The identification of MAXI J1659-152 as a black hole candidate

We report on the analysis of all 65 pointed Rossi X-ray Timing Explorer observations of the recently discovered soft X-ray transient MAXI J1659-152 (initially referred to as GRB 100925A). The source was studied in terms of its evolution through the hardness-intensity diagram (HID) as well as its X-ray variability properties. MAXI J1659-152 traced out an anti-clockwise loop in the HID, which is commonly seen in transient low-mass X-ray binaries. The variability properties of the source, in particular the detection of type-B and type-C low-frequency quasi-periodic oscillations, and the way they evolve along the HID track, indicate that MAXI J1659-152 is a black hole candidate. The spectral and variability properties of MAXI J1659-152 imply that the source was observed in the hard and soft intermediate states during the RXTE observations, with several transitions between these two states.Comment: 6 pages, 4 figures, accepted for publication in ApJ

The variable radio counterpart and possible large-scale jet of the new Z-source XTE J1701-462

We report radio observations, made with the Australia Telescope Compact Array, of the X-ray transient XTE J1701-462. This system has been classified as a new `Z' source, displaying characteristic patterns of behaviour probably associated with accretion onto a low magnetic field neutron star at close to the Eddington limit. The radio counterpart is highly variable, and was detected in six of sixteen observations over the period 2006 January -- April. The coupling of radio emission to X-ray state, despite limited sampling, appears to be similar to that of other `Z' sources, in that there is no radio emission on the flaring branch. The mean radio and X-ray luminosities are consistent with the other Z sources for a distance of 5--15 kpc. The radio spectrum is unusually flat, or even inverted, in contrast to the related sources, Sco X-1 and Cir X-1, which usually display an optically thin radio spectrum. Deep wide-field observations indicate an extended structure three arcminutes to the south which is aligned with the X-ray binary. This seems to represent a significant overdensity of radio sources for the field and so, although a background source remains a strong possibility, we consider it plausible that this is a large-scale jet associated with XTE J1701-462.Comment: Accepted for publication as a Letter in MNRA

On the nature of the first transient Z-source XTE J1701-462: its accretion disk structure, neutron star magnetic field strength, and hard tail

Using the data from the Rossi X-Ray Timing Explorer satellite, we investigate the spectral evolution along a "Z" track and a "v" track on the hardness-intensity diagrams of the first transient Z source XTE J1701-462. The spectral analyses suggest that the inner disk radius depends on the mass accretion rate, in agreement with the model prediction, R_in \propto ((dM/dt)_disk)^{2/7}, for a radiation pressure dominated accretion disk interacting with the magnetosphere of a neutron star (NS). The changes in the disk mass accretion rate (dM/dt)_disk are responsible for the evolution of the "Z" or "v" track. The radiation pressure thickens the disk considerably, and also produces significant outflows. The NS surface magnetic field strength, derived from the interaction between the magnetosphere and the radiation pressure dominated accretion disk, is ~(1--3)X10^9 G, which is possibly between normal atoll and Z sources. A significant hard tail is detected in the horizontal branches and we discuss several possible origins of the hard tail

Black hole candidate XTE J1752-223: Swift observations of canonical states during outburst

We present Swift broadband observations of the recently discovered black hole candidate, X-ray transient, XTE J1752-223, obtained over the period of outburst from October 2009 to June 2010. From Swift-UVOT data we confirm the presence of an optical counterpart which displays variability correlated, in the soft state, to the X-ray emission observed by Swift-XRT. The optical counterpart also displays hysteretical behaviour between the states not normally observed in the optical bands, suggesting a possible contribution from a synchrotron emitting jet to the optical emission in the rising hard state. We offer a purely phenomenological treatment of the spectra as an indication of the canonical spectral state of the source during different periods of the outburst. We find that the high energy hardness-intensity diagrams over two separate bands follows the canonical behavior, confirming the spectral states. Our XRT timing analysis shows that in the hard state there is significant variability below 10Hz which is more pronounced at low energies, while during the soft state the level of variability is consistent with being minimal. These properties of XTE J1752-223 support its candidacy as a black hole in the Galactic centre region.Comment: 8 pages, 8 figures; MNRAS in pres

Full-Stokes polarimetry with circularly polarized feeds - Sources with stable linear and circular polarization in the GHz regime

We present a pipeline that allows recovering reliable information for all four Stokes parameters with high accuracy. Its novelty relies on the treatment of the instrumental effects already prior to the computation of the Stokes parameters contrary to conventional methods, such as the M\"uller matrix one. The instrumental linear polarization is corrected across the whole telescope beam and significant Stokes $Q$ and $U$ can be recovered even when the recorded signals are severely corrupted. The accuracy we reach in terms of polarization degree is of the order of 0.1-0.2 %. The polarization angles are determined with an accuracy of almost 1$^{\circ}$. The presented methodology was applied to recover the linear and circular polarization of around 150 Active Galactic Nuclei. The sources were monitored from July 2010 to April 2016 with the Effelsberg 100-m telescope at 4.85 GHz and 8.35 GHz with a cadence of around 1.2 months. The polarized emission of the Moon was used to calibrate the polarization angle. Our analysis showed a small system-induced rotation of about 1$^{\circ}$ at both observing frequencies. Finally, we identify five sources with significant and stable linear polarization; three sources remain constantly linearly unpolarized over the period we examined; a total of 11 sources have stable circular polarization degree $m_\mathrm{c}$ and four of them with non-zero $m_\mathrm{c}$. We also identify eight sources that maintain a stable polarization angle over the examined period. All this is provided to the community for polarization observations reference. We finally show that our analysis method is conceptually different from the traditionally used ones and performs better than the M\"uller matrix method. Although it was developed for a system equipped with circularly polarized feeds it can easily be modified for systems with linearly polarized feeds as well.Comment: 19 pages, 17 figures, accepted for publication in Astronomy & Astrophysics on May 30, 201

INTEGRAL/RossiXTE high-energy observation of a state transition of GX 339-4

On 2004 August 15, we observed a fast (shorter than 10 hours) state transition in the bright black-hole transient GX 339-4 simultaneously with RossiXTE and INTEGRAL. This transition was evident both in timing and spectral properties. Combining the data from PCA, HEXTE and 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 cutoff 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 cutoff 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.Comment: Accepted for publication in MNRAS (9 pages, 6 figures

Discovery of two simultaneous non-harmonically related Quasi-Periodic Oscillations in the 2005 outburst of the black-hole binary GRO J1655-40

We studied the low-frequency quasi-periodic oscillations (LFQPOs) in the black hole GRO J1655-40 during the 2005 outburst, using data from the Rossi X-ray Timing Explorer. All LFQPOs could be identified as either type B or type C using previously proposed classification schemes. In the soft state of the outburst the type-C LFQPOs reached frequencies that are among the highest ever seen for LFQPOs in black holes. At the peak of the outburst, in the ultra-luminous state, the power spectrum showed two simultaneous, non-harmonically related peaks which we identified as a type-B and a type-C QPO. The simultaneous presence of a type-C and type-B QPO shows that at least two of the three known LFQPO types are intrinsically different and likely the result of distinct physical mechanisms. We also studied the properties of a broad peaked noise component in the power spectra of the ultra-luminous state. This noise component becomes more coherent with count rate and there are strong suggestions that it evolves into a type-B QPO at the highest observed count rates.Comment: 14 pages, 6 figures, 2 tables, accepted for publication in MNRA