kHz Quasi-Periodic Oscillations in the low-mass X-ray binary 4U 0614+09

We report on a comprehensive analysis of the kilohertz (above 300 Hz) quasi-periodic oscillations (kHz QPOs) detected from the neutron star low-mass X-ray binary 4U0614+09 with the Rossi X-ray Timing Explorer (RXTE). With a much larger data set than previously analyzed (all archival data from February 1996 up to October 2007), we first investigate the reality of the 1330 Hz QPO reported by van Straaten et al. (2000). This QPO would be of particular interest since it has the highest frequency reported for any source. A thorough analysis of the same observation fails to confirm the detection. On the other hand, over our extended data set, the highest QPO frequency we measure for the upper kHz QPO is at about 1224 Hz; a value which is fully consistent with the maximum values observed in similar systems. Second, we demonstrate that the frequency dependence of the quality factor and amplitude of the lower and upper kHz QPOs follow the systematic trends seen in similar systems (Barret et al., 2006). In particular, 4U0614+09 shows a drop of the quality factor of the lower kHz QPO above 700 Hz. If this is due to an approach to the innermost stable circular orbit, it implies a neutron star mass of about 1.9 solar masses. Finally, when analyzing the data over fixed durations, we have found a gap in the frequency distribution of the upper QPO, associated with a local minimum of its amplitude. A similar gap is not present in the distribution of the lower QPO frequencies, suggesting some cautions when interpreting frequency ratio distributions, based on the occurrence of the lower QPO only.Comment: 10 pages, 6 color figures, 2 tables, Accepted for publication in MNRA

Balmer-Like Series for Baryon Resonances

The pole positions of various baryon resonances have been found to reveal a well pronounced clustering, the so-called H"ohler cluster. In a previous work, the H"ohler clusters have been shown to be identical to Lorentz multiplets of the type (1/2+l', 1/2+l')*[(1/2,0)+(0,1/2)] with l' integer. Here we show that the cluster positions are well described by means of a Balmer-series like recursive mass formula.Comment: 5 pages LaTex, World Scientific style, two tables. A missing additive factor of +1 on the rhs of Eq. (2) has been inserted and thereby a misprint, not an error, correcte

Possible 38 day X-ray period of KS1731-260

We report the detection of a 38 day period in the X-ray flux of the transient burster KS1731-260. The narrow peak of periodicity was detected during ~TJD 10150--11050 when the source had a high and relatively stable X-ray flux. After \~TJD 11100 the source became strongly variable on a time scale of months that contaminates the search for the 38 day periodicity. The detected period can not be a binary period. The binary with Roche lobe overflow has in this case large radii of the secondary and of the accretion disk. Disk and secondary star illumination by X-ray flux from luminous neutron star would lead to high infrared brightness of the binary. That clearly contradicts the infrared data even for the brightest infrared sources within CHANDRA error box of KS1731-260. Remaining possibility is that observed periodicity is connected with the accretion disk precession, similar to that was observed for SS 433, Her X-1, Cyg X-1 etc.Comment: 4 pages, 5 figures. Submitted to A&A Letter

Simultaneous BeppoSAX and RXTE observations of the X-ray burst sources GX 3+1 and Ser X-1

We have obtained spectral and timing data on GX 3+1 and Ser X-1. Both sources were observed simultaneously with BeppoSAX and RXTE. The RXTE data is used to provide power spectra and colour-colour diagrams in order to constrain the state (and thus track $\dot M$) the sources are in. The BeppoSAX data provide the broad-band spectra. The spectra of both sources are reasonably well-fit using a model consisting of a disk-blackbody, a comptonized component and a Fe line, absorbed by interstellar absorption. The electron temperature (kT$_{\rm e}$) of the Comptonizing plasma is in both cases $\sim$2.5 keV. This implies that no strong high-energy tail from the Comptonized component is present in either of the sources. We discuss the similarities between these burst sources and the luminous X-ray sources located in globular clusters. We find that the spectral parameters of the comptonized component provide information about the mass-accretion rate, which agrees well with estimates from the timing and spectral variations.Comment: 8 pages, accepted by A&

Morphological analysis on the coherence of kHz QPOs

We take the recently published data of twin kHz quasi-period oscillations (QPOs) in neutron star (NS) lowmass X-ray binaries (LMXBs) as the samples, and investigate the morphology of the samples, which focuses on the quality factor, peak frequency of kHz QPOs, and try to infer their physical mechanism. We notice that: (1) The quality factors of upper kHz QPOs are low (2 ~ 20 in general) and increase with the kHz QPO peak frequencies for both Z and Atoll sources. (2) The distribution of quality factor versus frequency for the lower kHz QPOs are quite different between Z and Atoll sources. For most Z source samples, the quality factors of lower kHz QPOs are low (usually lower than 15) and rise steadily with the peak frequencies except for Sco X-1, which drop abruptly at the frequency of about 750 Hz. While for most Atoll sources, the quality factors of lower kHz QPOs are very high (from 2 to 200) and usually have a rising part, a maximum and an abrupt drop. (3) There are three Atoll sources (4U 1728-34, 4U 1636-53 and 4U 1608-52) of displaying very high quality factors for lower kHz QPOs. These three sources have been detected with the spin frequencies and sidebands, in which the source with higher spin frequency presents higher quality factor of lower kHz QPOs and lower difference between sideband frequency and lower kHz QPO frequency.Comment: 8 pages, 8 figures, publishe

RXTE Studies of X-ray Spectral Variations with Accretion Rate in 4U 1915-05

We present the results of detailed spectral studies of the ultra-compact low mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to exhibit a ~199-day modulation in its 2--12 keV flux. Observations were performed with the PCA and HEXTE instruments on RXTE at roughly one-month intervals to sample this long-term period and study accretion rate-related spectral changes. We obtain good fits with a model consisting of a blackbody and an exponentially cut-off power law. The spectral parameters are strongly correlated with both the broad-band (2--50 keV) luminosity and the position in the color-color diagram, with the source moving from a low hard state to a high soft state as the accretion rate increases. The blackbody component appears to drive the spectral evolution. Our results are consistent with a geometry in which the soft component arises from an optically thick boundary layer and the hard component from an extended Comptonizing corona. Comparing our results with those of a similar study of the brighter source 4U 1820-30 (Bloser et al. 2000), we find that the two ultra-compact LMXBs occupy similar spectral states even though the transitions occur at very different total luminosities.Comment: 27 pages LaTeX, 8 figures, accepted to the Astrophysical Journa

Ultrafast light-induced response of photoactive yellow protein chromophore analogues

The fluorescence decays of several analogues of the photoactive yellow protein (PYP) chromophore in aqueous solution have been measured by femtosecond fluorescence up-conversion and the corresponding time-resolved fluorescence spectra have been reconstructed. The native chromophore of PYP is a thioester derivative of p-coumaric acid in its trans deprotonated form. Fluorescence kinetics are reported for a thioester phenyl analogue and for two analogues where the thioester group has been changed to amide and carboxylate groups. The kinetics are compared to those we previously reported for the analogues bearing ketone and ester groups. The fluorescence decays of the full series are found to lie in the 1–10 ps range depending on the electron-acceptor character of the substituent, in good agreement with the excited-state relaxation kinetics extracted from transient absorption measurements. Steady-state photolysis is also examined and found to depend strongly on the nature of the substituent. While it has been shown that the ultrafast light-induced response of the chromophore in PYP is controlled by the properties of the protein nanospace, the present results demonstrate that, in solution, the relaxation dynamics and pathway of the chromophore is controlled by its electron donor–acceptor structure: structures of stronger electron donor–acceptor character lead to faster decays and less photoisomerisation