Anomalous X-ray Pulsars GRBs: The Brightest Explosions in the Universe Harvard University, May 23, 2002 Vicky Kaspi Montreal, Canada
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.
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
AXP Catalog not confirmed 2(3)/5(6) associated with supernova remnants
RXTE AXP Pulse Profiles (A) 4U 0142+61 149 ks, 2.5-9 kev (B) 1RXS 1708-4009 185 ks, 2.5-9 kev (C) 1E 2259+586 469 ks, 2.2-5.5 kev Gavriil & VK 2002
AXP Long-Term Spin-Down 1E 1048-5937 Period (s) Deviations from simple spin down apparent. Oosterbroek et al. 1998
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 0142+61 reportedly pulsed (Kern & Martin 2001?) LMXB inconsistent with SNR associations
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)
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?
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)
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?
AXP Long-Term Flux Stability 1E 1048-5937 Note data from different telescopes, having different energy responses, some imaging, some not. Note Eta Carina ~40 away. Oosterbroek et al 1998
AXP Pulse Profile Change? Same instrument (GINGA) for both observations. Iwasawa et al. 1992
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
Phase-Coherent Timing: Stable! 1E 2259+586 True of 4U0142+61, RXS 1708-4009, 1E 1841-045 too! Kaspi, Chakrabarty & Steinberger 1999; Gavriil et al. 2002
Pulsed Flux Monitoring: Stable! 1E 2259+586 True of 4U0142+61, 1708-4009, 1E 1841-045 too! Gavriil et al. 2002
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.
Anomalous AXP: 1E 1048-5937 Cannot be phase connected for more than a few months at a time. Very noisy rotator. Kaspi et al. 2001
1E 1048-5937 RXTE Pulsed Fluxes No large variations. Hard to reconcile with past reported variability. Agrees with results of Tiengo et al. 2002. No correlations with spin-down variations. Kaspi et al. 2001
1E 1048-5937: 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???
X-ray Bursts from 1E 1048-5937? 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
Possible AXP Bursts 1E 1048-5937 Burst 1 Burst 2 Gavriil, Kaspi & Woods, in prep.
Bursts from 1E 1048-5937? Burst 1 Burst 2 Rise time ~64 ms Rise time ~8 ms Gavriil, Kaspi & Woods, in prep
SGR Burst Durations Gogus et al. 2001
SGR Rise time/duration Gogus et al. 2001
SGR Burst Fluence vs Duration SGR 1900+14 Bursts also harder than SGR hardness vs duration predicts. Gogus et al. 2001
X-ray Bursts from 1E 1048-5937? 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 20-30 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)
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