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We studied time variability and spectral evolution of the Galactic black hole transient Swift J174510.8-262411 during the first phase of its outburst. INTEGRAL and Swift observations collected from 2012 September 16 until October 30 have been used. The total squared fractional rms values did not drop below 5 per cent and quasi-periodic oscillations (QPOs), when present, were type-C, indicating that the source never made the transition to the soft-intermediate state. Even though the source was very bright (up to 1 Crab in hard X-rays), it showed a so called failed outburst as it never reached the soft state. XRT and IBIS broad-band spectra, well represented by a hybrid thermal/non-thermal Comptonization model, showed physical parameters characteristic of the hard and intermediate states. In particular, the derived temperature of the geometrically thin disc blackbody was about 0.6 keV at maximum. We found a clear decline of the optical depth of the corona electrons (close to values of 0.1), as well as of the total compactness ratio lh/ls. The hard-to-hard/intermediate state spectral transition is mainly driven by the increase in the soft photon flux in the corona, rather than small variations of the electron heating. This, associated with the increasing of the disc temperature, is consistent with a disc moving towards the compact object scenario, i.e. the truncated-disc model. Moreover, this scenario is consistent with the decreasing fractional squared rms and increasing of the noise and QPO frequency. In our final group of observations, we found that the contribution from the non-thermal Comptonization to the total power supplied to the plasma is 0.59+0.02 -0.05 and that the thermal electrons cool to kTe < 26 keV. [ABSTRACT FROM AUTHOR]

We report on deep, coordinated radio and X-ray observations of the black hole X-ray binary XTE J1118+480 in quiescence. The source was observed with the Karl G. Jansky Very Large Array for a total of 17.5 h at 5.3 GHz, yielding a 4.8 ± 1.4 μJy radio source at a position consistent with the binary system. At a distance of 1.7 kpc, this corresponds to an integrated radio luminosity between 4 and 8 × 1025 erg s−1, depending on the spectral index. This is the lowest radio luminosity measured for any accreting black hole to date. Simultaneous observations with the Chandra X-ray Telescope detected XTE J1118+480 at 1.2 × 10−14 erg s−1 cm−2 (1–10 keV), corresponding to an Eddington ratio of ∼4 × 10−9 for a 7.5 M⊙ black hole. Combining these new measurements with data from the 2005 and 2000 outbursts available in the literature, we find evidence for a relationship of the form ℓr = α+βℓX (where ℓ denotes logarithmic luminosities), with β = 0.72 ± 0.09. XTE J1118+480 is thus the third system – together with GX339-4 and V404 Cyg – for which a tight, non-linear radio/X-ray correlation has been reported over more than 5 dex in ℓX. Confirming previous results, we find no evidence for a dependence of the correlation normalization of an individual system on orbital parameters, relativistic boosting, reported black hole spin and/or black hole mass. We then perform a clustering and linear regression analysis on what is arguably the most up-to-date collection of coordinated radio and X-ray luminosity measurements from quiescent and hard-state black hole X-ray binaries, including 24 systems. At variance with previous results, a two-cluster description is statistically preferred only for random errors ≲0.3 dex in both ℓr and ℓX, a level which we argue can be easily reached when the known spectral shape/distance uncertainties and intrinsic variability are accounted for. A linear regression analysis performed on the whole data set returns a best-fitting slope β = 0.61 ± 0.03 and intrinsic scatter σ0 = 0.31 ± 0.03 dex. [ABSTRACT FROM PUBLISHER]

In X-ray binaries, compact jets are known to commonly radiate at radio to infrared frequencies, whereas at optical to γ-ray energies, the contribution of the jet is debated. The total luminosity, and hence power of the jet, is critically dependent on the position of the break in its spectrum, between optically thick (self-absorbed) and optically thin synchrotron emission. This break, or turnover, has been reported in just one black hole X-ray binary (BHXB) thus far, GX 339−4, and inferred via spectral fitting in two others, A0620−00 and Cyg X−1. Here, we collect a wealth of multi-wavelength data from the outbursts of BHXBs during hard X-ray states, in order to search for jet breaks as yet unidentified in their spectral energy distributions. In particular, we report the direct detection of the jet break in the spectrum of V404 Cyg during its 1989 outburst, at νb = (1.8 ± 0.3) × 1014 Hz (1.7 ± 0.2 μm). We increase the number of BHXBs with measured jet breaks from three to eight. Jet breaks are found at frequencies spanning more than two orders of magnitude, from νb = (4.5 ± 0.8) × 1012 Hz for XTE J1118+480 during its 2005 outburst, to νb > 4.7 × 1014 Hz for V4641 Sgr in outburst. A positive correlation between jet break frequency and luminosity is expected theoretically; νb∝L∼ 0.5ν, jet if other parameters are constant. With constraints on the jet break in a total of 12 BHXBs including two quiescent systems, we find a large range of jet break frequencies at similar luminosities and no obvious global relation (but such a relation cannot be ruled out for individual sources). We speculate that different magnetic field strengths and/or different radii of the acceleration zone in the inner regions of the jet are likely to be responsible for the observed scatter between sources. There is evidence that the high-energy cooling break in the jet spectrum shifts from UV energies at LX ∼ 10−8LEdd (implying the jet may dominate the X-ray emission in quiescence) to X-ray energies at ∼10−3LEdd. Finally, we find that the jet break luminosity scales as Lν, jet∝L0.56 ± 0.05X (very similar to the radio–X-ray correlation), and radio-faint BHXBs have fainter jet breaks. In quiescence the jet break luminosity exceeds the X-ray luminosity. [ABSTRACT FROM AUTHOR]

ABSTRACT We report on Chandra, Rossi X-ray Timing Explorer ( RXTE), Swift/Burst Alert Telescope (BAT) and MAXI observations of an X-ray flare of ∼1 d and subsequent outburst of a transient X-ray source observed in 2011 October-November in the globular cluster Terzan 5. We show that the source is the same as the transient that was active in 2000, i.e. the neutron star low-mass X-ray binary EXO 1745−248. For the X-ray flare we estimate a 6-11 h exponential decay time and a radiated energy of 2-9 × 1042 erg. These properties, together with strong evidence of decreasing blackbody temperature during the flare decay, are fully consistent with what is expected for a thermonuclear superburst. We use the most recent superburst models and estimate an ignition column depth of ≈1012 g cm−2 and an energy release between 0.1 × 1018 and 2 × 1018 erg g−1, also consistent with expected superburst values. We conclude therefore that the flare was most probably a superburst. We discuss our results in the context of theoretical models and find that even when assuming a few days of low-level accretion before the superburst onset (which is more than what is suggested by the data), the observations of this superburst are very challenging for current superburst ignition models. [ABSTRACT FROM AUTHOR]

ABSTRACT In this paper we report on Expanded Very Large Array radio and Chandra and Swift X-ray observations of the outburst decay of the transient black hole candidate MAXI J1659-152 in 2011. We discuss the distance to the source taking the high inclination into account and conclude that the source distance is probably 6 ± 2 kpc. The lowest observed flux corresponds to a luminosity of erg s−1. This, together with the orbital period of 2.4 h reported in the literature, suggests that the quiescent X-ray luminosity is higher than predicted on the basis of the orbital period-quiescent X-ray luminosity relationship. It is more in line with that expected for a neutron star, although the outburst spectral and timing properties reported in the literature strongly suggest that MAXI J1659-152 harbours a black hole. This conclusion is subject to confirmation of the lowest observed flux as the quiescent flux. The relation between the accretion and ejection mechanisms can be studied using the observed correlation between the radio and X-ray luminosities as these evolve over an outburst. We determine the behaviour of MAXI J1659-152 in the radio-X-ray diagram at low X-ray luminosities using the observations reported in this paper and at high X-ray luminosities using values reported in the literature. At high X-ray luminosities, the source lies closer to the sources that follow a correlation index steeper than 0.6-0.7. However, when compared to other sources that follow a steeper correlation index, the X-ray luminosity in MAXI J1659-152 is also lower. The latter can potentially be explained by the high inclination of MAXI J1659-152 if the X-ray emission comes from close to the source and the radio emission is originating in a more extended region. However, it is probable that the source was not in the canonical low-hard state during these radio observations and this may affect the behaviour of the source as well. At intermediate X-ray luminosities, the source makes the transition from the radio underluminous sources in the direction of the relation traced by the 'standard' correlation similar to what has been reported for H 1743−322 in the literature. However, MAXI J1659-152 remains underluminous with respect to this 'standard' correlation. [ABSTRACT FROM AUTHOR]

ABSTRACT We present optical, X-ray and radio observations of the black hole transient (BHT) XTE J1752−223 towards and in quiescence. Optical photometry shows that the quiescent magnitude of XTE J1752−223 is fainter than 24.4 mag in the i′ band. A comparison with measurements of the source during its 2009-2010 outburst shows that the outburst amplitude is more than 8 mag in the i′ band. Known X-ray properties of the source combined with the faintness of the quiescence optical counterpart and the large outburst optical amplitude point towards a short orbital-period system ( Porb≲ 6.8 h) with an M type (or later) mass donor, at a distance of 3.5 ≲ d≲ 8 kpc. Simultaneous X-ray and radio data were collected with Chandra and the Expanded Very Large Array (EVLA), allowing constraints to be placed on the quiescent X-ray and radio flux of XTE J1752−223. Furthermore, using data covering the final stage of the outburst decay, we investigated the low-luminosity end of the X-ray-radio correlation for this source and compared it with other BHTs. We found that XTE J1752−223 adds to the number of outliers with respect to the 'standard' X-ray-radio luminosity relation. Furthermore, XTE J1752−223 is the second source, after the BHT H1743−322, that shows a transition from the region of the outliers towards the 'standard' correlation at low luminosity. Finally, we report on a faint, variable X-ray source we discovered with Chandra at an angular distance of ∼2.9 arcsec to XTE J1752−223 and at a position angle consistent with that of the radio jets previously observed from the BHT. We discuss the possibility that we detected X-ray emission associated with a jet from XTE J1752−223. [ABSTRACT FROM AUTHOR]

In this paper, we report on radio (Very Large Array and Austrialian Telescope Compact Array) and X-ray ( RXTE, Chandra and Swift) observations of the outburst decay of the transient black hole candidate H 1743-322 in early 2008. We find that the X-ray light curve followed an exponential decay, levelling off towards its quiescent level. The exponential decay time-scale is 4 days and the quiescent flux corresponds to a luminosity of erg s−1. This together with the relation between quiescent X-ray luminosity and orbital period reported in the literature suggests that H 1743-322 has an orbital period longer than ≈10 h. Both the radio and X-ray light curve show evidence for flares. The radio–X-ray correlation can be well described by a power-law with index 0.18. This is much lower than the index of 0.6–0.7 found for the decay of several black hole transients before. The radio spectral index measured during one of the radio flares while the source is in the low–hard state is , which indicates that the radio emission is optically thin. This is unlike what has been found before in black hole sources in the low–hard state. We attribute the radio flares and the low index for the radio–X-ray correlation to the presence of shocks downstream the jet flow, triggered by ejection events earlier in the outburst. We find no evidence for a change in X-ray power-law spectral index during the decay, although the relatively high extinction of limits the detected number of soft photons and thus the accuracy of the spectral fits. [ABSTRACT FROM AUTHOR]

We report on X-ray observations of the black hole candidate GX during its 2006/2007 outburst. The hardness–intensity diagram (HID) of all RXTE/Proportional Counter Array data combined shows a q-shaped track similar to that observed in previous outbursts. The evolution through the HID suggests that in the early phase of the outburst the source underwent a sequence of state transitions, from the hard to the soft state, which is supported by our timing analysis. Broad-band (4–200 keV) spectra, fitted with an exponentially cut-off power law, show that the hard spectral component steepens during the transition from the hard to the soft state. The high-energy cut-off decreased monotonically from 120 to 60 keV during the brightening of the hard state, but again increased to 100 keV during the softening in the hard intermediate state. In the short-lived soft intermediate state the cut-off energy was ∼130 keV, but was no longer detected in the soft state. This is one of the first times that the high-energy cut-off has been followed in such detail across several state transitions. We find that in comparison to several other spectral parameters, the cut-off energy changes more rapidly, just like the timing properties. The observed behaviour of the high-energy cut-off of GX is also similar to that observed with RXTE– INTEGRAL– Swift during the 2005 outburst of GRO . These results constitute a valuable reference to be considered when testing theoretical models for the production of the hard component in these systems. [ABSTRACT FROM AUTHOR]

We report on X-ray and soft γ-ray observations of the black hole candidate GX 339−4 during its 2007 outburst, performed with the RXTE and INTEGRAL satellites. The hardness–intensity diagram of all RXTE/PCA data combined shows a q-shaped track similar to that observed in previous outbursts. The evolution in the diagram suggested that a transition from hard- to soft-intermediate state occurred, simultaneously with INTEGRAL observations performed in March. The transition is confirmed by the timing analysis presented in this work, which reveals that a weak type-A quasi-periodic oscillation (QPO) replaces a strong type-C QPO. At the same time, spectral analysis shows that the flux of the high-energy component shows a significant decrease in its flux. However, we observe a delay (roughly one day) between variations of the spectral parameters of the high-energy component and changes in the flux and timing properties. The changes in the high-energy component can be explained either in terms of the high-energy cut-off or in terms of variations in the reflection component. We compare our results with those from a similar transition during the 2004 outburst of GX . [ABSTRACT FROM AUTHOR]

We observed the neutron star X-ray transient 2S 1803−245 in quiescence with the X-ray satellite XMM–Newton, but did not detect it. An analysis of the X-ray bursts observed during the 1998 outburst of 2S 1803−245 gives an upper limit to the distance of ≤7.3 kpc, leading to an upper limit on the quiescent 0.5–10 keV X-ray luminosity of (3σ). Since the expected orbital period of 2S 1803−245 is several hours, this limit is not much higher than those observed for the quiescent black hole transients with similar orbital periods. [ABSTRACT FROM AUTHOR]

We have monitored the atoll-type neutron star low-mass X-ray binary 4U 1636−53 with the Rossi X-ray Timing Explorer ( RXTE) for more than 1.5 yr. Our campaign consisted of short (∼2 ks) pointings separated by 2 d, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ∼30–40 d, already seen in the light curves from the RXTE All-Sky Monitor, which corresponded to regular transitions between the hard (island) and soft (banana) states. We detected kilohertz (kHz) quasi-periodic oscillations (QPOs) in about a third of the observations, most of which were in the soft (banana) state. The distribution of the frequencies of the peak identified as the lower kHz QPO is found to be different from that previously observed in an independent data set. This suggests that the kHz QPOs in the system shows no intrinsically preferred frequency. [ABSTRACT FROM AUTHOR]

We re-examine the correlation between the frequencies of upper and lower kHz quasi-periodic oscillations (QPO) in bright neutron star low-mass X-ray binaries. By including the kHz QPO frequencies of the X-ray binary Cir X-1 and two accreting millisecond pulsars in our sample, we show that the full sample does not support the class of theoretical models based on a single resonance, while models based on relativistic precession or Alfvén waves describe the data better. Moreover, we show that the fact that all sources follow roughly the same correlation over a finite frequency range creates a correlation between the linear parameters of the fits to any subsample. [ABSTRACT FROM AUTHOR]

The optical/near-infrared (OIR) region of the spectra of low-mass X-ray binaries (XBs) appears to lie at the intersection of a variety of different emission processes. In this paper we present quasi-simultaneous OIR–X-ray observations of 33 XBs in an attempt to estimate the contributions of various emission processes in these sources, as a function of X-ray state and luminosity. A global correlation is found between OIR and X-ray luminosity for low-mass black hole candidate XBs (BHXBs) in the hard X-ray state, of the form . This correlation holds over eight orders of magnitude in LX and includes data from BHXBs in quiescence and at large distances [Large Magellanic Cloud (LMC) and M31]. A similar correlation is found in low-mass neutron star XBs (NSXBs) in the hard state. For BHXBs in the soft state, all the near-infrared (NIR) and some of the optical emissions are suppressed below the correlation, a behaviour indicative of the jet switching off/on in transition to/from the soft state. We compare these relations to theoretical models of a number of emission processes. We find that X-ray reprocessing in the disc and emission from the jets both predict a slope close to 0.6 for BHXBs, and both contribute to the OIR in BHXBs in the hard state, the jets producing ∼90 per cent of the NIR emission at high luminosities. X-ray reprocessing dominates the OIR in NSXBs in the hard state, with possible contributions from the jets (only at high luminosity) and the viscously heated disc. We also show that the optically thick jet spectrum of BHXBs extends to near the K band. OIR spectral energy distributions of 15 BHXBs help us to confirm these interpretations. We present a prediction of the behaviour of a BHXB outburst that enters the soft state, where the peak LOIR in the hard state rise is greater than in the hard state decline (the well-known hysteretical behaviour). In addition, it is possible to estimate the X-ray, OIR and radio luminosity and the mass accretion rate in the hard state quasi-simultaneously, from observations of just one of these wavebands, since they are all linked through correlations. Finally, we have discovered that the nature of the compact object, the mass of the companion and the distance/reddening can be constrained by quasi-simultaneous OIR and X-ray luminosities. [ABSTRACT FROM AUTHOR]

On 2004 August 15, we observed a fast (shorter than 10 h) state transition in the bright black hole transient GX 339–4 simultaneously with Rossi X-Ray Timing Explorer ( RXTE) and INTEGRAL. This transition was evident both in timing and spectral properties. Combining the data from the Proportional Counter Array (PCA), the High-Energy X-ray Timing Experiment (HEXTE) and the Imager on Board the INTEGRAL Satellite (IBIS), we obtained good quality broad-band (3–200 keV) energy spectra before and after the transition. These spectra indicate that the hard component steepened. Also, the high-energy cut-off that was present at ∼70 keV before the transition was not detected after the transition. This is the first time that an accurate determination of the broad-band spectrum across such a transition has been measured on a short time-scale. It shows that, although some spectral parameters do not change abruptly through the transition, the high-energy cut-off increases/disappears rather fast. These results constitute a benchmark on which to test theoretical models for the production of the hard component in these systems. [ABSTRACT FROM AUTHOR]

We report on a detailed analysis of data obtained over nearly four years with the Rossi X-ray Timing Explorer of the Z source GX 5–1. From a spectral analysis using a hardness–intensity diagram it was found that the source traced out the typical Z-shaped pattern. The study of the power spectral properties showed that when the source moved on the horizontal branch towards the normal branch the fractional rms amplitudes and time-scales of all variability decreased, while their FWHMs increased. The frequency separation of the two kHz QPO peaks decreased from 344±12 to 232±13 Hz , while the frequency of the lower and upper kHz QPO increased from 172±10 to 608±6 Hz and from 516±10 to 840±12 Hz , respectively. At low frequencies, besides the horizontal branch oscillation (HBO) and its second harmonic, two additional broad Lorentzian components were needed to obtain acceptable fits. These broad Lorentzians have Q-values of ∼1–2 and frequencies 0.5 and 1.5 times the HBO frequency. When interpreted as being related to the HBO, they seem to favour disc models for the HBO over the magnetic beat-frequency model. The frequency of the normal branch oscillations changed slightly and non-monotonically, while on the normal branch between ∼6 Hz at both ends and 5.25±0.05 Hz near the middle of the branch. It evolved into a flat-topped noise component on the flaring branch. We compared the timing properties of the some of the Z sources. We also compare the timing properties and colour–colour diagrams (CDs) of GX 5–1 with those of the back hole candidate XTE J1550–564 and the atoll source 4U 1608–52 . The CDs are strikingly similar when a colour scheme commonly employed in back hole studies is used. However, this may be a degeneracy as the CDs turn out to be more complicated when colours common in neutron star studies are employed. Apart from some remarkable similarities between the CD of XTE J1550–564 and that of 4U 1608–52 , several differences can be seen between these CDs and that of GX 5–1. Conclusions on spectral states or properties based solely on the use of CDs using the ‘black hole scheme’ should be regarded with caution. [ABSTRACT FROM AUTHOR]

The quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748−676 recently started the transition to quiescence following an accretion outburst that lasted more than 24 years. We report on two Chandra and 12 Swift observations performed within five months after the end of the outburst. The Chandra spectrum is composed of a soft, thermal component that fits to a neutron star atmosphere model with , joined by a hard power-law tail that contributes ∼20 per cent of the total 0.5–10 keV unabsorbed flux. The combined Chandra/Swift data set reveals a relatively hot and luminous quiescent system with a temperature of and a bolometric thermal luminosity of . We discuss our results in the context of cooling neutron star models. [ABSTRACT FROM AUTHOR]