8 InterstellarCh aptermedium 1. Introduction effects S P A C E Dispersion Scintillation At source At Earth f f Rotational phase of pulsar Figure 1.4 One pulse from the low-DM pulsar PSR J1456–6843. Thelowerpanelshowsa characteristic spectrogram, the power in greyscaleScattering as a function of frequency and the pulsar’s rotational decorrelation bandwidth Pulse is phase. The white lines trace DM = 8.6pccm−3.Severaleffects of the ISM are clear here: dispersed particularly the dispersion of the pulse and the frequency-dependent amplitude modulation (“scintles”) caused by wavefront distortions in the ISM. The upper panel shows the pulse t t profile after dedispersion, integrated over the path traced600MHz by the white lines. electron density model of Cordes & Lazio (2002). Pulse dispersion is generally a property thatLorimer is beneficial to surveys for pulsing objects; it creates400MHz a characteristic signal with which toand distinguish local (esp. man-made) signals from pulses of astrophysical origin. This will Frequency (MHz) beKramer discussed further in Section 1.4 and is a central element325MHz to what makes the properties of(2005) the discoveries in Chapter 6τ so unexpected. There are two remaining ISM effects which can influence pulsar measurements that 240MHz S. Ransom will play a role later in this thesis: the effects of pulse scattering, and scintillation. These are mathematically and conceptually developed in detail by both Manchester & Taylor Rotational phase of pulsar (1977) and Lorimer & Kramer (2004)time based on the “thin screen” model of Scheuer (1968) (in which observed ISM effects are caused by a single screen positioned halfway between observer and emitter). The reader is referred to those texts for details; we report1 only on 2 their derived results here. Essentially, interstellar pulse broadening and scintillation both are caused by the fact that the interstellar medium is both inhomogeneous and turbulent. A propagating wave- front incident upon a scattering screen in the ISM will undergo random phase delays, the magnitude of which are dependent upon the scale, a,ofrefractiveindexinhomogeneities (that is, electron density variations, ∆ne). These delays cause several effects. First, they Dispersion trouble 0-10 pc/cc Pulse broadening Instrumental broadening [Detected width] 2 = [Intrinsic width] 2 + [Scattering] 2 + [Sampling Time] 2 + [Dispersion error] 2 100 pc/cc 1000 pc/cc Frequency Cordes+Lazio02 A rough dispersion map of the Milky Way for a source @ 30kpc Time 3 4 S P A C E D. Frail Transients Grid Scattering and Scintillation Tidal disruption events Gamma-ray bursts but extremely intense Here be dragons AGN variability Cataclysmic or repeated Supernovae 600MHz characteristic decorrelation bandwidth 400MHz X-ray Binaries 325MHz Many varieties of pulsar τ 240MHz Flare Stars Frequency (MHz) Repeated Brown Dwarfs Solar bursts Novae/Cepheid Variables Lorimer+Kramer05 Extrasolar planets Soft gamma-ray repeaters S. Ransom τ ∼ f –4.0 ± 0.4 Rotational phase of pulsar Coherent Incoherent Image credits: GRB, ESO/A. Roquette; Supernova, flare star, XRB: NASA; Pulsar, M. Kramer 5 6-1 D. Frail Transients Grid Tidal disruption events Gamma-ray bursts Far, bright but extremely intense Here be dragons Supernovae AGN variability Cataclysmic or repeated Near, X-ray Binaries faint Many varieties of pulsar Flare Stars Brown Dwarfs Repeated Solar bursts Novae/Cepheid Variables Extrasolar planets Soft gamma-ray repeaters Coherent Incoherent Image credits: GRB, ESO/A. Roquette; Supernova, flare star, XRB: NASA; Pulsar, M. Kramer Fast Slow 6-2 7 Extragalactic pulse history Fast Radio Bursts History Small Megellanic ■ 30 Jy burst Cloud ! 100x det. Limit 1967 1979 ¼ century gap! 2003 ! 12 ms duration Phinney & Taylor: McLaughlin & Cordes: Discovery of First conscious Fourier pulsar Pulsar surveys = pulsars extragalactic pulse search searching reigns extragalactic searches! ■ High Dispersion Measure ! 375 pc/cc ! 600 Mpc! Single pulse search resurgence: ~2003 – present 1.7o Archival pulsar surveys New targeted surveys New INSTRUMENTS for single pulses! Lorimer et al. 2007, Science Instrument: Parkes Multibeam Receiver, analog backend 8 9-1 Fast Radio Bursts History τ ∼ f –4.0 ± 0.4 Small Megellanic ■ 30 Jy burst Cloud dt α f -2 ! 100x det. Limit -4.8±0.4 Δw α f ! 12 ms duration ■ High Dispersion Measure ! 375 pc/cc ! 600 Mpc! 1.7o dt ∼ f –2.000 ± 0.006 Wow!!! Lorimer et al. 2007, Science Instrument: Parkes Multibeam Receiver, analog backend Thornton et al. (2013) 9-2 10 Molonglo: 2 GBT: 1 Arecibo: 1 Parkes: 23 ASKAP: ~25 (unpublished since mid-last-year)!!! Repeater Pulsars FRBs Unknown Known dispersion (Milky Way) Flux Density (Jy) contribution dispersion contribution ~50 sources detected (17 published) f = 0.7 - 1.5 GHz 0 20 40 60 80 100 120 140 Time (ms) 2500-10000/sky/day* Online FRB Catalog: http://astronomy.swin.edu.au/pulsar/frbcat/ 11 12 FRB Distances Inoue (2004) model IGM: 24% He, 76% H ) Ionization fraction 1.0 3 FRB light travel time (1ms) All excess attributed to contribution (pc/cm IGM DispersionMeasure IGM TB ~ 1038 K L1GHz ~ 1042 erg/s (Radio flux: up to tens of Jansky) 13 14-1 FRB Distances Inoue (2004) model IGM: 24% He, 76% H ) Ionization fraction 1.0 3 50% excess attributed to contribution (pc/cm IGM DispersionMeasure IGM 14-2.