Swift J1858.6–0814 is a recently discovered X-ray binary notable for extremely strong variability (by factors of >100 in soft X-rays) in its discovery state. We present the detection of five ...thermonuclear (Type I) X-ray bursts from Swift J1858.6–0814, implying that the compact object in the system is a neutron star (NS). Some of the bursts show photospheric radius expansion, so their peak flux can be used to estimate the distance to the system. The peak luminosity, and hence distance, can depend on several system parameters; for the most likely values, a high inclination and a helium atmosphere, D = 12.8+0.8−0.6 kpc, although systematic effects allow a conservative range of 9–18 kpc. Before one burst, we detect a QPO at 9.6 ± 0.5 mHz with a fractional rms amplitude of 2.2 ± 0.2 per cent (0.5–10 keV), likely due to marginally stable burning of helium; similar oscillations may be present before the other bursts but the light curves are not long enough to allow their detection. We also search for burst oscillations but do not detect any, with an upper limit in the best case of 15 per cent fractional amplitude (over 1–8 keV). Finally, we discuss the implications of the NS accretor and this distance on other inferences which have been made about the system. In particular, we find that Swift J1858.6–0814 was observed at super-Eddington luminosities at least during bright flares during the variable stage of its outburst.
Aims. We present a multiwavelength study of the Be/X-ray binary system SAX J2103.5+4545 with the goal of better characterizing the transient behaviour of this source. Methods. SAX J2103.5+4545 was ...observed by Swift/XRT four times in 2007 from April 25 to May 5, and during quiescence in 2012 August 31. In addition, this source has been monitored from the ground-based astronomical observatories of El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain), and Sierra Nevada (Granada, Spain) since 2011 August, and from the TÜBİTAK National Observatory (Antalya, Turkey) since 2009 June. We performed spectral and photometric temporal analyses to investigate the different states exhibited by SAX J2103.5+4545. Results. In X-rays, an absorbed power-law model provided the best fit for all the XRT spectra. An iron-line feature at ~6.42 keV was present in all the observations except for that taken during quiescence in 2012. The photon indexes are consistent with previous studies of SAX J2103.5+4545 in high/low-luminosity states. Pulsations were found in all the XRT data from 2007 (2.839(2) mHz; MJD 54 222.02), but not during quiescence. The two optical outbursts in 2010 and 2012 lasted for about eight or nine months (as the one in 2007 probably did and the current one in 2014 might do) and were most probably caused by mass-ejection events from the Be star that eventually fed the circumstellar disc. All of these outbursts started about three months before the triggering of the X-ray activity, and at about the same period before the maximum of the Hα line equivalent width (in emission) was reached at only ~–5 Å. The global correlation between the BV variability and the X-ray intensity was also observed at longer wavelengths in the IR domain.
ABSTRACT
Swift J1858.6-0814 is a transient neutron star X-ray binary discovered in 2018 October. Multiwavelength follow-up observations across the electromagnetic spectrum revealed many interesting ...properties, such as erratic flaring on minute time-scales and evidence for wind outflows at both X-ray and optical wavelengths, strong and variable local absorption, and an anomalously hard X-ray spectrum. Here, we report on a detailed radio observing campaign consisting of one observation at 5.5/9 GHz with the Australia Telescope Compact Array, and nine observations at 4.5/7.5 GHz with the Karl G. Jansky Very Large Array. A radio counterpart with a flat to inverted radio spectrum is detected in all observations, consistent with a compact jet being launched from the system. Swift J1858.6-0814 is highly variable at radio wavelengths in most observations, showing significant variability when imaged on 3-to-5-min time-scales and changing up to factors of 8 within 20 min. The periods of brightest radio emission are not associated with steep radio spectra, implying they do not originate from the launching of discrete ejecta. We find that the radio variability is similarly unlikely to have a geometric origin, be due to scintillation, or be causally related to the observed X-ray flaring. Instead, we find that it is consistent with being driven by variations in the accretion flow propagating down the compact jet. We compare the radio properties of SwiftJ1858.6-0814 with those of Eddington-limited X-ray binaries with similar X-ray and optical characteristics, but fail to find a match in radio variability, spectrum, and luminosity.
Aims. We present a study of the Be/X-ray binary system V 0332+53 with the main goal of characterizing its behaviour mainly during the intermediate-luminosity X-ray event in 2008. In addition, we aim ...to contribute to the understanding of the behaviour of the donor companion by including optical data from our dedicated campaign starting in 2006. Methods. V 0332+53 was observed by RXTE and Swift during the decay of the intermediate-luminosity X-ray outburst of 2008, and with Suzaku before the rising of the third normal outburst of the 2010 series. In addition, we present recent data from the Spanish ground-based astronomical observatories of El Teide (Tenerife), Roque de los Muchachos (La Palma), and Sierra Nevada (Granada), and since 2006 from the Turkish TÜBİTAK National Observatory (Antalya). We have performed temporal analyses to investigate the transient behaviour of this system during several outbursts. Results. Our optical study revealed that continuous mass ejection episodes from the Be star have been taking place since 2006 and another is currently ongoing. The broad-band 1–60 keV X-ray spectrum of the neutron star during the decay of the 2008 outburst was well fitted with standard phenomenological models that were enhanced by an absorption feature of unknown origin at about 10 keV and a narrow iron K-alpha fluorescence line at 6.4 keV. For the first time in V 0332+53 we tentatively see an increase in the cyclotron line energy with increasing flux (although further and more sensitive observations are needed to confirm this). The fast aperiodic variability shows a quasi-periodic oscillation (QPO) at 227 ± 9 mHz only during the lowest luminosities, which might indicate that the inner regions surrounding the magnetosphere are more visible during the lowest flux states.
We present the discovery of eclipses in the X-ray light curves of the X-ray binary Swift J1858.6–0814. From these, we find an orbital period of P= 76841.3(+1.3,−1.4) s (≈21.3 hours) and an eclipse ...duration of t(ec)= 4098(+17,−18) s (≈1.14 hours).We also find several absorption dips during the pre-eclipse phase. From the eclipse duration to orbital period ratio, the inclination of the binary orbit is constrained to i >70◦. The most likely range for the companion mass suggests that the inclination is likely to be closer to this value than 90◦. The eclipses are also consistent with earlier data, in which strong variability (‘flares’) and the long orbital period prevent clear detection of the period or eclipses. We also find that the bright flares occurred preferentially in the post-eclipse phase of the orbit, likely due to increased thickness at the disc-accretion stream interface preventing flares being visible during the pre-eclipse phase. This supports the notion that variable obscuration is responsible for the unusually strong variability in Swift J1858.6–0814.
ABSTRACT
We present results from quasi-simultaneous multiwavelength observations of the Galactic black hole X-ray transient MAXI J1820+070 during the decay of the 2018 outburst and its entire ...subsequent mini-outburst in March 2019. We fit the X-ray spectra with phenomenological and Comptonizaton models and discuss the X-ray spectral evolution in comparison with the multiwavelength behaviour of the system. The system showed a rebrightening in UV/Optical/NIR bands 7 d after the soft-to-hard transition during the main outburst decay while it was fading in X-rays and radio. In contrast, the mini-outburst occurred 165 d after the hard state transition of the initial outburst decay and was detected in all wavelengths. For both events, the measured time-scales are consistent with those observed in other black hole systems. Contemporaneous hard X-ray/soft γ-ray observations indicate a non-thermal electron energy distribution at the beginning of the UV/Optical/NIR rebrightening, whereas a thermal distribution can fit the data during the hard mini-outburst activity. The broad-band spectral energy distributions until the rebrightening are consistent with the irradiated outer accretion disc model. However, both the SEDs produced for the peak of rebrightening and close to the peak of mini-outburst provided good fits only with an additional power law component in the UV/Optical/NIR frequency ranges, which is often interpreted with a jet origin.
We present the discovery of an optical accretion disk wind in the X-ray transient Swift J1858.6-0814. Our 90-spectrum data set, taken with the 10.4 m Gran Telescopio Canarias telescope over eight ...different epochs and across five months, reveals the presence of conspicuous P-Cyg profiles in He i at 5876 and H . These features are detected throughout the entire campaign, albeit their intensity and main observational properties are observed to vary on timescales as short as 5 minutes. In particular, we observe significant variations in the wind velocity, between a few hundreds and ∼2400 . In agreement with previous reports, our observations are characterized by the presence of frequent flares, although the relation between the continuum flux variability and the presence/absence of wind features is not evident. The reported high activity of the system at radio waves indicates that the optical wind of Swift J1858.6-0814 is contemporaneous with the radio jet, as is the case for the handful of X-ray binary transients that have shown so far optical P-Cyg profiles. Finally, we compare our results with those of other sources showing optical accretion disk winds, with emphasis on V404 Cyg and V4641 Sgr, since they also display strong and variable optical wind features as well as similar flaring behavior.
We characterized the broad-band X-ray spectra of Swift J1745-26 during the decay of the 2013 outburst using INTEGRAL ISGRI, JEM-X and Swift XRT. The X-ray evolution is compared to the evolution in ...optical and radio. We fit the X-ray spectra with phenomenological and Comptonization models. We discuss possible scenarios for the physical origin of an ~50 d flare observed both in optical and X-rays ~170 d after the peak of the outburst. We conclude that it is a result of enhanced mass accretion in response to an earlier heating event. We characterized the evolution in the hard-X-ray band and showed that for the joint ISGRI-XRT fits, the e-folding energy decreased from 350 to 130 keV, while the energy where the exponential cut-off starts increased from 75 to 112 keV as the decay progressed. We investigated the claim that high-energy cut-offs disappear with the compact jet turning on during outburst decays, and showed that spectra taken with HEXTE on RXTE provide insufficient quality to characterize cut-offs during the decay for typical hard-X-ray fluxes. Long INTEGRAL monitoring observations are required to understand the relation between the compact jet formation and hard-X-ray behaviour. We found that for the entire decay (including the flare), the X-ray spectra are consistent with thermal Comptonization, but a jet synchrotron origin cannot be ruled out.
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