Aims. Despite recent progress in the field, there are still many open questions regarding γ-ray binaries. In this paper we provide an overview of non-transient γ-ray binaries and discuss how ...observations with the Cherenkov Telescope Array (CTA) will contribute to their study. Methods. We simulated the spectral behaviour of the non-transient γ-ray binaries using archival observations as a reference. With this we tested the CTA capability to measure the spectral parameters of the sources and detect variability on various timescales. Results. We review the known properties of γ-ray binaries and the theoretical models that have been used to describe their spectral and timing characteristics. We show that the CTA is capable of studying these sources on timescales comparable to their characteristic variability timescales. For most of the binaries, the unprecedented sensitivity of the CTA will allow studying the spectral evolution on a timescale as short as 30 min. This will enable a direct comparison of the TeV and lower energy (radio to GeV) properties of these sources from simultaneous observations. We also review the source-specific questions that can be addressed with these high-accuracy CTA measurements.
ABSTRACT
Neutron stars accreting from OB supergiants are often divided between persistently and transiently accreting systems, called supergiant X-ray binaries (SgXBs) and supergiant fast X-ray ...transients (SFXTs). This dichotomy in accretion behaviour is typically attributed to systematic differences in the massive stellar wind, binary orbit, or magnetic field configuration, but direct observational evidence for these hypotheses remains sparse. To investigate their stellar winds, we present the results of pilot 100-GHz observations of one SFXT and one SgXB with the Northern Extended Millimetre Array. The SFXT, IGR J18410-0535, is detected as a point source at 63.4 ± 9.6 μJy, while the SgXB, IGR J18410-0535 remains undetected. Radio observations of IGR J18410-0535 imply a flat or inverted low-frequency spectrum, arguing for wind emission and against non-thermal flaring. Due to the uncertain SFXT distance, however, the observations do not necessarily imply a difference between the wind properties of the SFXT and SgXB. We compare the mm constraints with other HMXBs and isolated OB supergiants, before considering how future mm campaigns can constrain HMXB wind properties by including X-ray measurements. Specifically, we discuss caveats and future steps to successfully measure wind mass-loss rates and velocities in HMXBs with coordinated mm, radio, and X-ray campaigns.
ABSTRACT We report on a 40 ks long, uninterrupted X-ray observation of the candidate supergiant fast X-ray transient (SFXT) IGRJ16418-4532 performed with XMM-Newton on 2011 February 23. This ...high-mass X-ray binary lies in the direction of the Norma arm, at an estimated distance of 13kpc. During the observation, the source showed strong variability exceeding two orders of magnitudes, never observed before from this source. Its X-ray flux varied in the range from 0.1 to 15 counts s-1, with several bright flares of different durations (from a few hundred to a few thousand seconds) and sometimes with a quasi-periodic behaviour. This finding supports the previous suggestion that IGRJ16418-4532 is a member of the SFXT class. In our new observation we measured a pulse period of 1212 ± 6s, thus confirming that this binary contains a slowly rotating neutron star. During the periods of low luminosity the source spectrum is softer and more absorbed than during the flares. A soft excess is present below 2keV in the cumulative flare spectrum, possibly due to ionized wind material at a distance similar to the neutron star accretion radius. The kind of X-ray variability displayed by IGRJ16418-4532, its dynamic range and time-scale, together with the sporadic presence of quasi-periodic flaring, are all suggestive of a transitional accretion regime between pure wind accretion and full Roche lobe overflow. We discuss here for the first time this hypothesis to explain the behaviour of IGRJ16418-4532 and, possibly, of other SFXTs with short orbital periods.
We analysed data from five XMM-Newton observations of GX 13 + 1 to investigate the variability of the photo-ionised absorber in this source. We fitted EPIC and RGS spectra obtained from the ...“least-variable” intervals with a model consisting of disc-blackbody and blackbody components together with a Gaussian emission feature at ~6.55–6.7 keV modified by absorption due to cold and photo-ionised material. We found a significant correlation between the hard, ~6–10 keV, flux, the ionisation and column density of the absorber and the equivalent width of the broad iron line. We interpret the correlation in a scenario in which a disc wind is thermally driven at large, ~1010 cm, radii and the broad line results from reprocessed emission in the wind and/or hot atmosphere. The breadth of the emission line is naturally explained by a combination of scattering, recombination and fluorescence processes. We attribute the variations in the absorption and emission along the orbital period to the view of different parts of the wind, possibly located at slightly different inclination angles. We constrain the inclination of GX 13 + 1 to be between 60 and 80° from the strong absorption in the line of sight, which obscures up to 80% of the total emission in one observation, and the absence of eclipses. We conclude that either a disc wind and/or a hot atmosphere can explain the current observations of narrow absorption and broad iron emission features in neutron star low-mass X-ray binaries as a class.
We investigate the nature of CXOU\,J005440.5-374320 ( a peculiar bright ($ and soft X-ray transient in the spiral galaxy NGC\,300 with a six-hour periodic flux modulation that was detected in a 2014 ...observation. Subsequent observations with and as well as a large observational campaign of NGC\,300 and its sources performed with the Swift Neil Gehrels Observatory showed that this source exhibits recurrent flaring activity: four other outbursts were detected across sim 8 years of monitoring. Using data from the archive and from the and catalogues, we determined that the source is likely associated with a bright blue optical/ultraviolet counterpart. This prompted us to perform follow-up observations with the Southern African Large Telescope in December 2019. With the multi-wavelength information at hand, we discuss several possibilities for the nature of Although none is able to account for the full range of the observed peculiar features, we found that the two most promising scenarios are a stellar-mass compact object in a binary system with a Wolf--Rayet star companion, or the recurrent tidal stripping of a stellar object trapped in a system with an intermediate-mass (sim 1000\,M$_ black hole.
We report the results provided by the XMM-Newton observation of the X-ray binary pulsar SXP59.0 during its most recent outburst in April 2017. The source was detected at fX(0.2–12 keV) = 8 × 10−11erg ...cm−2 s−1, one of its highest flux levels reported to date. The measured pulse period was Pspin = 58.949(1) s, very similar to the periods measured in most of the previous observations. The pulsed emission was clearly detected over the whole energy range between 0.2 and 12 keV, but the pulse profile is energy dependent and the pulsed fraction increases as the energy increases. Although the time-averaged EPIC spectrum is dominated by a power-law component (with photon index Γ = 0.76 ± 0.01), the data show an evident soft excess, which can be described with the sum of a black-body and a hot thermal plasma component (with temperatures kTBB = 171+11−14 k T BB = 171 − 14 + 11 $ kT_{\rm BB} = 171^{+11}_{-14} $ eV and kTAPEC = 1.09+0.16−0.09 k T APEC = 1.09 − 0.09 + 0.16 $ kT_{\rm APEC} = 1.09^{+0.16}_{-0.09} $ keV, respectively). Moreover, the EPIC and RGS spectra show narrow emission lines due to N, O, Ne, Mg, and Fe. The phase-resolved spectral analysis of the EPIC data shows that the flux of the black-body component varies with the pulse phase, while the plasma component is almost constant. We show that the black-body component can be attributed to the reprocessing of the primary emission by the optically thick material at the inner edge of the accretion disc, while the hot plasma component is due to a diffuse gas far from the accretion region and the narrow emission lines of the RGS spectrum are most probably due to photoionized matter around the accreting source.
Abstract
We present the results from INTEGRAL and Swift/XRT observations of the hitherto poorly studied unidentified X-ray transient AX J1949.8+2534, and on archival multiwavelength observations of ...field objects. Bright hard X-ray outbursts have been discovered above 20 keV for the first time, the measured duty cycle and dynamic range are of the order of ∼4 per cent and ≥ 630, respectively. The source was also detected during a low soft X-ray state (∼2 × 10−12 erg cm−2 s−1) thanks to a Swift/XRT followup, which allowed for the first time to perform a soft X-ray spectral analysis as well as significantly improve the source positional uncertainty from arcminute to arcsecond size. From archival near-infrared data, we pinpointed two bright objects as most likely counterparts whose photometric properties are compatible with an early-type spectral nature. This strongly supports a high-mass X-ray binary (HMXB) scenario for AX J1949.8+2534, specifically a Supergiant Fast X-ray Transient (more likely) or alternatively a Be HMXB.
We report on a 40 ks long, uninterrupted X-ray observation of the candidate supergiant fast X-ray transient (SFXT) IGR J16418−4532 performed with XMM-Newton on 2011 February 23. This high-mass X-ray ...binary lies in the direction of the Norma arm, at an estimated distance of 13 kpc. During the observation, the source showed strong variability exceeding two orders of magnitudes, never observed before from this source. Its X-ray flux varied in the range from ∼0.1 to 15 counts s−1, with several bright flares of different durations (from a few hundred to a few thousand seconds) and sometimes with a quasi-periodic behaviour. This finding supports the previous suggestion that IGR J16418−4532 is a member of the SFXT class. In our new observation we measured a pulse period of 1212 ± 6 s, thus confirming that this binary contains a slowly rotating neutron star. During the periods of low luminosity the source spectrum is softer and more absorbed than during the flares. A soft excess is present below 2 keV in the cumulative flare spectrum, possibly due to ionized wind material at a distance similar to the neutron star accretion radius. The kind of X-ray variability displayed by IGR J16418−4532, its dynamic range and time-scale, together with the sporadic presence of quasi-periodic flaring, are all suggestive of a transitional accretion regime between pure wind accretion and full Roche lobe overflow. We discuss here for the first time this hypothesis to explain the behaviour of IGR J16418−4532 and, possibly, of other SFXTs with short orbital periods.
We report on the discovery with Chandra of a strong modulation (~90 per cent pulsed fraction) at ~6.4 h from the source CXOU J123030.3+413853 in the star-forming, low-metallicity spiral galaxy NGC ...4490, which is interacting with the irregular companion NGC 4485. This modulation, confirmed also by XMM-Newton observations, is interpreted as the orbital period of a binary system. The spectra from the Chandra and XMM-Newton observations can be described by a power-law model with photon index ... During these observations, which span from 2000 November to 2008 May, the source showed a long-term luminosity variability by a factor of ~5, between ~2 x ... and 1.1 x ... erg s... (for a distance of 8 Mpc). The maximum X-ray luminosity, exceeding by far the Eddington limit of a neutron star, indicates that the accretor is a black hole. Given the high X-ray luminosity, the short orbital period and the morphology of the orbital light curve, we favour an interpretation of CXOU J123030.3+413853 as a rare high-mass X-ray binary system with a Wolf-Rayet star as a donor, similar to Cyg X-3. This would be the fourth system of this kind known in the local Universe. CXOU J123030.3+413853 can also be considered as a transitional object between high-mass X-ray binaries and ultraluminous X-ray sources (ULXs), the study of which may reveal how the properties of persistent black hole binaries evolve entering the ULX regime. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
We report on the results of the XMM–Newton observation of IGR J01572−7259 during its most recent outburst in 2016 May, the first since 2008. The source reached a flux f ∼ 10−10 erg cm−2 s−1, ...which allowed us to perform a detailed analysis of its timing and spectral properties. We obtained a pulse period Pspin = 11.58208(2) s. The pulse profile is double peaked and strongly energy dependent, as the second peak is prominent only at low energies and the pulsed fraction increases with energy. The main spectral component is a power-law model, but at low energies, we also detected a soft thermal component, which can be described with either a blackbody or a hot plasma model. Both the EPIC and RGS spectra show several emission lines, which can be identified with the transition lines of ionized N, O, Ne, and Fe and cannot be described with a thermal emission model. The phase-resolved spectral analysis showed that the flux of both the soft excess and the emission lines vary with the pulse phase: the soft excess disappears in the first pulse and becomes significant only in the second, where also the Fe line is stronger. This variability is difficult to explain with emission from a hot plasma, while the reprocessing of the primary X-ray emission at the inner edge of the accretion disc provides a reliable scenario. On the other hand, the narrow emission lines can be due to the presence of photoionized matter around the accreting source.
PDF
