Discovery of a Second Kilohertz QPO in the X-ray Binary 4U 1735-44

In recent observations with the Rossi X-Ray Timing Explorer we have detected two simultaneous quasi-periodic oscillation (QPO) peaks in the low mass X-ray binary and atoll source 4U 1735-44. The lower and higher frequency QPOs have frequencies varying between 632 and 729 Hz, and 982 and 1026 Hz, respectively. The fractional rms amplitudes are 3.7 to 8.1% and 5.0 to 5.8%. The frequency separation between the two QPOs changes from 341+-7 Hz to 296+-12 Hz. The inferred mass accretion rate during our observations is relatively low compared to that during the previous observations, where only a single QPO was present. There is weak evidence that the frequency of the QPOs correlates with the mass accretion rate, as observed in other binaries. Five X-ray bursts were recorded with no detectable oscillations with upper limits for the rms fraction of 4% to 13%.Comment: accepted by ApJ Letters, for more information see the QPO page at http://www.astro.uva.nl/ecford/qpos.htm

On the masses of neutron stars

We analyze the currently available observations of X-ray binaries in a consistent way, to re-determine the masses of the neutron stars in these systems. In particular, our attention is focussed on a realistic and consistent assessment of observational uncertainties and sources of systematic error. Confidence limits for these new mass estimates are generally less optimistic than previously assumed. The available observations, including data on six radio pulsars, do not firmly constrain the equation of state of neutron star matter. In particular, a firm upper mass limit cannot yet be established. An improvement of the accuracy of optical data holds the key to further progress.Comment: 5 pages of compressed, uuencoded postscript (text+figure). Accepted for publication in A&A. In case of problems, contact [email protected]

The eclipsing Cataclysmic Variable GS Pavonis: Evidence for disk radius changes

We have obtained differential time series photometry of the cataclysmic variable GS Pavonis over a timespan of 2 years. These show that this system is deeply eclipsing (~2-3.5 mag) with an orbital period of 3.72 hr. The eclipse depth and out-of-eclipse light levels are correlated. From this correlation we deduce that the disk radius is changing and that the eclipses in the low state are total. The derived distance to GS Pav is 790+/-90 pc, with a height above the galactic plane of 420+/-60 pc. We classify GS Pav as a novalike system.Comment: 4 pages, including 2 figures, accepted for publication in A&A Letter

Simultaneous Measurements of X-Ray Luminosity and Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries

We measure simultaneously the properties of the energy spectra and the frequencies of the kilohertz quasi-periodic oscillations (QPOs) in fifteen low mass X-ray binaries covering a wide range of X-ray luminosities. In each source the QPO frequencies cover the same range of approximately 300 Hz to 1300 Hz, though the sources differ by two orders of magnitude in their X-ray luminosities (as measured from the unabsorbed 2-50 keV flux). So the X-ray luminosity does not uniquely determine the QPO frequency. This is difficult to understand since the evidence from individual sources indicates that the frequency and luminosity are very well correlated at least over short timescales. Perhaps beaming effects or bolometric corrections change the observed luminosities, or perhaps part of the energy in mass accretion is used to power outflows reducing the energy emitted in X-rays. It is also possible that the parameters of a QPO model are tuned in such a way that the same range of frequencies appears in all sources. Different modes of accretion may be involved for example (disk and radial) or multiple parameters may conspire to yield the same frequencies.Comment: 14 pages, 2 figures (1 in color), accepted by ApJ, see the 'QPO page': http://www.astro.uva.nl/~ecford/qpos.htm

Non-detection of kHz QPOs in GX 9+1 and GX 9+9

In numerous low-mass X-ray binaries kHz quasi-periodic oscillations (kHz QPOs) are detected. We observed the atoll sources GX 9+1 and GX 9+9 with the RXTE satellite. Both sources were on the (upper) banana branch during our observations. No kHz QPOs were detected, with upper limits of 1.3-1.8 %. It is known from other atoll sources (e.g. 4U 1636-53 and 4U 1820-30) that when they are in the upper banana branch the kHz QPOs are not detected. Thus, it remains possible that when GX 9+1 and GX 9+9 are observed longer on the lower banana, or even in the island state, kHz QPOs are detected in these sources.Comment: 2 pages, 1 figure. To appear in the proceedings of IAU Symposium 188 "The Hot Universe

Timing Properties of Magnetars

We study the pulse morphologies and pulse amplitudes of thermally emitting neutron stars with ultrastrong magnetic fields. The beaming of the radiation emerging from a magnetar was recently shown to be predominantly non-radial, with a small pencil and a broad fan component. We show that the combination of this radiation pattern with the effects of strong lensing in the gravitational field of the neutron star yields pulse profiles that show a qualitatively different behavior compared to that of the radially-peaked beaming patterns explored previously. Specifically, we find that: (i) the pulse profiles of magnetars with a single hot emission region on their surface exhibit 1-2 peaks, whereas those with an antipodal emission geometry have 1-4 peaks, depending on the neutron star compactness, the observer's viewing angle, and the size of the hot regions; (ii) the energy dependence of the beaming pattern may give rise to weakly or strongly energy-dependent pulse profiles and may introduce phase lags between different energy bands; (iii) the non-radial beaming pattern can give rise to high pulsed fractions even for very relativistic neutron stars; (iv) the pulsed fraction may not vary monotonically with neutron star compactness; (v) the pulsed fraction does not decrease monotonically with the size of the emitting region; (vi) the pulsed fraction from a neutron star with a single hot pole has, in general, a very weak energy dependence, in contrast to the case of an antipodal geometry. Comparison of these results to the observed properties of anomalous X-ray pulsars strongly suggests that they are neutron stars with a single hot region of ultrastrong magnetic field.Comment: 22 pages, 13 color figures, ApJ in pres

The mass of X-Ray Nova Scorpii 1994 (=GRO J1655--40)]

We have obtained high and intermediate resolution optical spectra of the black-hole candidate Nova Sco 1994 in May/June 1998, when the source was in complete (X-ray) quiescence. We measure the radial velocity curve of the secondary star and obtain a semi-amplitude of K_2=215.5+/-2.4 km/s, which is 6 per cent lower than the only previously determined value. This new value for K_2 thus reduces the binary mass function to f(M) = 2.73+/-0.09 Mo. Using only the high resolution spectra we constrain the rotational broadening of the secondary star, vsini, to lie in the range 82.9-94.9 km/s (95 per cent confidence) and thus constrain the binary mass ratio to lie in the range 0.337--0.436 (95 per cent confidence). We can also combine our results with published limits for the binary inclination to constrain the mass of the compact object and secondary star to the ranges 5.5 -- 7.9 and 1.7 -- 3.3 Mo respectively (95 per cent confidence). Finally, we report on the detection of the Lithium resonance line at 6707.8 A, with an equivalent width of 55+/-8 mA.Comment: 5 pages, 5 figures, accepted by MNRA

Pulse Profile Change Possibly Associated with a Glitch in an Anomalous X-Ray Pulsar 4U 0142+61

We report a glitch-like pulse frequency deviation from the simple spin-down law in an anomalous X-ray pulsar (AXP) 4U 0142+61 detected by ASCA observations. We also found a significant pulse profile change after the putative glitch. The glitch parameters resemble those found in another AXP 1RXS J170849.0$-$400910, in the Vela pulsar, and in other radio pulsars. This suggests that the radio pulsars and AXPs have the same internal structure and glitch mechanism. It must be noted, however, that the pulse frequency anomaly can also be explained by a gradual change of the spin-down rate ($\dot{P}$) without invoking a glitch.Comment: 14 pages, 4 figures, accepted by Ap

The Optical Counterpart of NGC 1313 X-1

We identify the optical counterpart of the ultraluminous X-ray source (ULX) NGC 1313 X-1 and discuss constraints on its physical nature from multiband optical spectra. There is a single object on Hubble Space Telescope (HST) images within the aspect-corrected Chandra X-ray error circle; a fainter, possibly extended, feature lies near the edge of the error circle. The brighter object showed prominent variation in the F555W band, but was constant in the F814W band. The spectrum was consistent with a single power-law on 2003 Nov 17, but deviated from this on 2004 Jul 17, suggestive of more than one emission component. Based on the location, magnitudes, spectral shape, and variability of the bright object, it is likely the ULX counterpart. The red wing of the spectrum around F814W may be due to emission from the companion star, and the blue wing is likely from disk emission. The stellar population around X-1 has an age older than 30 Myr, without very blue stars or young clusters. This places a constraint on the companion mass of the ULX as no more than 10 solar masses.Comment: 6 pages, 6 figures, accepted for publication in the Astrophysical Journa

Difference Frequency of Kilohertz QPOs Not Equal to Half the Burst Oscillation Frequency in 4U 1636-53

We have analyzed data obtained during two observations with the Rossi X-ray Timing Explorer on January 5 and 8, 1997, of the low-mass X-ray binary (LMXB) and atoll source 4U 1636-53. We measure the frequency separation of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source to be 253.7 +/- 4.7 and 246.4 +/- 5.4 Hz, respectively. These values are inconsistent with being equal to 0.5 times the frequency of the 581-Hz oscillations that have been detected previously in 4U 1636-53 during type I bursts. The weighted average discrepancy is 39.5 +/- 3.5 Hz. This result shows that a simple beat-frequency interpretation of the kHz QPOs, in which the frequency of the oscillations detected during type I bursts equals the separation between the two kHz QPOs (or twice that value), is incorrect.Comment: Accepted for publication in The Astrophysical Journal Letters. 4 pages plus 1 postscript figur