Oleg Kargaltsev

Pulsar Wind Nebulae in the Chandra Era

Pulsar winds shocked in the ambient medium produce spectacular nebulae observable from the radio through γ‐rays. The shape and the spectrum of a pulsar wind nebula (PWN) depend on the angular distribution, magnetization and energy spectrum of the wind streaming from the pulsar magnetosphere, as well as on the pulsar velocity and the properties of the ambient medium. The advent of Chandra, with its unprecedented angular resolution and high sensitivity, has allowed us not only to detect many new PWNe, but also study their spatial and spectral structure and dynamics, which has significantly advanced our understanding of these objects. Here we overview recent observational results on PWNe, with emphasis on Chandra observations.

The Variable Jet of the Vela Pulsar

Observations of the Vela pulsar-wind nebula (PWN) with the Chandra X-ray Observatory have revealed a complex, variable PWN structure, including inner and outer arcs, a jet in the direction of the pulsar’s proper motion, and a counter-jet in the opposite direction, embedded in diffuse nebular emission. The jet consists of a bright, 8 ′′ -long inner jet, between the pulsar and the outer arc, and a dim, curved outer jet that extends up to ∼ 100 ′′ in approximately the same direction. From the analysis of thirteen Chandra observations spread over ≈ 2.5 years we found that this outer jet shows particularly stron g variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3‐0.6) c and fading on time-scales of days to weeks. If the blobs are carried away by a flow along the jet, the observed variations suggest mildly relativistic flow velocities, ab out (0.3‐0.7) c. The spectrum of the outer jet fits a power-law model with a photon index = 1.3 ± 0.1. For a distance of 300 pc, the apparent average luminosity of the outer jet in the 1‐8 keV band is about 3 × 10 30 erg s -1 , compared to 6 × 10 32 from the whole PWN within 42 ′′ from the pulsar. The X-ray emission of the outer jet can be interpreted as synchrotron radiation of ultrarelativistic electrons/positrons. This interpreta tion allows one to estimate the magnetic field, ∼ 100 µG, maximum energy of X-ray emitting electrons, ∼ 2 × 10 14 eV, and energy injection rate, ∼ 8 × 10 33 erg s -1 , for the outer jet. In the summed PWN image, we see a faint, strongly bent, extension of the outer jet. This bending could be caused by combined action of a wind within the supernova remnant, with a velocity of a few ×10 km s -1 , along with the ram pressure due to the pulsar’s proper motio n. The more extreme bends closer to the pulsar, as well as the apparent side motions of the outer j et, can be associated with kink instabilities of a magnetically confined, pinched jet flow. Another feature fou nd in the summed image is a dim, ∼ 2 ′ -long outer counter-jet, which also shows a power-law spectrum with ≈ 1.2‐1.5. Southwest of the jet/counter-jet (i.e., approximately perpendicular to the direction of pulsar’s p roper motion), an extended region of diffuse emission is seen. Relativistic particles responsible for this radia tion are apparently supplied by the outer jet. Subject headings: ISM: jets and outflows — pulsars: individual (Vela) — stars: n eutron — stars: winds, outflows — supernova remnants: individual (Vela) — X-rays: s tars

Variability of the Vela Pulsar Wind Nebula Observed with Chandra

The observations of the pulsar wind nebula (PWN) around the Vela pulsar with the Advanced CCD Imaging Spectrometer aboard the Chandra X-Ray Observatory, taken on 2000 April 30 and November 30, reveal its complex morphology reminiscent of that of the Crab PWN. Comparison of the two observations shows changes up to 30% in the surface brightness of the PWN features. Some of the PWN elements show appreciable shifts, up to a few arcseconds (~1016 cm), and/or spectral changes. Such variations hold the potential to provide a new diagnostics for the study of PWNe.

Ultraviolet, X-Ray, and Optical Radiation from the Geminga Pulsar*

We observed the � -ray pulsar Geminga with the FUV-MAMA and NUV-MAMA detectors of the Space Telescope Imaging Spectrometer to measure Geminga’s spectrum and pulsations in the ultraviolet. The slope of the far-ultraviolet (FUV) spectrum is close to that of a Rayleigh-Jeans spectrum, suggesting that the FUV radiation is dominated by thermal emissionfromthe neutron star(NS) surface.ThemeasuredFUVflux,FFUV ¼ (3:7 � 0:2) ; 10 � 15 ergscm � 2 s � 1 inthe1155–17028band,correspondstoabrightnesstemperatureTRJ � (0:3–0:4)(d200/R13) 2 MK,dependingon the interstellarextinction(d ¼ 200d200 pcandR ¼ 13R13 kmarethedistanceandtheNSradius,respectively).Thesoft thermal component of Geminga’s X-ray spectrum measured with the XMM-Newton observatory corresponds to a temperature Ts ¼ 0:49 � 0:01 MK and radius Rs ¼ (12:9 � 1:0)d200 km. Contrary to other NSs detected in the UV-optical,forwhich theextrapolation ofthe X-ray thermalcomponentinto theopticalunderpredicts the observed flux of thermal radiation, the FUV spectrum of Geminga lies slightly below the extrapolation of the soft thermal component,which might beassociatedwith Geminga’s very low temperature.Surprisingly, thethermalFUVradiation is strongly pulsed, showing a narrow dip at a phase close to that of a broader minimum of the soft X-ray light curve. The strong pulsations might be attributed to partial occultations of the thermal UV radiation by regions of the magnetosphere filled with electron/positron plasma. In contrast to the FUV spectrum, the near-infrared (NIR) through near-ultraviolet (NUV) spectrum of Geminga is clearly nonthermal. It can be described by a power-law model, F� / � � � þ1 , with a photon index � ¼ 1:43 � 0:15, close to the slope � ¼ 1:56 � 0:24 of the hard X-ray (E > 2:5 keV) magnetospheric component. The extrapolation of the X-ray magnetospheric spectrum into the optical is marginally consistent with (or perhaps lies slightly above) the observed NIR-optical-NUV spectrum. The NUV pulsations, however, do not show a clear correlation with the hard X-ray pulsations. Subject headingg pulsars: individual (Geminga) — stars: neutron — ultraviolet: stars

Ultraviolet emission from the millisecond pulsar j0437-4715

We observed PSR J0437-4715 with the FUV-MAMA detector of the Space Telescope Imaging Spectrometer (STIS) to measure the pulsars spectrum and pulsations. For the first time, UV emission from a millisecond pulsar has been detected. The measured flux, (2.0 ? 0.2) ? 10-15 ergs s-1 cm-2 in the 1150-1700 ? range, corresponds to the luminosity LFUV = (4.7 ? 0.5) ? 1027 ergs s-1, for the distance of 140 pc and negligible interstellar extinction. The shape of the observed spectrum suggests thermal emission from the neutron star surface with a surprisingly high temperature of about 1 ? 105 K, above the upper limit on the surface temperature of the younger ordinary pulsar J0108-1431. For the few-Gigayear-old J0437-4715, such a temperature requires a heating mechanism to operate. The spectrum of J0437-4715 shows marginal evidence of an emission line at 1372 ?, which might be a gravitationally redshifted Zeeman component of the hydrogen Ly? line in a magnetic field of ~7 ? 108 G. No pulsations are detected, with a 3 ? upper limit of 50% on the pulsed fraction.

New constraints on the cooling of the Central Compact Object in Cas A

To examine the previously claimed fast cooling of the Central Compact Object (CCO) in the Cas A supernova remnant (SNR), we analyzed two Chandra observations of this CCO, taken in a setup minimizing instrumental spectral distortions. We fit the two CCO X-ray spectra from 2006 and 2012 with hydrogen and carbon neutron star atmosphere models. The temperature and flux changes in the 5.5 years between the two epochs depend on the adopted constraints on the fitting parameters and the uncertainties of the effective area calibrations. If we allow a change of the equivalent emitting region size, R_Em, the effective temperature remains essentially the same. If R_Em is held constant, the best-fit temperature change is negative, but its statistical significance ranges from 0.8sigma to 2.5sigma, depending on the model. If we assume that the optical depth of the ACIS filter contaminant in 2012 was +/-10% different from its default calibration value, the significance of the temperature drop becomes 0.8sigma to 3.1sigma, for the carbon atmospheres with constant R_Em. Thus, we do not see a statistically significant temperature drop in our data, but the involved uncertainties are too large to firmly exclude the previously reported fast cooling. Our analysis indicate a decrease of 4%-6% (1.9-2.9sigma significance) for the absorbed flux in the energy range 0.6-6keV between 2006 and 2012, most prominent in the 1.4-1.8 keV energy range. It could be caused by unaccounted changes of the detector response or contributions from unresolved SNR material along the line of sight to the CCO.

THE COMPACT CENTRAL OBJECT IN THE SUPERNOVA REMNANT G266.2 1.2

We observed the compact central object CXOU J085201.4-461753 in the supernova remnant G266.2-1.2 (RX J0852.0-4622) with the Chandra ACIS detector in timing mode. The spectrum of this object can be described by a blackbody model with the temperature kT = 404 ± 5 eV and radius of the emitting region R = 0.28 ± 0.01 km at a distance of 1 kpc. Power-law and thermal plasma models do not fit the source spectrum. The spectrum shows a marginally significant feature at 1.68 keV. A search for periodicity yields two candidate periods, about 301 and 33 ms, both significant at a 2.1 σ level; the corresponding pulsed fractions are 13% and 9%, respectively. We find no evidence for long-term variability of the source flux, nor do we find extended emission around the central object. We suggest that CXOU J085201.4-461753 is similar to CXOU J232327.9+584842, the central source of the supernova remnant Cas A. It could be either a neutron star with a low or regular magnetic field, slowly accreting from a fossil disk or, more likely, an isolated neutron star with a superstrong magnetic field. In either case, a conservative upper limit on surface temperature of a 10 km radius neutron star is about 90 eV, which suggests accelerated cooling for a reasonable age of a few thousand years.

X-Ray Emission from the Nearby PSR B1133+16 and Other Old Pulsars

We detected a nearby (d 360 pc), old (τ 5 Myr) pulsar B1133+16 with Chandra. The observed pulsars flux is (0.8 ± 0.2) × 10-14 ergs cm-2 s-1 in the 0.5-8 keV band. Because of the small number of counts detected, the spectrum can be described by various models. A power-law fit of the spectrum gives a photon index Γ ≈ 2.5 and an isotropic luminosity of 1.4 × 1029 ergs s-1 in the 0.5-8 keV band, which is about 1.6 × 10-3 of the spin-down power . The spectrum can also be fitted by a blackbody model with a temperature of ≈2.8 MK and a projected emitting area of ~500 m2, possibly a hot polar cap. The X-ray properties of PSR B1133+16 are similar to those of other old pulsars observed in X-rays, particularly the drifting pulsar B0943+10.

Pulsar-Wind Nebulae

In this review we describe recent observational and theoretical developments in our understanding of pulsar winds and pulsar-wind nebulae (PWNe). We put special emphasis on the results from observations of well-characterized PWNe of various types (e.g., torus-jet and bowshock-tail), the most recent MHD modeling efforts, and the status of the flaring Crab PWN puzzle.

OPTICAL-ULTRAVIOLET SPECTRUM AND PROPER MOTION OF THE MIDDLE-AGED PULSAR B1055−52 ∗

PSR B1055−52 is a middle-aged (τ = 535 kyr) radio, X-ray, and γ -ray pulsar showing X-ray thermal emission from the neutron star (NS) surface. A candidate optical counterpart to PSR B1055−52 was proposed by Mignani and coworkers based on Hubble Space Telescope (HST) observations performed in 1996, in one spectral band only. We report on HST observations of this field carried out in 2008, in four spectral bands. The astrometric and photometric analyses of these data confirm the identification of the proposed candidate as the pulsar’s optical counterpart. Similar to other middle-aged pulsars, its optical–UV spectrum can be described by the sum of a power-law (PLO) component (Fν ∝ ν −αO ), presumably emitted from the pulsar magnetosphere, and a Rayleigh–Jeans (RJ) component emitted from the NS surface. The spectral index of the PLO component, αO = 1.05 ± 0.34, is larger than for other pulsars with optical counterparts. The RJ component, with a brightness temperature TO = (0.66 ± 0.10) d 2 350 R −2 O,13 MK (whered350 and RO,13 are the distance to the pulsar in units of 350 pc and the radius of the emitting area in units of 13 km, respectively), shows a factor of 4 excess with respect to the extrapolation of the X-ray thermal component into the UV–optical. This hints that the RJ component is emitted from a larger, colder area, and suggests that the distance to the pulsar is smaller than previously thought. From absolute astrometry of the HST images, we measured the pulsar coordinates with a position accuracy of 0. 15. From comparison with previous observations, we measured the pulsar proper motion, μ = 42 ± 5 mas yr −1 , which corresponds to a transverse velocity Vt = (70 ± 8) d350 km s −1 .