Optical and infrared polarimetry of the transient LMXB Cen X-4 in quiescence

We present the first optical and infrared polarimetric study of the low mass transient X-ray binary Cen X-4 during its quiescent phase. This work is aimed to search for an intrinsic linear polarisation component in the system emitted radiation that might be due, e.g., to synchrotron emission from a compact jet, or to Thomson scattering with free electrons in an accretion disc. Multiband (BVRI) optical polarimetric observations were obtained during two nights in 2008 at the ESO La Silla 3.6 m telescope (EFOSC2) in polarimetric mode. These observations cover about the 30% of the 15.1 hours orbital period. J-band observations were obtained in 2007 with the NICS (TNG) instrument at La Palma, for a totality of 1 hour observation. We obtained 3-sigma upper limits to the polarisation degree in all the optical bands, with the most constraining one being in the I-band (P<0.5%). No phase-correlated variability has been noticed in all the filters. The J-band observations provided a 6% upper limit on the polarisation level. The constraining upper limits to the polarisation in the optical allowed us to evaluate the contribution of the possible emission of a relativistic particles jet to the total system radiation to be less then the 10%. This is in agreement with the observation of a spectral energy distribution typical of a single black body of a K-spectral type main sequence star irradiated from the compact object. Due to the low S/N ratio it was not possible to investigate the possible dependency of the polarisation degree from the wavelength, that could be suggestive of polarisation induced by Thomson scattering of radiation with free electrons in the outer part of the accretion disc. Observations with higher S/N ratio are required to examine in depth this hypothesis, searching for significant phase-correlated variability.Comment: 7 pages, 9 figures, accepted for publication in section 7. Stellar structure and evolution of Astronomy and Astrophysic

The long-term evolution of the X-ray pulsar XTE J1814-338: a receding jet contribution to the quiescent optical emission?

We present a study of the quiescent optical counterpart of the Accreting Millisecond X-ray Pulsar XTE J1814-338, carrying out multiband (BVR) orbital phase-resolved photometry using the ESO VLT/FORS2. The optical light curves are consistent with a sinusoidal variability modulated with the orbital period, showing evidence for a strongly irradiated companion star, in agreement with previous findings. The observed colours cannot be accounted for by the companion star alone, suggesting the presence of an accretion disc during quiescence. The system is fainter in all analysed bands compared to previous observations. The R band light curve displays a possible phase offset with respect to the B and V band. Through a combined fit of the multi-band light curves we derive constraints on the companion star and disc fluxes, on the system distance and on the companion star mass. The irradiation luminosity required to account for the observed day-side temperature of the companion star is consistent with the spin-down luminosity of a millisecond radio pulsar. The flux decrease and spectral evolution of the quiescent optical emission observed comparing our data with previous observations, collected over 5 years, cannot be well explained with the contribution of an irradiated companion star and an accretion disc alone. The progressive flux decrease as the system gets bluer could be due to a continuum component evolving towards a lower, bluer spectrum. While most of the continuum component is likely due to the disc, we do not expect it to become bluer in quiescence. Hence we hypothesize that an additional component, such as synchrotron emission from a jet was contributing significantly in the earlier data obtained during quiescence and then progressively fading or moving its break frequency toward longer wavelengths.Comment: 7 pages, 8 figures, accepted for publication in Section 7. Stellar structure and evolution of Astronomy and Astrophysic

1RXS J180408.9-342058: an ultra compact X-ray binary candidate with a transient jet

We present a detailed NIR/optical/UV study of the transient low mass X-ray binary 1RXS J180408.9-342058 performed during its 2015 outburst, aimed at determining the nature of its companion star. We obtained three optical spectra at the 2.1 m San Pedro Martir Observatory telescope (Mexico). We performed optical and NIR photometric observations with both the REM telescope and the New Technology Telescope (NTT) in La Silla. We obtained optical and UV observations from the Swift archive. Finally, we performed optical polarimetry of the source by using the EFOSC2 instrument mounted on the NTT. The optical spectrum of the source is almost featureless since the hydrogen and He I emissions lines, typically observed in LMXBs, are not detected. Similarly, carbon and oxygen lines are neither observed. We marginally detect the He II 4686 AA emission line, suggesting the presence of helium in the accretion disc. No significant optical polarisation level was observed. The lack of hydrogen and He I emission lines in the spectrum implies that the companion is likely not a main sequence star. Driven by the tentative detection of the He II 4686 AA emission line, we suggest that the system could harbour a helium white dwarf. If this is the case, 1RXS J180408.9-342058 would be an ultra-compact X-ray binary. By combining an estimate of the mass accretion rate together with evolutionary tracks for a He white dwarf, we obtain a tentative orbital period of ~ 40 min. On the other hand, we also built the NIR-optical-UV spectral energy distribution (SED) of the source at two different epochs. One SED was gathered when the source was in the soft X-ray state, and it is consistent with the presence of a single thermal component. The second SED, obtained when the source was in the hard X-ray state, shows a thermal component together with a tail in the NIR, likely indicating the presence of a (transient) jet.Comment: 8 pages, 5 figures, 4 tables. Accepted for publication in Astronomy & Astrophysics (Section 7

Different twins in the millisecond pulsar recycling scenario: optical polarimetry of PSR J1023+0038 and XSS J12270-4859

We present the first optical polarimetric study of the two transitional pulsars PSR J1023+0038 and XSS J12270-4859. This work is focused on the search for intrinsical linear polarisation (LP) in the optical emission from the two systems. We carried out multiband optical and NIR photo-polarimetry of the two systems using the ESO NTT at La Silla (Chile), equipped with the EFOSC2 and the SOFI instruments. XSS J12270-4859 was observed during its radio-pulsar state; we did not detect LP in all bands, with 3 sigma upper limits of, e.g., 1.4% in the R-band. We built the NIR-optical averaged spectral energy distribution (SED) of the system, that could be well described by an irradiated black body with radius $R_{*} = 0.33\pm0.03\,R_{\odot}$ and albedo $\eta=0.32\pm0.05$, without the need of further components (thus excluding the visible presence of an extended accretion disc and/or of relativistic jets). The case was different for PSR J1023+0038, that was in its accretion phase during our campaign. We measured a LP of $1.09\pm0.27\%$ and $0.90\pm 0.17\%$ in the V and R bands, respectively. The phase-resolved polarimetric curve of the source in the R-band reveals a hint of a sinusoidal modulation at the source 4.75 hr orbital period, peaked at the same orbital phase as the light curve. The measured LP of PSR J1023+0038 could in principle be interpreted as scattering with free electrons (that can be found in the accretion disc of the system or even in the hot corona that surrounds the disc itself) or to synchrotron emission from a relativistic particles jet or outflow. However, the NIR-optical SED of the system built starting from our dataset did not suggest the presence of a jet. We conclude that the optical LP observed for PSR J1023+0038 is possibly due to Thomson scattering with electrons in the disc, as also suggested from the possible modulation of the R-band LP at the system orbital period.Comment: 10 pages, 8 figures, 4 tables. Accepted for publication in Sec. 7. Stellar structure and evolution of Astronomy and Astrophysic

The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence

We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran Telescopio Canarias (GTC) in August 2014. Despite the source being in quiescence at the time of our observations, it showed a strong optical flaring activity, more pronounced at higher frequencies (i.e. the g band). Once the flares were subtracted, we tentatively recovered a sinusoidal modulation at the system orbital period in all bands, even if a significant phase shift with respect to an irradiated star, typical of accreting millisecond X-ray pulsars is detected. We conclude that the observed flaring could be a manifestation of the presence of an accretion disc in the system. The observed light curve variability could be explained by the presence of a superhump, which might be another proof of the formation of an accretion disc. In particular, the disc at the time of our observations was probably preparing to the new outburst of the source, that happened just a few months later, in 2015.Comment: 6 pages, 2 figures, 1 table. Accepted for publication in A&

Magnetically-Coupled Cantilevers with Antiphase Bistable Behavior for Kinetic Energy Harvesting

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Engulfing a radio pulsar: the case of PSR J1023+0038

The binary millisecond radio pulsar PSR J1023+0038 has been recently the subject of multiwavelength monitoring campaigns which revealed that an accretion disc has formed around the neutron star (since 2013 June). We present here the results of X-ray and UV observations carried out by the Swift satellite between 2013 October and 2014 May, and of optical and NIR observations performed with the REM telescope, the Liverpool Telescope, the 2.1-m telescope at the San Pedro M\'artir Observatory and the 1.52-m telescope at the Loiano observing station. The X-ray spectrum is well described by an absorbed power law, which is softer than the previous quiescent epoch (up to 2013 June). The strong correlation between the X-ray and the UV emissions indicates that the same mechanism should be responsible for part of the emission in these bands. Optical and infrared photometric observations show that the companion star is strongly irradiated. Double-peaked emission lines in the optical spectra provide compelling evidence for the presence of an outer accretion disc too. The spectral energy distribution from IR to X-rays is well modelled if the contributions from the companion, the disc and the intra-binary shock emission are all considered. Our extensive data set can be interpreted in terms of an engulfed radio pulsar: the radio pulsar is still active, but undetectable in the radio band due to a large amount of ionized material surrounding the compact object. X-rays and gamma-rays are produced in an intra-binary shock front between the relativistic pulsar wind and matter from the companion and an outer accretion disc. The intense spin-down power irradiates the disc and the companion star, accounting for the UV and optical emissions.Comment: 11 pages, 8 figures, 5 tables; accepted for publication on MNRA

Polarimetric and spectroscopic optical observations of the ultra-compact X-ray binary 4U 0614+091

Aims: We present a polarimetric and spectroscopic study of the persistent ultra compact X-ray binary 4U 0614+091 aimed at searching for the emission of a relativistic particle jet and at unveiling the orbital period of the system. Methods: We obtained r-band polarimetric observations with the Telescopio Nazionale Galileo (TNG) equipped with the PAOLO polarimeter and with the Nordic Optical Telescope (NOT) equipped with the ALFOSC instrument, covering ~ 2 hours and ~ 0.5 hours observations, respectively. We carried out low resolution spectroscopy of the system using the ESO Very Large Telescope equipped with FORS1 for ~ 1.5 hours (16 spectra covering the range 430-800 nm). Results: The polarimetric analysis performed starting from the TNG dataset revealed a polarisation degree in the r-band of 3 % +/- 1 %. From the NOT dataset, due to the lower S/N ratio, we could obtain only a 3 sigma upper limit of 3.4 %. From the joining of a spectroscopic and photometric analysis, through the study of the equivalent width variations of the CII 724 nm line and the r-band light curve, we could find a hint of a ~ 45 min periodicity. Conclusions: A polarisation degree P of ~ 3 % in the r-band is consistent with the emission of a relativistic particle jet, which is supposed to emit intrinsically linearly polarised synchrotron radiation. Since no variations of P with time have been detected, and the accretion disc of the system does not contain ionised hydrogen, scattering by free electrons in the accretion disc has been rejected. The period of ~ 45 min obtained through the analysis of the system light curve and of the equivalent width variations of the selected spectral line is probably linked to the presence of a hot spot or a superhump in the accretion disc, and lead to an orbital period > 1 hour for the binary system.Comment: 8 pages, 6 figures, 4 tables. Accepted for publication in Astronomy & Astrophysics (Section 7

Peering at the outflow mechanisms in the transitional pulsar PSR J1023+0038: simultaneous VLT, XMM-Newton, and Swift high-time resolution observations

We report on a NIR, optical and X-ray campaign performed in 2017 with the XMM-Newton and Swift satellites and the VLT/HAWK-I instrument on the transitional MSP PSR J1023+0038. NIR observations were performed in fast-photometric mode in order to detect any fast variation of the flux and correlate them with the optical and X-ray light curves. The optical curve shows the typical sinusoidal modulation at the orbital period (4.75hr). No flaring or flickering is found in the optical, neither signs of transitions between active and passive states. On the contrary, the NIR curve displays a bimodal behaviour, showing strong flares in the first part of the curve, and an almost flat trend in the rest. The X-ray curves show a few low/high mode transitions, but no flaring activity. One of the low/high mode transition is found to happen at the same time as the emission of an infrared flare. This can be interpreted as the emission of a jet: the NIR flare could be due to the evolving spectrum of the jet, which possesses a break frequency that moves from higher (NIR) to lower (radio) frequencies after the launching, that has to happen at the low/high mode transition. We also present the cross correlation function between the optical and near infrared curves. Due to the bimodality of the NIR curve, we divided it in two parts (flaring and quiet). While the CCF of the quiet part is found to be flat, the one referring to the flaring part shows a narrow peak at ~10s, which indicates a delay of the NIR emission with respect to the optical. This lag can be interpreted as reprocessing of the optical emission at the light cylinder radius with a stream of matter spiraling around the system due to a phase of radio-ejection. This strongly supports a different origin of the NIR flares observed for PSR J1023+0038 with respect to the optical and X-ray flaring activity reported in other works on the same source.Comment: 10 pages, 9 figures. Accepted for publication in Astronomy&Astrophysic

Assessment of the stability of exogenous gamma hydroxybutyric acid (GHB) in stored blood and urine specimens

OBJECTIVE: The aim of this work is to test the stability of exogenous GHB in whole blood and urine samples collected from living and deceased GHB free-users, spiked with known concentrations of GHB and stored at different temperatures (–20°C, 4°C and 20°C) up to 4 weeks. MATERIALS AND METHODS: GHB was added to GHB-free ante-mortem blood and urine samples at the concentration of 5 and 10 mg/L, respectively whereas in post-mortem blood and urine specimens at 50 and 10 mg/L respectively. All samples were stored at three different temperatures: –20°C, 4°C and 20°C and extracted and analyzed at three days, 1 week, 2 weeks, 3 and 4 weeks in duplicate. No preservatives were added. GHB was quantified by GC-MS after LLE according to a previously published method. RESULTS: Post-mortem blood specimens showed a reduction of GHB levels higher than 10% only after a period of 4 weeks of storage for samples kept at +4°C and +20°C, whereas samples stored at –20°C showed a mean reduction of 8.7%. In post-mortem urine samples, there was a mean reduction of GHB levels higher than 20% at all storage temperatures, after 4 weeks of storage. Antemortem blood samples showed a reduction of GHB levels lower than 10% only after 3 days of storage at –20°C and at +4°C (samples stored at +20°C showed a mean reduction of 10.4%). After 4 weeks of storage, there was a mean reduction of GHB concentrations higher than 20% at all storage temperatures. Ante-mortem urine samples showed a reduction of GHB levels higher than 10% after just 3 days of storage for samples kept at all tested temperatures. After 4 weeks of storage, there was a mean reduction of GHB concentrations higher than 25% at all storage temperatures. CONCLUSIONS: According to our findings, it would be useful to perform GHB analysis both in blood and urine specimens within 3 days of sampling and the specimens should be stored at –20°C or 4°C in order to avoid instability issues