1 Anomalous X-ray Pulsars GRBs: The Brightest Explosions in the Universe Harvard University, May 23, 2002 Vicky Kaspi Montreal, Canada

2 What are Anomalous X-ray Pulsars? exotic class of objects 1st discovered 20 yr ago unclear why they shine currently best explanations: magnetars is this correct?? Important implications for understanding young neutron stars in general.

3 Anomalous X-ray Pulsars 5(6) known all in Galactic Plane ( b < 1 degree) periods from 6-12 s broad pulse profiles all spinning down spectra described by two component model: Black body (kt ~ 0.4 kev) + power law (index ~ -3) modest X-ray luminosities anomalous as energy source unclear

4 AXP Catalog not confirmed 2(3)/5(6) associated with supernova remnants

5 RXTE AXP Pulse Profiles (A) 4U ks, kev (B) 1RXS ks, kev (C) 1E ks, kev Gavriil & VK 2002

6 AXP Long-Term Spin-Down 1E Period (s) Deviations from simple spin down apparent. Oosterbroek et al. 1998

7 AXP Models: Accretion anomalous: Lx >> spin-down luminosity not rotation powered like Crab-like pulsars accretion powered X-ray binary? Unlikely. no Doppler shifts (Mereghetti et al. 1998) softer spectra than accreting X-ray pulsars all spinning down regularly no plausible optical/ir companion counterparts (Hulleman et al. 1999, 2000) faint optical counterpart of 4U reportedly pulsed (Kern & Martin 2001?) LMXB inconsistent with SNR associations

8 Accretion Models continued... Accretion from a supernova fall-back disk (Chatterjee, Narayan & Hernquist 1999) neutron star accretes in a propeller mode: extracts angular momentum but only modest X-ray luminosity all young neutron stars have such disks similarity to SGRs coincidental (no bursting mechanism suggested) faint optical/ir counterparts argue against disk (Hulleman et al. 1999, 2000)

9 AXP Models:Magnetars Magnetars (Thompson & Duncan 1996): spin down due to magnetic dipole braking as in radio pulsars B implied by spin down 2-3 orders of magnitude larger than radio pulsars emission similar to that of soft gamma repeaters in quiescence: two populations related Similar periods Similar spin down rates Similar quiescent X-ray properties expect AXPs to burst occasionally?

10 Magnetar Model continued... emission powered either by heat due to B field decay (Thompson & Duncan 96) initial cooling (Heyl & Hernquist 97) both produce thermal spectra non-thermal component from - vacuum polarization effects in high B field (Ozel 2001) - detailed modeling of highly magnetized neutron star atmsophere (Ho & Lai 2002) - scattering in magnetosphere due to twisted B-field (Thompson, Lyutikov & Kulkarni 2002)

11 Long-Term AXP Monitoring understand deviations from spin-down glitches? long-term spin-up? timing noise? look for flux, spectral variability look for pulse profile variations look for SGR-like bursts Establish basic long-term properties. Look for evidence of activity and correlations between properties. ARE AXPs MAGNETARS?

12 AXP Long-Term Flux Stability 1E Note data from different telescopes, having different energy responses, some imaging, some not. Note Eta Carina ~40 away. Oosterbroek et al 1998

13 AXP Pulse Profile Change? Same instrument (GINGA) for both observations. Iwasawa et al. 1992

14 RXTE Monitoring of AXPs observe each AXP monthly/weekly with PCA since 1998 using brief snapshots permits phase coherent timing: monitor evolution of pulse phase account for every rotation of the neutron star can determine spin parameters with high precision monitor also flux, pulse profile stability, look for SGR-like activity Fotis Gavriil, McGill University

15 Phase-Coherent Timing: Stable! 1E True of 4U , RXS , 1E too! Kaspi, Chakrabarty & Steinberger 1999; Gavriil et al. 2002

16 Pulsed Flux Monitoring: Stable! 1E True of 4U , , 1E too! Gavriil et al. 2002

17 Summary of RXTE AXP Results Data sets range from 2-5 yrs 4 AXPs are very stable rotators (apart from glitch) RMS phase residuals ~1% of pulse period 5 AXPS have stable pulsed fluxes to within ~20-30%, consistent with systematic uncertainties 5 AXPs show no evidence for pulse profile changes In spite of past evidence for activity, we see none for 4 of the 5 AXPs.

18 Anomalous AXP: 1E Cannot be phase connected for more than a few months at a time. Very noisy rotator. Kaspi et al. 2001

19 1E RXTE Pulsed Fluxes No large variations. Hard to reconcile with past reported variability. Agrees with results of Tiengo et al No correlations with spin-down variations. Kaspi et al. 2001

20 1E : AXP/SGR Transition Object? Noisiest rotator: most like SGR Hardest spectrum: most like SGR softest spectrum SGR also may be a transition object (Kaplan et al. 2001) Largest pulsed fraction Most sinusoidal profile Most likely to one day burst???

21 X-ray Bursts from 1E ? Work in progress (Gavriil, Kaspi & Woods) Discovered 2 bursts in RXTE/PCA data total exposure ~425 ks over ~5 yr Bursts ~2 weeks apart apart in 2001 No similar events seen from any other AXPs in RXTE monitoring data Similar to SGR bursts: Rise time, burst profile, spectra

22 Possible AXP Bursts 1E Burst 1 Burst 2 Gavriil, Kaspi & Woods, in prep.

23 Bursts from 1E ? Burst 1 Burst 2 Rise time ~64 ms Rise time ~8 ms Gavriil, Kaspi & Woods, in prep

24 SGR Burst Durations Gogus et al. 2001

25 SGR Rise time/duration Gogus et al. 2001

26 SGR Burst Fluence vs Duration SGR Bursts also harder than SGR hardness vs duration predicts. Gogus et al. 2001

27 X-ray Bursts from 1E ? Other origins cannot be ruled out but are unlikely Type I XRB? rise times too short; spectra too hard (kt~3-4 kev) no spectral softening; no known LMXB in FOV faint - implied distance kpc X-ray Rich GRBs? in Galactic plane; 2 in 1 deg PCA FOV within 2 weeks fast rise, slow decay atypical short duration (~2 s)

28 Conclusions Strong evidence that SGRs are magnetars AXPs similar to SGRs in several ways similar periods, spin-down rates, spectra but clearly less active Why don t AXPs burst? maybe they do! AXP bursts require confirmation with imaging telescope if confirmed, strong support for magnetar model If bursts not from AXP, still interesting phenomenon