George G. Pavlov

The Thermal X-Ray Spectra of Centaurus X-4, Aquila X-1 , and 4U 1608-522 in Quiescence

We reanalyze the available X-ray spectral data of the type I bursting neutron star transients Aql X-1, Cen X-4, and 4U 1608-522 using realistic hydrogen atmosphere models. Previous spectral fits assumed a blackbody spectrum; because the free-free-dominated photospheric opacity decreases with increasing frequency, blackbody spectral fits overestimate the effective temperature and underestimate, by as much as 2 orders of magnitude, the emitting area. Hydrogen atmosphere spectral models, when fit to the available observational data, imply systematically larger emission area radii, consistent with the canonical 10 km radius of a neutron star. This suggests that a substantial fraction of the quiescent luminosity is thermal emission from the surface of the neutron star. The magnitude of the equivalent hydrogen column density toward these systems, however, presents a considerable systematic uncertainty, which can be eliminated only by high signal-to-noise X-ray spectral measurements (e.g., with AXAF or XMM) that would permit simultaneous determination of the equivalent hydrogen column density, emission area, and thermal temperature.

Discovery of absorption features in the X-ray spectrum of an isolated neutron star

We observed 1E 1207.4-5209, a neutron star in the center of the supernova remnant PKS 1209-51/52, with the ACIS detector aboard the Chandra X-Ray Observatory and detected two absorption features in the source spectrum. The features are centered near 0.7 and 1.4 keV; their equivalent widths are about 0.1 keV. We discuss various possible interpretations of the absorption features and exclude some of them. A likely interpretation is that the features are associated with atomic transitions of once-ionized helium in the neutron star atmosphere with a strong magnetic field. The first clear detection of absorption features in the spectrum of an isolated neutron star provides an opportunity to measure the mass-to-radius ratio and to constrain the equation of state of the superdense matter.

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

Crustal emission and the quiescent spectrum of the neutron star in ks 1731-260

The type-I X-ray bursting low mass X-ray binary KS 1731-260 was recently detected for the first time in quiescence by Wijnands et al., following a τoutburst≈13 yr outburst which ended in Feb 2001. We show that the emission area radius for a H atmosphere spectrum (possibly with a hard power-law component that dominates the emission above 3.5 keV) is consistent with that observed from other quiescent neutron star transients, R∞=23 +30 -15(d/8 kpc) km, and examine possible IR counterparts for KS 1731-260. Unlike all other known transient neutron stars (NS), the duration of this recent (and the only observed) outburst is as long as the thermal diffusion time of the crust. The large amount of heat deposited by reactions in the crust will have heated the crust to temperatures much higher than the equilibrium core temperature. As a result, the thermal luminosity currently observed from the neutron star is dominated not by the core, but by the crust. This scenario implies that the mean outburst recurrence timescale found by Wijnands et al. (∼ 200 yr) is a lower limit. Moreover, because the thermal emission is dominated by the hot crust, the level and the time evolution of quiescent luminosity is determined mostly by the amount of heat deposited in the crust during the most recent outburst (for which reasonable constraints on the mass accretion rate exist), and is only weakly sensitive to t he core temperature. Using estimates of the outburst mass accretion rate, our calculations of the quiescent flux i mmediately following the end of the outburst agree with the observed quiescent flux to within a factor of a few. In this paper, we present simulations of the evolution of the quiescent lightcurve for different scenarios of the c rust microphysics, and demonstrate that monitoring observations (with currently flying instruments) spanning fr om 1‐30 yr can measure the crust cooling timescale and the total amount of heat stored in the crust. These quantitie s have not been directly measured for any neutron star. This makes KS 1731-260 a unique laboratory for studying the thermal properties of the crust by monitoring the luminosity over the next few years to decades. Subject headings: stars: atmospheres — stars: individual (KS 1731-260) — stars: neutron — x-rays: binaries

Variable thermal emission from aquila X-1 in quiescence

We obtained four Chandra/ACIS-S observations beginning 2 weeks after the end of the 2000 November outburst of the neutron star (NS) transient Aql X-1. Over the 5 month span in quiescence, the X-ray spectra are consistent with thermal emission from a NS with a pure hydrogen photosphere and R∞ = 15.9 (d/5 kpc) km at the optically implied X-ray column density. We also detect a hard power-law tail during two of the four observations. The intensity of Aql X-1 first decreased by 50% ± 4% over 3 months, then increased by 35% ± 5% in 1 month, and then remained constant (<6% change) over the last month. These variations in the first two observations cannot be explained by a change in either the power-law spectral component or the X-ray column density. Presuming a pure hydrogen atmosphere and that R∞ is not variable, the long-term changes can only be explained by variations in the NS effective temperature, from kTeff,∞ = 130 eV, down to 113 eV, and finally increasing to 118 eV for the final two observations. During one of these observations, we observe two phenomena that were previously suggested as indicators of quiescent accretion onto the NS: short-timescale (<104 s) variability (at 32% rms) and a possible absorption feature near 0.5 keV. The possible absorption feature can potentially be explained as being due to a time-variable response in the ACIS detector. Even so, such a feature has not been detected previously from a NS and, if confirmed and identified, can be exploited for simultaneous measurements of the photospheric redshift and NS radius.

The Quiescent X-Ray Spectrum of the Neutron Star in Centaurus X-4 Observed with Chandra/ACIS-S

We report on spectral and intensity variability analysis from a Chandra/ACIS-S observation of the transient, type I X-ray bursting low-mass X-ray binary Cen X-4. The quiescent X-ray spectrum during this observation is statistically identical to one observed previously with BeppoSAX and close, but not identical, to one observed previously with ASCA. The X-ray spectrum is best described as a pure hydrogen atmosphere thermal spectrum plus a power-law component that dominates the spectrum above 2 keV. The best-fit radius of the neutron star is r = 12.9 ± 2.6 (d/1.2 kpc) km if the interstellar absorption is fixed at the value implied by the optical reddening. Allowing the interstellar absorption to be a free parameter yields r = 19 ^(+45)_(-10) (d/1.2 kpc) km (90% confidence). The thermal spectrum from the neutron star surface is inconsistent with a solar metallicity. We find a 3 σ upper limit of rms variability ≤18% (0.2-2.0 keV; 0.0001-1 Hz) during the observation. On the other hand, the 0.5-10.0 keV luminosity decreased by 40 ± 8% in the 4.9 yr between the ASCA and Chandra observations. This variability can be attributed to the power-law component. Moreover, we limit the variation in thermal temperature to ≾10% over these 4.9 yr. The stability of the thermal temperature and emission-area radius supports the interpretation that the quiescent thermal emission is caused by the hot neutron star core.

Quiescent Thermal Emission from the Neutron Star in Aquila X-1

We report on the quiescent spectrum measured with Chandra ACIS-S of the transient, type I, X-ray-bursting neutron star Aql X-1, immediately following an accretion outburst. The neutron star radius, assuming a pure hydrogen atmosphere and a hard power-law spectrum, is R∞ = 13.4(d/5 kpc) km. Based on the historical outburst record of the Rossi X-Ray Timing Explorer All-Sky Monitor, the quiescent luminosity is consistent with that predicted by Brown, Bildsten, and Rutledge from deep crustal heating, lending support to this theory for providing a minimum quiescent luminosity of transient neutron stars. While not required by the data, the hard power-law component can account for 18% ± 8% of the 0.5-10 keV thermal flux. Short-timescale intensity variability during this observation is less than 15% rms (3 σ; 0.0001-1 Hz, 0.2-8 keV). Comparison between the Chandra spectrum and three X-ray spectral observations made between 1992 October and 1996 October find all spectra consistent with a pure H atmosphere, but with temperatures ranging from 145 to 168 eV, spanning a factor of 1.87 ± 0.21 in observed flux. The source of variability in the quiescent luminosity on long timescales (greater than years) remains a puzzle. If from accretion, then it remains to be explained why the quiescent accretion rate provides a luminosity so nearly equal to that from deep crustal heating.

Discovery of 424 Millisecond Pulsations from the Radio-quiet Neutron Star in the Supernova Remnant PKS 1209–51/52

The central source of the supernova remnant PKS 1209-52 was observed with the Advanced CCD Imaging Spectrometer aboard Chandra X-ray observatory on 2000 January 6-7. The use of the Continuos Clocking mode allowed us to perform the timing analysis of the data with time resolution of 2.85 ms and to find a period P=0.42412927+/-2.3e-7 s. The detection of this short period proves that the source is a neutron star. It may be either an active pulsar with unfavorably directed radio beam or a truly radio-silent neutron star whose X-ray pulsations are caused by a nonuniform distribution of surface temperature. To infer the actual properties of this neutron star, the period derivative should be measured.The central source of the supernova remnant PKS 1209-51/52 was observed with the Advanced CCD Imaging Spectrometer aboard Chandra X-Ray Observatory on 2000 January 6-7. The use of the continuous clocking mode allowed us to perform the timing analysis of the data with a time resolution of 2.85 ms and to find a period P = 0.42412924 s ± 0.23 μs. The detection of this short period proves that the source is a neutron star. It may be either an active pulsar with an unfavorably directed radio beam or a truly radio-silent neutron star whose X-ray pulsations are caused by a nonuniform distribution of surface temperature. To infer the actual properties of this neutron star, the period derivative should be measured.

The Compact Central Object in Cassiopeia A: A Neutron Star with Hot Polar Caps or a Black Hole?

The central pointlike X-ray source of the Cassiopeia A supernova remnant was discovered in the Chandra first light observation and found later in the archival ROSAT and Einstein images. The analysis of these data does not show statistically significant variability of the source. Because of the small number of photons detected, different spectral models can fit the observed spectrum. The power-law fit yields the photon index gamma=2.6-4.1, and luminosity L(0.1-5.0 keV&parr0;=&parl0;2-60&parr0;x1034 ergs s-1 for d=3.4 kpc. The power-law index is higher, and the luminosity lower, than those observed from very young pulsars. One can fit the spectrum equally well with a blackbody model with T=6-8 MK, R=0.2-0.5 km, and Lbol=&parl0;1.4-1.9&parr0;x1033 ergs s-1. The inferred radii are too small, and the temperatures too high, for the radiation to be interpreted as emitted from the whole surface of a uniformly heated neutron star. Fits with the neutron star atmosphere models increase the radius and reduce the temperature, but these parameters are still substantially different from those expected for a young neutron star. One cannot exclude, however, the possibility that the observed emission originates from hot spots on a cooler neutron star surface. An upper limit on the (gravitationally redshifted) surface temperature is Tinfinitys<1.9-2.3 MK, depending on the chemical composition of the surface and the stars radius. Among several possible interpretations, we favor a model of a strongly magnetized neutron star with magnetically confined hydrogen or helium polar caps (Tinfinitypc approximately 2.8 MK, Rpc approximately 1 km) on a cooler iron surface (Tinfinitys approximately 1.7 MK). Such temperatures are consistent with the standard models of neutron star cooling. Alternatively, the observed radiation may be interpreted as emitted by a compact object (more likely, a black hole) accreting from a residual disk or from a late-type dwarf in a close binary.