ISOLATED NEUTRON STARS Andrea Tiengo INAF IASFMilano on

ISOLATED NEUTRON STARS Andrea Tiengo INAF, IASF-Milano on behalf of Roberto Turolla Dept. of Physics, University of Padova NHXM - ESRIN, November 12 -13 2009 1

INSs in the X-rays • At present > 110 INSs detected in X-rays • Large majority are “normal” (rotation powered) radiopulsars (~ 90 sources, including high-B pulsars and ms pulsars) • Radio-silent (quasi), X/γ-ray bright INSs – Anomalous X-ray pulsars and Soft γ-ray repeaters (AXPs and SGRs, the magnetar candidates) – Central compact objects in SNRs (CCOs) – Rotating radio transients (RRa. Ts) – X-ray dim neutrons stars (XDINSs) – Geminga and Geminga-like objects • “Isolated” NSs in binaries: Soft X-ray transients (SXTs) NHXM - ESRIN, November 12 -13 2009 2

• X-ray emission ubiquitous over the P-Ṗ diagram • Detected from objects with extremely different – Ages: from young Crab-like to old PSRs – Magnetic fields: from low B ms PSRs to magnetars Kondratiev et al. (2009) NHXM - ESRIN, November 12 -13 2009 3

Different mechanisms powering X-ray emission SGR+AXPs XDINSs HBPSRs Rotation RRa. Ts LX = Ė rot Cooling Magnetic from Becker (2009) NHXM - ESRIN, November 12 -13 2009 4

INS spectrum: thermal plus non-thermal component (magnetospheric, if rotation-powered LNT ~ Ė ~ t-β, β ≈ 2 -4) total non-thermal Young, < 1 kyr: non-thermal component dominates (Crab, PSR 1509 -58) Middle-aged, 10 -100 kyr: thermal emission from the star surface (Vela, Geminga) + SGRs/AXPs NHXM - ESRIN, November 12 -13 2009 Old, > 1 Myr: no magnetospheric activity (XDINSs) + SXTs 5

Some Like It Hot Page, Geppert & Weber (2006) • NSs are born very hot (T ≈ 1011 K) and progressively cool down • Surface temperature drops quickly and then stays at ≈ 105 – 106 K for about 1 Myr • Thermal emission at ≈ 100 e. V, L ≈ 1031 – 1032 erg/s • Thermal component detected ~ 40 INSsmagnetic dissipation, inflowing Heatingin processes: magnetospheric currents, accretion NHXM - ESRIN, November 12 -13 2009 6

BB or Not BB ? Turolla, Zane & Drake (2004) BB at Teff Zavlin & Pavlov (2002) Magnetic H atmosphere models Bare NS H models Whatever the state of the surface, the emitted hard photons decouple deeper k. T ~ hνp, e: emission strongly suppressed thermal radiation is NOT a blackbody in the atmopsphere where T is higher NHXM - ESRIN, November 12 -13 2009 7

Fast (and Furious) • Electrons accelerated by electric field • High-energy CR/ICS photons produce pairs via γ + B e+ + e • e emit synchrotron radiation (+ ICS) • Hard power-law spectrum (Γ ≈ 1. 5 – 2) Hirotani (2006) NHXM - ESRIN, November 12 -13 2009 8

Twist, Twist Little Star • In “normal” PSRs currents flow along open field lines • In an ultramagnetized NS the “wound up” internal field is > 1015 G • It stresses and deforms the crust • The external field twists up ( ) • Currents flow along the closed field lines NHXM - ESRIN, November 12 -13 2009 Thompson & Duncan (2001) 9

The Seven Thermally Emitting Neutron Stars (XDINSs) • 7 known (the “Magnificent Seven”), plus 2 candidates • Nearby (D 100 – 500 pc) • Radio-quiet, no association with SNRs • X-ray pulsations (P 3 – 10 s) • Ṗ 10 -14 s/s → B (1 – 4) x 1013 G, τc a few Myr • Very faint optical counterparts (f. X/fopt > 104) • Soft, purely thermal spectrum (k. T 50 – 100 e. V), LX 1031 1032 erg/s • Broad spectral feature(s) at 100 – 700 e. V NHXM - ESRIN, November 12 -13 2009 10

Rotating Radio Transients (RRa. Ts) • 11 sources detected in the Parkes Multibeam Survey (Mc. Laughlin et al 2006) • Burst duration 2 -30 ms, interval 4 min-3 hr • Periods in the range 0. 4 -7 s • Period derivative measured in 3 sources: B 1012 -1014 G, τc 0. 1 – 3 Myr • RRa. T J 1819 -1458 detected in the X-rays, spectrum soft and thermal (k. T ~ 114 e. V), absorption feature at ~ 1 ke. V (Reynolds et al 2006; Mc. Laughlin et al 2008) P, Ṗ, spatial distribution, lack of pulsed radio emission (and X-ray properties) are very similar to those of the XDINSs NHXM - ESRIN, November 12 -13 2009 11

Central Compact Objects in SNRs (CCOs) • 7 bona fide sources • Located near the center of young SNRs; no radio emission At least two absorption lines detected; no PWNs De Luca et al. (2004) 33 erg/s • Mostly thermal spectrum (k. T 0. 7 0. 3 – 0. 4 ke. V), L 10 X ke. V • P 0. 1 – 0. 4 s (in 3 sources); limits on Ṗ → B < 1011 G (Halpern et al. 2007; Gotthelf & Halpern 2007, 2009) 1. 4 ke. V • Cas A may have similar properties (Pavlov & Luna 2009) • Absorption features in the spectrum of 1 E 1207. 4 -5209 supports low B, if electron cyclotron Are CCOs “antimagnetars” ? Need to ne born with P 0 P to have τc τSNR 1 kyr. RCW 103 has P = 6. 67 hr. Are CCOs a class ? NHXM - ESRIN, November 12 -13 2009 12

Soft γ Repeaters and Anomalous X-ray Pulsars (SGRs and AXPs) • 6 SGRs and 10 AXPs known (plus a few candidates); some AXPs are transients (TAXPs) • X-ray pulsations (P 2 – 12 s) • Ṗ 10 -10 – 10 -12 s/s → B 1014 – 1015 G, τc 1 – 10 kyr • Emission of short bursts (Δt 0. 1 s, LX 1039 – 1041 erg/s); giant flares (L up to 1047 erg/s) from 3 SGRs • Persistent X-ray flux (0. 1 – 10 ke. V): thermal (k. T 0. 5 ke. V) plus non-thermal component (Γs 1. 5 – 4); LX 1033 – 1035 erg/s; variable spectral and timing properties (months/yrs) • Transient radio emission in 2 AXPs; IR/opt counterparts • Hard power-law tails (20 – 200 ke. V), Γh < Γs in AXPs NHXM - ESRIN, November 12 -13 2009 13

What is producing the persistent X-ray spectrum ? • Thermal emission from part of the surface kept hot by returning currents/magnetic dissipation (corroborated by TAXPs evolution) • Soft PL from resonant upscattering of thermal photons by e± moving along the closed field lines; quantitative models fit well, but unclear if charges can have γ 1 • No definite model for the hard PL. Does it come from a different region ? What is the origin of the IR/optical radiation ? • Passive disk ? IR, but not optical pulsations • Inner magnetosphere (CR from moderately relativistic particles) ? Possibly Bursts 1 E 1841 -045 (Rea et al. 2008) • Double BB spectrum and/or absorption lines ? • Crustal or magnetospheric origin? XTE J 1810 (Israel et al. 2004) NHXM - ESRIN, November 12 -13 2009 14

Unity in the Diversity ? Why are INSs different ? • “Intrinsic” diversity – Progenitors with different properties (P, B, M, …) → differences in their formation/very early evolution – Magnetars require peculiar conditions to form, either in the fossil field or in dynamo scenario – Antimagnetar CCOs need to be born with long P and low B • Evolution – RRa. Ts XDINSs (Popov et al 2006), no RRa. T-like emission detected from XDINSs yet (Kondratiev et al. 2009) – HBPSRs XDINSs (Kaplan & Van Kerkwijk 2009) – quiescent TAXPs may look quite similar to XDINSs • An incomplete knowledge of their properties (SGRs and AXPs !) NHXM - ESRIN, November 12 -13 2009 15

The Future and NHXM: a few ideas Broad-band (phase-resolved) X-ray spectroscopy of rotation-powered pulsars with NHXM – Constraining non-thermal emission in young and middleaged pulsars allows to better constrain thermal emission models. (Non-simultaneous) observations at soft X-rays are also required – Observations of Fermi pulsars in hard X-rays: test of pulsar non-thermal emission models (SEDs and phase-resolved SEDs) NHXM - ESRIN, November 12 -13 2009 16

The Future and NHXM: a few ideas Broad-band (phase-resolved) X-ray spectroscopy of AXPs/SGRs with NHXM – Spectral analysis in the poorly explored ~ 10 -20 ke. V band, where the hard tails emerge from softer X-ray component – High sensitivity coverage of the 1 -80 ke. V energy band allows to study the (possibly correlated) variability of the soft and hard components in magnetars NHXM - ESRIN, November 12 -13 2009 17

AXP 4 U 0142+61 XMM-Newton data (spectra integrated over few hours) INTEGRAL data (spectra integrated over several weeks) NHXM - ESRIN, November 12 -13 2009 18

NHXM: AXP 4 U 0142+61, 20 ks NHXM - ESRIN, November 12 -13 2009 19

The Future and NHXM: a few ideas Broad-band (phase-resolved) X-ray spectroscopy of AXPs/SGRs with NHXM – Spectral analysis in the poorly explored ~10 -20 ke. V band, where the hard tails emerge from softer X-ray component – High sensitivity coverage of the 1 -80 ke. V energy band allows to study the (possibly correlated) variability of the soft and hard components in magnetars – Broad band spectra of bursts: pile-up at low energies and low sensitivity in hard X-rays have limited these studies up to now NHXM - ESRIN, November 12 -13 2009 20

NHXM: SGR burst (BB+BB or BB+PL) NHXM - ESRIN, November 12 -13 2009 21

The Future and NHXM: a few ideas Broad-band X-ray imaging with NHXM – Pulsar Wind Nebulae (PWN): ~10 -15 detected by INTEGRAL; NHXM can discover tens of new PWN, and study them with better spatial resolution disentangle PWN and pulsar hard X-ray emission – If low background in LED, dust-scatterings from bright magnetar bursts (or GRBs): up to now very rare, but allow to constrain dust properties, burst spectrum and source distance NHXM - ESRIN, November 12 -13 2009 22

Swift/XRT: t=1. 37 d XMM/EPIC: t=12. 3 d Tiengo et al. 2009, submitted to Ap. J NHXM - ESRIN, November 12 -13 2009 23

The Future and NHXM: a few ideas X-ray polarimetry with NHXM – Non-thermal emission of X-ray pulsars: different beaming geometry from polar caps, slot gaps and outer magnetosphere models, with different energy dependence. Up to now: polarization detected in radio and optical only for Crab pulsar, with slot gap model favored – Thermal emission of X-ray pulsars: polarization is different for atmosphere or condensed surface. NHXM - ESRIN, November 12 -13 2009 24

Magnetized H atmosphere Condensed H surface Pavlov & Zavlin (2000) Van Adelsberg et al. (2005) NHXM - ESRIN, November 12 -13 2009 25

The Future and NHXM: a few ideas X-ray polarimetry with NHXM – Non-thermal emission of X-ray pulsars: different beaming geometry from polar caps, slot gaps and outer magnetosphere models, with different energy dependence. Up to now: polarization detected in radio and optical only for Crab, with slot gap model favored – Thermal emission of X-ray pulsars: polarization is different for atmosphere or condensed surface – Magnetars: test for up-scattering emission models NHXM - ESRIN, November 12 -13 2009 26

Nobili, Turolla & Zane (2008) Up-scattering in twisted magnetosphere should suppress the polarization of the (strongly polarized) seed X-ray photons NHXM - ESRIN, November 12 -13 2009 27

The Future and NHXM: a few ideas Spatially-resolved X-ray polarimetry with NHXM Linear polarization in Crab nebula detected at soft (Weisskopf et al. 1978) and hard X-rays (Dean et al. 2008), but imaging is needed to map B and estimate maximum particle energy ) L A r cto R G E NT I e v n r la o (P NHXM - ESRIN, November 12 -13 2009 tio a iz Dean et al. 2008 28

The Future and NHXM: a few ideas Spatially-resolved X-ray polarimetry with NHXM Weisskopf 2008 Linear polarization in Crab nebula detected at soft (Weisskopf et al. 1978) and hard X-rays (Dean et al. 2008), but imaging is needed to map B and estimate maximum particle energy NHXM - ESRIN, November 12 -13 2009 29

Conclusions What requirements to study INSs with NHXM? • Disentangle thermal/non-thermal emission spectral components Broad energy band (good Aeff <1 ke. V desirable but not required) • Phase-resolved spectroscopy and polarimetry Good time resolution (~1 s for magnetars, ~10 ms for pulsars, ~1 ms for ms pulsars) • Magnetar variability fast (hours/days, the sooner the better) response to To. O and high pile-up limit for bursts • PWN and dust halo good PSF and low background; spatially-resolved polarimetry NHXM - ESRIN, November 12 -13 2009 30

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Geminga (XMM; Caraveo et al. 2004) PSR J 1930+1852 (Chandra; Lu et al. 2006) RX J 1856 -3754 (XMM+Chandra; Burwitz et al. 2003) AXP 1 E 2259+586 (XMM; Rea et al. 2008) NHXM - ESRIN, November 12 -13 2009 34

Middle-aged, cooling INSs (right T for age) Origin in the O-B associations of the Gould Belt can explain local overabundance Why a blackbody spectrum ? Are these “bare” INSs ? What is the nature of the absorption features ? RX J 0720 Epic • Proton cyclotron ? No, if multiple lines are detected, as in RX J 1308 Haberl et al. (2004) • Atomic transitions in the strong B- field ? Ok, but requires an atmosphere… • Absorption edges in the emission from the condensed surface ? Possibly RBS 1774 (Zane et al 2008) Why the “optical excess” ? • Thermal emission from a cooler region ? No, RBBcold too large • A “thin” H atmosphere on top the condensed surface ? Possibly What is producing the long-term changes in RXHohle J 0720 ? Optical excess ~ 35 et al. (2009) Motch (2007) A precessing INS, the first seen in X-rays ? NHXM - ESRIN, November 12 -13 2009 35

Tapping the NS surface Comparison of observations with theoretical models provides Ts, Bs, chemical composition, R (and M) • Ts(t) → thermal evolution • R, M → NS matter equation of state • Chemical composition → NS formation and interaction with surrounding medium What is the correct model for thermal emission ? NHXM - ESRIN, November 12 -13 2009 36

A “Hard” Surface ? • NS surface composition: either H (accreted from ISM Turolla et al (2004) Fe H or from fallback of SN material) or heavy elements condensation (C, Ne, Fe) (Lai 2001) • Under typical conditions surface layers are in gaseous state: NS atmosphere Fe C He condensation • For sufficiently low T and large B (and depending on H (Medin & Lai Fe the surface can be in a condensed 2006, 2007) composition) state: “bare” NS NHXM - ESRIN, November 12 -13 2009 37