First X-ray observations of the young pulsar J1357-6429

The first short Chandra and XMM-Newton observations of the young and energetic pulsar J1357-6429 provided strong indications of a tail-like pulsar-wind nebula associated with this object, as well as pulsations of its X-ray flux with a pulsed fraction above 50% and a thermal component dominating at lower photon energies (below 2 keV). The elongated nebula is very compact in size and may be interpreted as evidence for a pulsar jet. The thermal radiation is most plausibly emitted from the entire neutron star surface of a 10 km radius and a 1.0+/-0.1 K temperature, covered with a magnetized hydrogen atmosphere. At higher energies the pulsar's emission is of a nonthermal (magnetospheric) origin, with a power-law spectrum of a photon index of 1.1-1.3. This makes the X-ray properties of PSR J1357-6429 very similar to those of the youngest pulsars J1119-6127 and Vela with a detected thermal radiation.Comment: Minor changes, including the reference to Esposito et al. (2007 A&A, 467, L45). Accepted by ApJ Letter

Variations in the spin period of the radio-quiet pulsar 1E 1207.4-5209

The X-ray source 1E 1207.4-5209 is a compact central object in the G296.5+10.0 supernova remnant. Its spin period of 424 ms, discovered with the Chandra X-ray Observatory, suggests that it is a neutron star. The X-ray spectrum of this radio-quiet pulsar shows at least two absorption lines, first spectral features discovered in radiation from an isolated neutron star. Here we report the results of timing analysis of Chandra and XMM-Newton observations of this source showing a non-monotonous behavior of its period. We discuss three hypotheses which may explain the observational result. The first one assumes that 1E 1207.$-5209 is a glitching pulsar, with frequency jumps of \Delta f > 5 \muHz occurring every 1-2 years. The second hypothesis explains the deviations from a steady spin-down as due to accretion, with accretion rate varying from \sim 10^{13} to >10^{16} g s^{-1}, from a disk possibly formed from ejecta produced in the supernova explosion. Finally, the period variations could be explained assuming that the pulsar is in a wide binary system with a long period, P_orb \sim 0.2-6 yr, and a low-mass companion, M_2 < 0.3 M_\odot.Comment: 20 pages, 5 figures, accepted for publications in ApJ. 2004 ApJ, in pres

XMM-Newton observations of four millisecond pulsars

I present an analysis of the XMM-Newton observations of four millisecond pulsars, J0437-4715, J2124-3358, J1024-0719, and J0034-0534. The new data provide strong evidence of thermal emission in the X-ray flux detected from the first three objects. This thermal component is best interpreted as radiation from pulsar polar caps covered with a nonmagnetic hydrogen atmosphere. A nonthermal power-law component, dominating at energies E>3 keV, can also be present in the detected X-ray emission. For PSR J0437-4715, the timing analysis reveals that the shape and pulsed fraction of the pulsar light curves are energy dependent. This, together with the results obtained from the phase-resolved spectroscopy, supports the two-component (thermal plus nonthermal) interpretation of the pulsar's X-ray radiation. Highly significant pulsations have been found in the X-ray flux of PSRs J2124-3358 and J1024-0719. For PSR J0034-0534, a possible X-ray counterpart of the radio pulsar has been suggested. The inferred properties of the detected thermal emission are compared with predictions of radio pulsar models.Comment: 33 pages, 13 figures (of them 4 are color); to be published in Ap

X-ray emission from the old pulsar B0950+08

We present the timing and spectral analyses of theXMM-newton data on the 17-Myr-old, nearby radio pulsar B0950+08. This observation revealed pulsations of the X-ray flux of the pulsar at its radio period. The pulse shape and pulsed fraction are apparently different at lower and higher energies of the observed 0.2-10 keV energy range, which suggests that the radiation cannot be explained by a single emission mechanism. The X-ray spectrum of the pulsar can be fitted with a power-law model with a photon index about 1.75 and an (isotropic) luminosity about 9.8e29 erg/s in the 0.2-10 keV. Better fits are obtained with two-component, power-law plus thermal, models with index of 1.30 and 9.7e29 erg/s for the power-law component that presumably originates from the pulsar's magnetosphere. The thermal component, dominating at E>0.7 keV, can be interpreted as radiation from heated polar caps on the neutron star surface covered with a hydrogen atmosphere. The inferred effective temperature, radius, and bolometric luminosity of the polar caps are about 1 MK, 250 m, and 3e29 erg/s. Optical through X-ray nonthermal spectrum of the pulsar can be described as a single power-law with index 1.3-1.4 for the two-component X-ray fit. The ratio of the nonthermal X-ray (1-10 keV) luminosity to the nonthermal optical (4000-9000 \AA) luminosity is within the range of 1e2-1e3 observed for younger pulsars, which suggests that the magnetospheric X-ray and optical emissions are powered by the same mechanism in all pulsars. An upper limit on the temperature of the bulk of the neutron star surface, inferred from the optical and X-ray data, is about 0.15 MK. We also analyze X-ray observations of several other old pulsars, B2224+65, J2043+2740, B0628-28, B1813-36, B1929+10, and B0823+26.Comment: To be published in ApJ. Nonthermal optical and X-ray luminosities of seven radio pulsars are updated and presented in a new Table. Figure 6 showing the ratios of the luminosities vs. spin-down energy is also update

XMM-Newton reveals a candidate period for the spin of the "Magnificent Seven" neutron star RX J1605.3+3249

The group of thermally emitting isolated neutron stars (INSs) known as the "Magnificent Seven" (M7) is unique among the various neutron star populations. Crustal heating by means of magnetic field decay and an evolutionary link with magnetars may explain why these objects rotate more slowly and have higher thermal luminosities and magnetic field intensities than standard pulsars of similar age. The third brightest INS, RX J1605.3+3249, is the only object amidst the seven still lacking a detected periodicity. We observed the source with the XMM-Newton Observatory for 60 ks aiming at unveiling the neutron star rotation rate and investigating its spectrum in detail. A periodic signal at P=3.387864(16) s, most likely the neutron star spin period, is detected at the 4-sigma confidence level. The coherent combination of the new data with a past XMM-Newton EPIC-pn observation of the source constrains the pulsar spin-down rate at the 2-sigma confidence level, implying a dipolar magnetic field of B~7.4e13 G. If confirmed, RX J1605.3+3249 would be the neutron star with the highest dipolar field amongst the M7. The spectrum of the source shows evidence of a cool blackbody component, as well as for the presence of two broad absorption features. Furthermore, high-resolution spectroscopy with the RGS cameras confirms the presence of a narrow absorption feature at energy 0.57 keV in the co-added spectrum of the source, also seen in other thermally emitting isolated neutron stars. Phase-resolved spectroscopy, as well as a dedicated observing campaign aimed at determining a timing solution, will give invaluable constraints on the neutron star geometry and will allow one to confirm the high value of spin down, which would place the source closer to a magnetar than any other M7 INS.Comment: 12 pages, 6 figures; accepted for publication in A&A (revised version after language editing; results unchanged

Timing analysis of the isolated neutron star RX J0720.4-3125

We present a combined analysis of XMM-Newton, Chandra and Rosat observations of the isolated neutron star RXJ0720.4-3125, spanning a total period of \sim 7 years. We develop a maximum likelihood periodogramme for our analysis based on the \Delta C-statistic and the maximum likelihood method, which are appropriate for the treatment of sparse event lists. Our results have been checked "a posteriori" by folding a further BeppoSAX dataset with the period predicted at the time of that observation: the phase is found to be consistent. The study of the spin history and the measure of the spin-down rate is of extreme importance in discriminating between the possible mechanisms suggested for the nature of the X-ray emission. The value of \dot P, here measured for the first time, is \approx 10^{-14} s/s. This value can not be explained in terms of torque from a fossil disk. When interpreted in terms of dipolar losses, it gives a magnetic field of B \approx 10^{13} G, making also implausible that the source is accreting from the underdense surroundings. On the other hand, we also find unlikely that the field decayed from a much larger value (B\approx 10^{15} G, as expected for a magnetar powered by dissipation of a superstrong field) since this scenario predicts a source age of \approx 10^4 yrs, too young to match the observed X-ray luminosity. The observed properties are more compatible with a scenario in which the source is \approx 10^6 yrs old, and its magnetic field has not changed substantially over the lifetime.Comment: 11 Pages, 6 Figures. Accepted for publication in MNRA

Identification of A Transient Neutron Star in Quiescence in the Globular Cluster NGC 5139

Using the Chandra/ACIS-I detector, we have identified an X-ray source (CXOU 132619.7-472910.8) in the globular cluster NGC 5139 with a thermal spectrum identical to that observed from transiently accreting neutron stars in quiescence. The absence of intensity variability on timescales as short as 4 seconds (< 25% rms variability) and as long as 5 years (<50% variability) supports the identification of this source as a neutron star, most likely maintained at a high effective temperature (approximately 1e6 K) by transient accretion from a binary companion. The ability to spectrally identify quiescent neutron stars in globular clusters (where the distance and interstellar column densities are known) opens up new opportunities for precision neutron star radius measurements.Comment: 13 pages, 4 figures, submitted to Ap

Quiescent Thermal Emission from the Neutron Star in Aql 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 hard power-law spectrum, is $R_\infty$=13.4{+5}{-4} (d/5 \kpc) km. Based on the historical outburst record of RXTE/ASM, 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 sigma; 0.0001-1 Hz, 0.2-8 keV). Comparison between the Chandra spectrum and three X-ray spectral observations made between Oct 1992 and Oct 1996 find all spectra consistent with a pure H atmosphere, but with temperatures ranging from 145--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.Comment: 15 pages, 1 figure, 2 tables; ApJ, accepte