Precise location of Sagittarius X ray sources with a rocket-borne rotating modulation collimator

Precise location of Sagittarius X ray sources with rocket-borne rotating modulation collimato

Relativistic Iron Line Emission from the Neutron Star Low-mass X-ray Binary 4U 1636-536

We present an analysis of XMM-Newton and RXTE data from three observations of the neutron star LMXB 4U 1636-536. The X-ray spectra show clear evidence of a broad, asymmetric iron emission line extending over the energy range 4-9 keV. The line profile is consistent with relativistically broadened Fe K-alpha emission from the inner accretion disk. The Fe K-alpha line in 4U 1636-536 is considerably broader than the asymmetric iron lines recently found in other neutron star LMXBs, which indicates a high disk inclination. We find evidence that the broad iron line feature is a combination of several K-alpha lines from iron in different ionization states.Comment: 7 pages, 2 figures, Published in the Astrophysical Journa

RXTE confirmation of the intermediate polar status of IGR J15094-6649

Aims. To establish the X-ray properties of the intermediate polar candidate IGR J15094-6649 and therefore confirm its inclusion into the class. Methods. 42 856 s of X-ray data from RXTE was analysed. Frequency analysis was used to constrain temporal variations and spectral analysis used to characterise the emission and absorption properties. Results. A spin period of 809.7+-0.6 s is present, revealed as a complex pulse profile whose modulation depth decreases with increasing X-ray energy. The spectrum is well fitted by either a 19+-4 keV Bremsstrahlung or Gamma=1.8+-0.1 power law, with an iron emission line feature and significant absorption in each case. Conclusions. IGR J15094-6649 is confirmed to be an intermediate polar.Comment: 3 pages, 5 figures. Submitted to A&

Neutron Star Masses and Radii as Inferred from kilo-Hertz QPOs

Kilo-Hertz (kHz) Quasi-periodic oscillations (QPOs) have been discovered in the X-ray fluxes of 8 low-mass X-ray binaries (LMXBs) with the Rossi X-ray Timing Explorer (RXTE). The characteristics of these QPOs are remarkably similar from one source to another. In particular, the highest observed QPO frequencies for 6 of the 8 sources fall in a very narrow range: 1,066 to 1,171 Hz. This is the more remarkable when one considers that these sources are thought to have very different luminosities and magnetic fields, and produce very different count rates in the RXTE detectors. Therefore it is highly unlikely that this near constancy of the highest observed frequencies is due to some unknown selection effect or instrumental bias. In this letter we propose that the highest observed QPO frequency can be taken as the orbital frequency of the marginally stable orbit. This leads to the conclusions that the neutron stars in these LMXBs are inside their marginally stable orbits and have masses in the vicinity of 2.0 solar masses. This mass is consistent with the hypothesis that these neutron stars were born with about 1.4 solar masses and have been accreting matter at a fraction of the Eddington limit for 100 million years.Comment: 7 pages, uses aas2pp4.sty, Accepted by ApJ

INTEGRAL High Energy Observations of 2S 0114+65

We report the first INTEGRAL timing and spectral analysis of the high mass X-ray binary source 2S 0114+65 at high energies (5-100 keV). The pulse period was found at 2.668 hr with a high pulsed fraction, ~80% in both the 20-40 keV and 40-80 keV energy bands. The spin-up trend over ~8 years was measured to be -8.9 * 10^{-7}. The hard X-ray spectrum obtained with JEM-X/ISGRI is well described by a high energy exponential cut-off power law model where the estimated luminosity is 1.8 * 10^{36} erg/s in the 5-100 keV energy band, for a source distance of 7.2 kpc. We tentatively identify a cyclotron resonance scattering feature at ~22 keV with one harmonic, implying a magnetic field of 2.5 * 10^{12} G.Comment: 4 pages, 5 figures, accepted to A&A Letter

Discovery of 16.6 and 25.5 s Pulsations from the Small Magellanic Cloud

We report the serendipitous detection of two previously unreported pulsars from the direction of the Small Magellanic Cloud, with periods of 16.6 and 25.5 seconds. The detections are based on archival PCA data from the Rossi X-ray Timing Explorer (RXTE). The observation leading to these detections occurred in September 2000 extending over 2.1 days with an exposure of 121 ks. A possible identification of the 16.6 s pulsar with an X-ray source RX J0051.8-7310 seen by both ROSAT and ASCA imaging X-ray satellites is presented.Comment: 9 pages with 3 figures. Submitted to ApJ Letter

Infrared Candidates for the Intense Galactic X-ray Source GX 17+2

We present new astrometric solutions and infrared Hubble Space Telescope observations of GX 17+2 (X1813-140), one of the brightest X-ray sources on the celestial sphere. Despite 30 years of intensive study, and the existence of a strong radio counterpart with a sub-arcsecond position, the object remains optically unidentified. The observed X-ray characteristics strongly suggest that it is a so-called "Z-source," the rare but important category that includes Sco X-1 and Cyg X-2. Use of the USNO-A2.0 catalog enables us to measure the position of optical and infrared objects near the radio source to sub-arcsecond precision within the International Celestial Reference Frame, for direct comparison with the radio position, which we also recompute using modern calibrators. With high confidence we eliminate the V~17.5 star NP Ser, often listed as the probable optical counterpart of the X-ray source, as a candidate. Our HST NICMOS observations show two faint objects within our 0.5" radius 90% confidence error circle. Even the brighter of the two, Star A, is far fainter than expected (H~19.8), given multiple estimates of the extinction in this field and our previous understanding of Z sources, but it becomes the best candidate for the counterpart of GX 17+2. The probability of a chance coincidence of an unrelated faint object on the radio position is high. However, if the true counterpart is not Star A, it is fainter still, and our conclusion that the optical counterpart is surprisingly underluminous is but strengthened.Comment: 15 pages including 3 figures and 3 tables. Accepted for publication in The Astrophysical Journa

Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients

We propose a hybrid model to fit the X-ray spectra of atoll-type X-ray transients in the soft and hard states. This model uniquely produces luminosity tracks that are proportional to T^4 for both the accretion disk and boundary layer. The model also indicates low Comptonization levels for the soft state, gaining a similarity to black holes in the relationship between Comptonization level and the strength of integrated rms variability in the power density spectrum. The boundary layer appears small, with a surface area that is roughly constant across soft and hard states. This result may suggestion that the NS radius is smaller than its inner-most stable circular orbit.Comment: 15 pages, 15 figures, accepted for publication in the Ap

Discovery of an Ultra-fast X-ray Pulsar in the Supernova Remnant N157B

We present the serendipitous discovery of 16 ms pulsed X-ray emission from the Crab-like supernova remnant N157B in the Large Magellanic Cloud. This is the fastest spinning pulsar associated with a supernova remnant (SNR). Observations with the Rossi X-ray Timing Explorer (RXTE), centered on the field containing SN1987A, reveal an X-ray pulsar with a narrow pulse profile. Archival ASCA X-ray data confirm this detection and locate the pulsar within 1' of the supernova remnant N157B, 14' from SN1987A. The pulsar manifests evidence for glitch(es) between the RXTE and ASCA observations which span 3.5 years; the mean linear spin-down rate is Pdot = 5.126 X 10E-14 s/s. The background subtracted pulsed emission is similar to other Crab-like pulsars with a power law of photon index of ~ 1.6. The characteristic spin-down age (~ 5000 years) is consistent with the previous age estimate of the SNR. The inferred B-field for a rotationally powered pulsar is ~ 1 X 10E12 Gauss. Our result confirms the Crab-like nature of N157B; the pulsar is likely associated with a compact X-ray source revealed by ROSAT HRI observations.Comment: 9 pages with 3 eps figure, LaTex (aas2pp2, psfig). Submitted to the Astrophysical Journal Letter

Scattering and Iron Fluorescence Revealed During Absorption Dips in Circinus X-1

We show that dramatic spectral evolution associated with dips occurring near phase zero in RXTE observations of Cir X-1 is well-fit by variable and at times heavy absorption (N_H > 10^24 cm^-2) of a bright component, plus an underlying faint component which is not attenuated by the variable column and whose flux is ~10% of that of the unabsorbed bright component. A prominent Fe emission line at ~6.5 keV is evident during the dips. The absolute line flux outside the dips is similar to that during the dips, indicating that the line is associated with the faint component. These results are consistent with a model in which the bright component is radiation received directly from a compact source while the faint component may be attributed to scattered radiation. Our results are also generally consistent with those of Brandt et al., who found that a partial- covering model could explain ASCA spectra of a low-to-high transition in Cir X-1. The relative brightness of the two components in our model requires a column density of ~2*10^23 cm^-2 if the faint component is due to Thomson scattering in material that mostly surrounds the source. We find that illumination of such a scattering cloud by the observed direct component would produce an Fe K-alpha fluorescence flux that is in rough agreement with the flux of the observed emission line. We also conclude that if the scattering medium is not highly ionized, our line of sight to the compact source does not pass through it. Finally, we discuss simple pictures of the absorbers responsible for the dips themselves.Comment: Accepted for publication in The Astrophysical Journal (23 pages, including 11 figures