X-Ray Eclipse Time Delays in 4U2129+47

4U 2129+47 was discovered in the early 80's and classified as an accretion disk corona source due to its broad and partial X-ray eclipses. The 5.24 hr binary orbital period was inferred from the X-ray and optical light curve modulation, implying a late K or M spectral type companion star. The source entered a low state in 1983, during which the optical modulation disappeared and an F8 IV star was revealed, suggesting that 4U 2129+47 might be part of a triple system. The nature of 4U 2129+47 has since been investigated, but no definitive conclusion has been reached. Here, we present timing and spectral analyses of two XMM-Newton observations of this source, carried out in May and June, 2005. We find evidence for a delay between two mid-eclipse epochs measured ~22 days apart, and we show that this delay can be naturally explained as being due to the orbital motion of the binary 4U 2129+47 around the center of mass of a triple system. This result thus provides further support in favor of the triple nature of 4U 2129+47.Comment: 6 pages, 5 figures, accepted by A&

The Quiescent Optical and Infrared Counterpart to EXO 0748-676 = UY Vol

We present optical and infrared photometry of the low-mass X-ray binary EXO 0748-676 in quiescence for the first time in 24 years since it became X-ray active in 1985. We find the counterpart at average magnitudes of R=22.4 and J=21.3. We monitored the source approximately nightly through 2008 November to 2009 January. During this time there was considerable night-to-night optical variability but no long term trends were apparent. The night-to-night variability reveals a periodicity of P=0.159331+/-0.000012d, consistent with the X-ray orbital period to within 0.01%. This indicates that the quiescent optical modulation is indeed orbital in nature rather than a superhump. Interestingly, the modulation remains single-peaked with a deep minimum coincident with the times of X-ray eclipse, and there is no indication of a double-peaked ellipsoidal modulation. This indicates that even in `quiescence' emission from the accretion disk and/or X-ray heated inner face of the companion star dominate the optical emission, and implies that obtaining an accurate dynamical mass estimate in quiescence will be challenging.Comment: Accepted for publication by the Astrophysical Journal Letter

Quiescent X-ray variability from the neutron star transient Aql X-1

A number of studies have revealed variability from neutron star low-mass X-ray binaries during quiescence. Such variability is not well characterised, or understood, but may be a common property that has been missed due to lack of multiple observations. One such source where variability has been observed is Aql X-1. Here, we analyse 14 Chandra and XMM-Newton observations of Aql X-1 in quiescence, covering a period of approximately 2 years. There is clear variability between the epochs, with the most striking feature being a flare-like increase in the flux by a factor of 5. Spectral fitting is inconclusive as to whether the power-law and/or thermal component is variable. We suggest that the variability and flare-like behaviour during quiescence is due to accretion at low rates which might reach the neutron star surface.Comment: 8 pages, 5 figures, accepted for publication in MNRA

The faint neutron star soft X-ray transient SAX J1810.8-2609 in quiescence

We present the analysis of a 35 ksec long Chandra observation of the neutron star soft X-ray transient (SXT) SAX J1810.8-2609. We detect three sources in the field of view. The position of one of them is consistent with the location of the ROSAT error circle of SAX J1810.8-2609. The accurate Chandra position of that source coincides with the position of the proposed optical counterpart, strengthening the identification as the counterpart. We detected the neutron star SXT system in quiescence at an unabsorbed luminosity of ~1x10^32 erg s^-1 (assuming a distance of 4.9 kpc). This luminosity is at the low-end of quiescent luminosities found in other neutron star SXTs. This renders support to the existence of a group of faint soft X-ray transients of which the accreting millisecond X-ray pulsar SAX J1808.4-3658 is the most prominent member. The quiescent spectrum of SAX J1810.8-2609 is well-fit with an absorbed power law with photon index of 3.3+-0.5. With a value of 3.3x10^21 cm^-2 the Galactic absorption is consistent with the value derived in outburst. Since the spectra of quiescent neutron star SXTs are often fit with an absorbed blackbody or neutron star atmosphere plus power-law model we also fitted the spectrum using those fit functions. Both models provide a good fit to the data. If cooling of the neutron star core and/or crust is responsible for the soft part of the spectrum the time averaged mass accretion rate must have been very low (~5.7x10^-13 Msun yr^-1; assuming standard core cooling only) or the neutron star must be massive. We also discuss the possibility that the thermal spectral component in neutron stars in quiescence is produced by residual accretion.Comment: 5 pages, 1 figure, accepted for publication by MNRA

XMM-Newton Finds That SAX J1750.8-2900 May Harbor the Hottest, Most Luminous Known Neutron Star

We have performed the first sensitive X-ray observation of the low-mass X-ray binary SAX J1750.8-2900 in quiescence with XMM-Newton. The spectrum was fit to both a classical black body model, and a non-magnetized, pure hydrogen neutron star atmosphere model. A power law component was added to these models, but we found that it was not required by the fits. The distance to SAX J1750.8-2900 is known to be D = 6.79 kpc from a previous analysis of photospheric radius expansion bursts. This distance implies a bolometric luminosity (as given by the NS atmosphere model) of (1.05 +/- 0.12) x 10^34 (D/6.79 kpc)^2 erg s^-1, which is the highest known luminosity for a NS LMXB in quiescence. One simple explanation for this surprising result could be that the crust and core of the NS were not in thermal equilibrium during the observation. We argue that this was likely not the case, and that the core temperature of the NS in SAX J1750.8-2900 is unusually high

Spectral Fits to the 1999 Aql X-1 Outburst Data

We present analysis and spectral fits of the RXTE data from the May/June 1999 outburst of Aql X-1. These data include observations in the rising portion of the hard state, in the soft state, and in the falling portion of the hard state. We show that the data can be fit by a purely thermal Comptonization model for all the observations, but that more complicated models cannot be ruled out. Up to 60% of the corona's power in the soft state may be injected into non-thermal electrons. The soft state data show approximately constant optical depth and coronal temperatures over a range of $\sim10$ in luminosity, while they show evidence for a reduction of seed photon temperature with reduced luminosity and indicate that the characteristic size of the seed photon emitting region is roughly constant throughout the soft state. The hard state before the soft state shows a higher luminosity, higher optical depth, and lower electron temperature than the hard state after the soft state. We find a reduction of the hard (30-60 keV) X-ray flux during a type I burst and show that it requires a total corona energy reservoir of less than $\sim 10^{38}$ ergs.Comment: 10 pages, 7 figues, accepted to A+

Orbital Period and Outburst Luminosity of Transient Low Mass X-ray Binaries

In this paper we investigate the relation between the maximal luminosity of X-ray outburst and the orbital period in transient low mass X-ray binaries (or soft X-ray transients) observed by the Rossi X-ray Timing Explorer (RXTE) in the past decade. We find that the maximal luminosity (3-200 keV) in Eddington unit generally increases with increasing orbital period, which does not show a luminosity saturation but in general agrees with theoretical prediction. The peak luminosities in ultra-compact binaries might be higher than those with orbital period of 2-4 h, but more data are needed to make the claim. We also find that there is no significant difference in the 3-200 keV outburst peak luminosity between neutron star systems and black hole systems with orbital periods above 4 h; however, there might be significant difference at smaller orbital period where only neutron star systems are observed and radiatively ineffcient accretion flow is expected to work at the low luminosities for black hole accreters.Comment: 28 pages, 5 figures, accepted by Ap

Radio / X-ray correlation in the low/hard state of GX 339--4

We present the results of a long-term study of the black hole candidate GX 339-4 using simultaneous radio (from the Australia Telescope Compact Array) and X-ray (from the Rossi X-ray Timing Explorer and BeppoSAX) observations performed between 1997 and 2000. We find strong evidence for a correlation between these two emission regimes that extends over more than three decades in X-ray flux, down to the quiescence level of GX 339-4. This is the strongest evidence to date for such strong coupling between radio and X-ray emission. We discuss these results in light of a jet model that can explain the radio/X-ray correlation. This could indicate that a significant fraction of the X-ray flux that is observed in the low-hard state of black hole candidates may be due to optically thin synchrotron emission from the compact jet.Comment: 8 pages. Accepted for publication in Astronomy & Astrophysics, 200

Centralities in the city border: a method to identify strategic urban-rural interventions

City borders perform an essential role in connecting towns and their surroundings. Being more a fuzzy area than a thin line, these urban borders gather together residential, tertiary and infrastructural uses, places awaiting development, agricultural fields, brown-field sites, abandoned areas etc. all named as ‘b-sites’. Within this context, we propose a method to identify places for strategic urban-rural interventions based on the assessment and identification of centralities in the urban-rural transition. Multiple centrality assessment is here presented as an innovative application considering both urban streets and rural road networks as a mixed network with identified central nodes. This innovative method has been tested in the city of Granada (Spain) allowing us to identify high centrality ‘b-sites’ where landscape project design and, urban-rural interventions could contribute to creating urban-rural transition continuity

Radio and X-ray observations during the outburst decay of the Black Hole Candidate XTE J1908+094

Obtaining simultaneous radio and X-ray data during the outburst decay of soft X-ray transients is a potentially important tool to study the disc - jet connection. Here we report results of the analysis of (nearly) simultaneous radio (VLA or WSRT) and Chandra X-ray observations of XTE J1908+094 during the last part of the decay of the source after an outburst. The limit on the index of a radio - X-ray correlation we find is consistent with the value of 0.7 which was found for other black hole candidates in the low/hard state. Interestingly, the limit we find seems more consistent with a value of 1.4 which was recently shown to be typical for radiatively efficient accretion flow models. We further show that when the correlation-index is the same for two sources one can use the differences in normalisation in the radio - X-ray flux correlation to estimate the distance towards the sources if the distance of one of them is accurately known (assuming black hole spin and mass and jet Lorentz factor differences are unimportant or minimal). Finally, we observed a strong increase in the rate of decay of the X-ray flux. Between March 23, 2003 and April 19, 2003 the X-ray flux decayed with a factor ~5 whereas between April 19, 2003 and May 13, 2003, the X-ray flux decreased by a factor ~750. The source (0.5-10 keV) luminosity at the last Chandra observation was L~3x10^32 (d/8.5 kpc)^2 erg s^-1.Comment: 7 pages, 3 figures, accepted for publication by MNRA