Light Curves of Rapidly Rotating Neutron Stars

We consider the effect of rapid rotation on the light curves of neutron stars with hot polar caps. For $P \approx 3$ms spin periods, the pulse fractions can be as much as an order of magnitude larger than with simple slowly-rotating (Schwarzschild) estimates. Doppler boosting, in particular, leads to characteristic distortion and ``soft lags'' in the pulse profiles, which are easily measurable in light curves with moderate energy resolution. With $\sim 10^5$ photons it should also be possible to isolate the more subtle distortions of light travel time variations and frame dragging. Detailed analysis of high quality millisecond pulsar data from upcoming X-ray missions must include these effects

Photon Propagation Around Compact Objects and the Inferred Properties of Thermally Emitting Neutron Stars

Anomalous X-ray pulsars, compact non-pulsing X-ray sources in supernova remnants, and X-ray bursters are three distinct types of sources for which there are viable models that attribute their X-ray emission to thermal emission from the surface of a neutron star. Inferring the surface area of the emitting regions in such systems is crucial in assessing the viability of different models and in providing bounds on the radii of neutron stars. We show that the inferred areas of the emitting regions may be over- or under-estimated by a factor of <=2, because of the geometry of the system and general relativistic light deflection, combined with the effects of phase averaging. Such effects make the determination of neutron-star radii uncertain, especially when compared to the ~5% level required for constraining the equation of state of neutron-star matter. We also note that, for a given spectral shape, the inferred source luminosities and pulse fractions are anticorrelated because they depend on the same properties of the emitting regions, namely their sizes and orientations, i.e., brighter sources have on average weaker pulsation amplitudes than fainter sources. We argue that this property can be used as a diagnostic tool in distinguishing between different spectral models. As an example, we show that the high inferred pulse fraction and brightness of the pulsar RXS J1708-40 are inconsistent with isotropic thermal emission from a neutron-star surface. Finally, we discuss the implication of our results for surveys in the soft X-rays for young, cooling neutron stars in supernova remnants and show that the absence of detectable pulsations from the compact source at the center of Cas A (at a level of >=30%) is not a strong argument againts its identification with a spinning neutron star.Comment: 6 pages, 6 figures, to appear in the Astrophysical Journal; minor change

Evidence for a Mid-Atomic-Number Atmosphere in the Neutron Star 1E1207.4-5209

Recently Sanwal et al. (2002) reported the first clear detection of absorption features in an isolated neutron star, 1E1207.4-5209. Remarkably their spectral modeling demonstrates that the atmosphere cannot be Hydrogen. They speculated that the neutron star atmosphere is indicative of ionized Helium in an ultra-strong (~1.5x10^{14} G) magnetic field. We have applied our recently developed atomic model (Mori & Hailey 2002) for strongly-magnetized neutron star atmospheres to this problem. We find that this model, along with some simp le atomic physics arguments, severely constrains the possible composition of the atmosphere. In particular we find that the absorption features are naturally associated with He-like Oxygen or Neon in a magnetic field of ~10^{12} G, comparable to the magnetic field derived from the spin parameters of the neutron star. This interpretation is consistent with the relative line strengths and widths and is robust. Our model predicts possible substructure in the spectral features, which has now been reported by XMM-Newton (Mereghetti et al. 2002). However we show the Mereghetti et al. claim that the atmosphere is Iron or some comparable high-Z element at ~ 10^{12} G is easily ruled out by the Chandra and XMM-Newton data.Comment: 5 pages, AASTeX, Revised version. Accepted for publication in ApJ Letter

The X-ray Spectrum of the Vela Pulsar Resolved with Chandra

We report the results of the spectral analysis of two observations of the Vela pulsar with the Chandra X-ray observatory. The spectrum of the pulsar does not show statistically significant spectral lines in the observed 0.25-8.0 keV band. Similar to middle-aged pulsars with detected thermal emission, the spectrum consists of two distinct components. The softer component can be modeled as a magnetic hydrogen atmosphere spectrum - for the pulsar magnetic field $B=3\times 10^{12}$ G and neutron star mass $M=1.4 M_\odot$ and radius $R^\infty =13$ km, we obtain \tef^\infty =0.68\pm 0.03 MK, $L_{\rm bol}^\infty = (2.6\pm 0.2)\times 10^{32}$ erg s$^{-1}$, $d=210\pm 20$ pc (the effective temperature, bolometric luminosity, and radius are as measured by a distant observer). The effective temperature is lower than that predicted by standard neutron star cooling models. A standard blackbody fit gives $T^\infty =1.49\pm 0.04$ MK, $L_{\rm bol}^\infty=(1.5\pm 0.4)\times 10^{32} d_{250}^2$ erg s$^{-1}$ ($d_{250}$ is the distance in units of 250 pc); the blackbody temperature corresponds to a radius, $R^\infty =(2.1\pm 0.2) d_{250}$ km, much smaller than realistic neutron star radii. The harder component can be modeled as a power-law spectrum, with parameters depending on the model adopted for the soft component - $\gamma=1.5\pm 0.3$, $L_x=(1.5\pm 0.4)\times 10^{31} d_{250}^2$ erg s$^{-1}$ and $\gamma=2.7\pm 0.4$, $L_x=(4.2\pm 0.6)\times 10^{31} d_{250}^2$ erg s$^{-1}$ for the hydrogen atmosphere and blackbody soft component, respectively ($\gamma$ is the photon index, $L_x$ is the luminosity in the 0.2--8 keV band). The extrapolation of the power-law component of the former fit towards lower energies matches the optical flux at $\gamma\simeq 1.35$--1.45.Comment: Submitted to ApJ, three figures; color figure 1 can be found at http://www.xray.mpe.mpg.de/~zavlin/pub_list.htm

The Low Quiescent X-Ray Luminosity of the Neutron Star Transient XTE J2123-058

We report on the first X-ray observations of the neutron star soft X-ray transient (SXT) XTE J2123-058 in quiescence, made by Chandra and BeppoSAX, as well as contemporaneous optical observations. In 2002, the Chandra spectrum of XTE J2123-058 is consistent with a power-law model, or the combination of a blackbody plus a power-law, but it is not well-described by a pure blackbody. Using the interstellar column density, the power-law fit gives photon index of 3.1 (+0.7,-0.6) and indicates a 0.3-8 keV unabsorbed luminosity of 9(+4,-3)E31 (d/8.5 kpc)^2 ergs/s (90% confidence errors). Fits with models consisting of thermal plus power-law components indicate that the upper limit on the temperature of a 1.4 solar mass, 10 km radius neutron star with a hydrogen atmosphere is kT_eff < 66 eV, and the upper limit on the bolometric luminosity is L_infinity < 1.4E32 ergs/s, assuming d = 8.5 kpc. Of the neutron star SXTs that exhibit short (< 1 year) outbursts, including Aql X-1, 4U 1608-522, Cen X-4, and SAX J1810.8-2609, the lowest temperatures and luminosities are found for XTE J2123-058 and SAX J1810.8-2609. From the BeppoSAX observation of XTE J2123-058 in 2000, we obtained an upper limit on the 1-10 keV unabsorbed luminosity of 9E32 ergs/s. Although this upper limit allows that the X-ray luminosity may have decreased between 2000 and 2002, that possibility is not supported by our contemporaneous R-band observations, which indicate that the optical flux increased significantly. Motivated by the theory of deep crustal heating by Brown and co-workers, we characterize the outburst histories of the five SXTs. The low quiescent luminosity for XTE J2123-058 is consistent with the theory of deep crustal heating without requiring enhanced neutron star cooling if the outburst recurrence time is >~ 70 years.Comment: 8 pages, accepted by Ap

A XMM-Newton observation during the 2000 outburst of SAX J1808.4-3658

I present a XMM-Newton observation of the accretion driven millisecond X-ray pulsar SAX J1808.4-3658 during its 2000 outburst. The source was conclusively detected, albeit at a level of only ~2 x 10^{32} erg/s. The source spectrum could be fitted with a power-law model (with a photon index of ~2.2), a neutron star atmosphere model (with a temperature of ~0.2 keV), or with a combination of a thermal (either a black-body or an atmosphere model) and a power-law component. During a XMM-Newton observation taken approximately one year later, the source was in quiescence and its luminosity was a factor of ~4 lower. It is possible that the source spectrum during the 2000 outburst was softer than its quiescent 2001 spectrum, however, the statistics of the data do not allow to make a firm conclusion. The results obtained are discussed in the context of the 2000 outburst of SAX J1808.4-3658 and the quiescent properties of the source.Comment: Accepted for publication in ApJ, 15 January 200

Constraints on Thermal X-ray Radiation from SAX J1808.4-3658 and Implications for Neutron Star Neutrino Emission

Thermal X-ray radiation from neutron star soft X-ray transients in quiescence provides the strongest constraints on the cooling rates of neutron stars, and thus on the interior composition and properties of matter in the cores of neutron stars. We analyze new (2006) and archival (2001) XMM-Newton observations of the accreting millisecond pulsar SAX J1808.4-3658 in quiescence, which provide the most stringent constraints to date. The X-ray spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a power-law of photon index 1.83\pm0.17, without requiring the presence of a blackbody-like component from a neutron star atmosphere. Our 2006 observation shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of 68^{+15}_{-13}% that inferred from the 2001 observation. Simultaneous fitting of all available XMM data allows a constraint on the quiescent neutron star (0.01-10 keV) luminosity of L_{NS}<1.1*10^{31} erg/s. This limit excludes some current models of neutrino emission mediated by pion condensates, and provides further evidence for additional cooling processes, such as neutrino emission via direct Urca processes involving nucleons and/or hyperons, in the cores of massive neutron stars.Comment: 5 pages, 2 figures; slight revisions, accepted by Ap

Chandra X-ray Sources in the Collapsed-Core Globular Cluster M30 (NGC 7099)

We report the detection of six discrete, low-luminosity (Lx < 10^33 erg/s) X-ray sources, located within 12 arcsec of the center of the collapsed-core globular cluster M30 (NGC 7099), and a total of 13 sources within the half-mass radius, from a 50 ksec Chandra ACIS-S exposure. Three sources lie within the very small upper limit of 1.9 arcsec on the core radius. The brightest of the three core sources has a luminosity of Lx (0.5-6 keV) = 6x10^32 erg/s and a blackbody-like soft X-ray spectrum, which are both consistent with it being a quiescent low-mass X-ray binary (qLMXB). We have identified optical counterparts to four of the six central sources and a number of the outlying sources, using deep Hubble Space Telescope and ground-based imaging. While the two proposed counterparts that lie within the core may represent chance superpositions, the two identified central sources that lie outside of the core have X-ray and optical properties consistent with being CVs. Two additional sources outside of the core have possible active binary counterparts. We discuss the X-ray source population of M30 in light of its collapsed-core status.Comment: 18 pages, 13 figures (8 color), resubmitted to ApJ after incorporating referee comment

Chandra observations of the accretion-driven millisecond X-ray pulsars XTE J0929-314 and XTE J1751-305 in quiescence

(Abridge) We observed the accreting millisecond X-ray pulsars XTE J0929-314 and XTE J1751-305 in their quiescent states using Chandra. From XTE J0929-314 we detected 22 photons (0.3-8 keV) in 24.4 ksec, resulting in a count rate of 9 x 10^{-4} c/s. The small number of photons detected did not allow for a detailed spectral analysis, but we can demonstrate that the spectrum is harder than simple thermal emission which is what is usually presumed to arise from a cooling neutron star that has been heated during the outbursts. Assuming a power-law model for the spectrum, we obtain a power-law index of ~1.8 and an unabsorbed flux of 6 x 10^{-15} ergs/s/cm^2 (0.5-10 keV), resulting in a luminosity of 7 x 10^{31} (d/10 kpc)^2 ergs/s, with d in kpc. No thermal component could be detected; such a component contributed at most 30% to the 0.5-10 keV flux. Variability in the count rate of XTE J0929-314 was observed at the 95% confidence level. We did not conclusively detect XTE J1751-305 in our 43 ksec observation, with 0.5-10 keV flux upper limits between 0.2 and 2.7 x 10^{-14} ergs/s/cm^2 depending on assumed spectral shape, resulting in luminosity upper limits of 0.2 - 2 x 10^{32} (d/8 kpc)^2 ergs/s. We compare our results with those obtained for other neutron-star X-ray transients in their quiescent state. Using simple accretion disk physics in combination with our measured quiescent luminosity of XTE J0929-314 and the luminosity upper limits of XTE J1751-305, and the known spin frequency of the neutron stars, we could constrain the magnetic field of the neutron stars in XTE J0929-314 and XTE J1751-305 to be less than 3 x 10^9 (d/10 kpc) and 3 - 7 x 10^8 (d/8 kpc) Gauss (depending on assumed spectral shape of the quiescent spectrum), respectively.Comment: Accepted for publication in ApJ, 29 September 2004. Added spectral variability search for the data of XTE J0929-314 and added the non-detection with Chandra of XTE J1751-30

X-ray observations of the compact central object in supernova remnant G347.3-0.5

We present Chandra, XMM-Newton and RXTE observations of 1WGA J1713.4-3949, a compact source at the center of the galactic supernova remnant (SNR) G347.3-0.5. The X-ray spectrum of the source is well-fitted by the sum of a blackbody component with a temperature of about 0.4 keV plus a power law component with photon index about 4. We found no pulsations down to 4% in the 0.01-0.16 Hz range and down to 25% in the 0.01-128 Hz range. This source resembles other compact central objects (CCOs) in SNRs, and we suggest that 1WGA J1713.4-3949 is the associated neutron star for G347.3--0.5. We also measured the properties of the adjacent radio pulsar PSR J1713-3945 with a 392 ms period and show that it is not associated with 1WGA J1713.4-3949 nor, most probably, with SNR G347.3-0.5 as well.Comment: 8 pages, 2 figures, accepted for publication in ApJ Letter