ABSTRACT
The coupling between radio and X-ray luminosity is an important diagnostic tool to study the connection between the accretion inflow and jet outflow for low-mass X-ray binaries (LMXBs). The ...radio/X-ray correlation for individual neutron star (NS) LMXBs is scattered, whereas for individual black hole (BH) LMXBs a more consistent correlation is generally found. Furthermore, jet quenching is observed for both types of LMXBs, but it is unclear whether jets in NS-LMXBs quench as strongly as those in BH-LMXBs. While additional soft X-ray spectral components can be detected in NS-LMXB spectra due to the presence of the NS surface, disentangling the individual X-ray spectral components has thus far not been considered when studying the radio/X-ray coupling. Here we present eleven epochs of Swift/XRT observations matched with quasi-simultaneous archival radio observations of the 2009 November outburst of Aql X-1. We decompose the thermal and Comptonized spectral components in the Swift/XRT spectra, with the aim of studying whether the presence of additional thermal emission affects the coupling of the radio/X-ray luminosity. We find that there is no evidence of a significant thermal contribution in Swift/XRT spectra that could cause scatter in the radio/X-ray coupling. To explore the role of potential spectral degeneracies in the X-ray models and consider the improvements from including hard X-rays, we perform joint fits with quasi-simultaneous RXTE/PCA spectra. Follow-up research using more sensitive, broad-band X-ray observations and densely sampled near-simultaneous radio observations is required to study this in more detail.
ABSTRACT
Accreting low-mass X-ray binaries (LMXBs) are capable of launching powerful outflows such as accretion disc winds. In disc winds, vast amounts of material can be carried away, potentially ...greatly impacting the binary and its environment. Previous studies have uncovered signatures of disc winds in the X-ray, optical, near-infrared, and recently even the ultraviolet (UV) band, predominantly in LMXBs with large discs (Porb ≥ 20 h). Here, we present the discovery of transient UV outflow features in UW CrB, a high-inclination (i ≥ 77°) neutron star LMXB (NS-LMXB) with an orbital period of only Porb ≈ 111 min. We present P-Cygni profiles identified for Si iv 1400 Å and tentatively for N v 1240 Å in one 15 min exposure, which is the only exposure covering orbital phase ϕ ≈ 0.7–0.8, with a velocity of ≈1500 km s−1. We show that due to the presence of blackbody emission from the neutron star surface and/or boundary layer, a thermal disc wind can be driven despite the short Porb, but explore alternative scenarios as well. The discovery that thermal disc winds may occur in NS-LMXBs with Porb as small as ≈111 min, and can potentially be transient on time-scales as short as ≈15 min, warrants further observational and theoretical work.
ABSTRACT
Many transient and variable sources detected at multiple wavelengths are also observed to vary at radio frequencies. However, these samples are typically biased towards sources that are ...initially detected in wide-field optical, X-ray, or gamma-ray surveys. Many sources that are insufficiently bright at higher frequencies are therefore missed, leading to potential gaps in our knowledge of these sources and missing populations that are not detectable in optical, X-rays, or gamma-rays. Taking advantage of new state-of-the-art radio facilities that provide high-quality wide-field images with fast survey speeds, we can now conduct unbiased surveys for transient and variable sources at radio frequencies. In this paper, we present an unbiased survey using observations obtained by MeerKAT, a mid-frequency (∼GHz) radio array in South Africa’s Karoo Desert. The observations used were obtained as part of a weekly monitoring campaign for X-ray binaries (XRBs) and we focus on the field of MAXI J1820+070. We develop methods to efficiently filter transient and variable candidates that can be directly applied to other data sets. In addition to MAXI J1820+070, we identify four likely active galactic nuclei, one source that could be a Galactic source (pulsar or quiescent XRB) or an AGN, and one variable pulsar. No transient sources, defined as being undetected in deep images, were identified leading to a transient surface density of <3.7 × 10−2 deg−2 at a sensitivity of 1 mJy on time-scales of 1 week at 1.4 GHz.
ABSTRACT
Transient surveys are a vital tool in exploring the dynamic Universe, with radio transients acting as beacons for explosive and highly energetic astrophysical phenomena. However, performing ...commensal transient surveys using radio imaging can require a significant amount of computing power, data storage, and time. With the instrumentation available to us, and with new and exciting radio interferometers in development, it is essential that we develop efficient methods to probe the radio transient sky. In this paper, we present results from a commensal short-duration transient survey, on time-scales of 8 s, 128 s, and 1 h, using data from the MeerKAT radio telescope. The data set used was obtained as part of a galaxy observing campaign, and we focus on the field of NGC 5068. We present a quick, wide-field imaging strategy to enable fast imaging of large data sets, and develop methods to efficiently filter detected transient candidates. No transient candidates were identified on the time-scales of 8 s, 128 s, and 1 h, leading to competitive limits on the transient surface densities of $6.7\, {\times }\, 10^{-5}$, $1.1\, {\times }\, 10^{-3}$, and $3.2\, {\times }\, 10^{-2}$ deg−1 at sensitivities of 56.4, 19.2, and 3.9 mJy following primary beam correction for the respective time-scales. We find one possible candidate that could be associated with a stellar flare, which was rejected due to strict image quality control. Further short time-scale radio observations of this candidate could give definite results about its origin.
ABSTRACT
The evolution of accreting X-ray binary systems is closely coupled to the properties of their donor stars. Consequently, we can constrain the evolutionary track a system is by establishing ...the nature of its donor. Here, we present far-ultraviolet (far-UV) spectroscopy of the transient neutron-star low-mass X-ray binary J1858 in different accretion states (low-hard, high-hard, and soft). All of these spectra exhibit anomalous N v, C iv, Si iv, and He ii lines, suggesting that its donor star has undergone CNO processing. We also determine the donor’s effective temperature, Td ≃ 5700 K, and radius, Rd ≃ 1.7 R⊙, based on photometric observations obtained during quiescence. Lastly, we leverage the transient nature of the system to set an upper limit of $\dot{M}_{\rm acc} \lesssim 10^{-8.5}~{\rm M}_{\odot }~\mathrm{ yr}^{-1}$ on the present-day mass-transfer rate. Combining these with the orbital period of the system, Porb = 21.3 h, we search for viable evolution paths. The initial donor masses in the allowed solutions span the range 1 M⊙ ≲ Md,i ≲ 3.5 M⊙. All but the lowest masses in this range are consistent with the strong CNO-processing signature in the UV line ratios. The present-day donor mass in the permitted tracks are 0.5 M⊙ ≲ Md,obs ≲ 1.3 M⊙, higher than suggested by eclipse modelling. Since Porb is close to the so-called bifurcation period, both converging and diverging binary tracks are permitted. If the former is confirmed, J1858 will end its life as an ultracompact system with a substellar donor.
The coupling between radio and X-ray luminosity is an important diagnostic tool to study the connection between the accretion inflow and jet outflow for low-mass X-ray binaries (LMXBs). When ...comparing NS- and BH-LMXBs, we find that the radio/X-ray correlation for individual NS-LMXBs is scattered, whereas for individual BH-LMXBs a more consistent correlation is generally found. Furthermore, we observe jet quenching for both types of LMXBs, but it is unclear what exactly causes this, and if jets in NS-LMXBs quench as strongly as those in BH-LMXBs. While additional soft X-ray spectral components can be present for NS-LMXBs due to the presence of the neutron star's surface, disentangling the individual X-ray spectral components has thus far not been considered when studying the radio/X-ray coupling. Here we present eleven epochs of Swift/XRT observations matched with quasi-simultaneous archival radio observations of the 2009 November outburst of Aql X-1. We decompose the thermal and Comptonised spectral components in the Swift/XRT spectra, with the aim of studying whether the presence of additional thermal emission affects the coupling of the radio/X-ray luminosity. We find that there is no evidence of a significant thermal contribution in Swift/XRT spectra that could cause scatter in the radio/X-ray coupling. However, a noteworthy finding is that the X-ray observation with the strongest statistically significant thermal component occurs around the same epoch as a bright radio detection. Follow-up research using more sensitive X-ray observations combined with densely sampled near-simultaneous radio observations is required to explore the radio/X-ray coupling for NS-LMXBs in more detail.
Accreting low mass X-ray binaries (LMXBs) are capable of launching powerful outflows such as accretion disc winds. In disc winds, vast amounts of material can be carried away, potentially greatly ...impacting the binary and its environment. Previous studies have uncovered signatures of disc winds in the X-ray, optical, near-infrared, and recently even the UV band, predominantly in LMXBs with large discs (\(P_{orb}{\geq}20\) hrs). Here, we present the discovery of transient UV outflow features in UW CrB, a high-inclination (\(i{\geq}77\)\deg) neutron star LMXB with an orbital period of only \(P_{orb}{\approx}111\) min. We present P-Cygni profiles detected for Si iv 1400Å and tentatively for N v 1240Å in one 15 min exposure, which is the only exposure covering orbital phase \(\phi{\approx}0.7{-}0.8\), with a velocity of \({\approx}1500\) km/s. We show that due to the presence of black body emission from the neutron star surface and/or boundary layer, a thermal disc wind can be driven despite the short \(P_{orb}\), but explore alternative scenarios as well. The discovery that thermal disc winds may occur in NS-LMXBs with \(P_{orb}\) as small as \({\approx}111\) min, and can potentially be transient on time scales as short as \({\approx}15\) min, warrants further observational and theoretical work.
The evolution of accreting X-ray binary systems is closely coupled to the properties of their donor stars. As a result, we can constrain the evolutionary track a system is by establishing the nature ...of its donor. Here, we present far-UV spectroscopy of the transient neutron-star low-mass X-ray binary Swift J1858 in three different accretion states (low-hard, high-hard and soft). All of these spectra exhibit anomalous N\,{\sc v}, C\,{\sc iv}, Si\,{\sc iv} and He\,{\sc ii} lines, suggesting that its donor star has undergone CNO processing. We also determine the donor's effective temperature, \(T_{d} \simeq 5700\)~K, and radius, \(R_d \simeq 1.7~R_{\odot}\), based on photometric observations obtained during quiescence. Lastly, we leverage the transient nature of the system to set an upper limit of \(\dot{M}_{\rm acc} \lesssim 10^{-8.5}~M_{\odot}~yr^{-1}\) on the present-day mass-transfer rate. Combining all these with the orbital period of the system, \(P_{\rm orb} = 21.3\)~hrs, we search for viable evolution paths. The initial donor masses in the allowed solutions span the range \(1~M_{\odot} \lesssim M_{d,i} \lesssim 3.5~M_{\odot}\). All but the lowest masses in this range are consistent with the strong CNO-processing signature in the UV line ratios. The present-day donor mass in the permitted tracks are \(0.5~M_{\odot}\lesssim M_{d,obs} \lesssim 1.3~M_{\odot}\), higher than suggested by recent eclipse modelling. Since \(P_{\rm orb}\) is close to the so-called bifurcation period, both converging and diverging binary tracks are permitted. If Swift J1858 is on a converging track, it will end its life as an ultra-compact system with a sub-stellar donor star.
Transient surveys are a vital tool in exploring the dynamic universe, with radio transients acting as beacons for explosive and highly energetic astrophysical phenomena. However, performing commensal ...transient surveys using radio imaging can require a significant amount of computing power, data storage and time. With the instrumentation available to us, and with new and exciting radio interferometers in development, it is essential that we develop efficient methods to probe the radio transient sky. In this paper, we present results from an commensal short duration transient survey, on time scales of 8 seconds, 128 seconds and 1 hour, using data from the MeerKAT radio telescope. The dataset used was obtained as part of a galaxy observing campaign, and we focus on the field of NGC 5068. We present a quick, wide field imaging strategy to enable fast imaging of large datasets, and develop methods to efficiently filter detected transient candidates. No transient candidates were identified on the time scales of 8 seconds, 128 seconds and 1 hour, leading to competitive limits on the transient surface densities of \(6.7{\times}10^{-5}\) deg\(^{-1}\), \(1.1{\times}10^{-3}\) deg\(^{-1}\), and \(3.2{\times}10^{-2}\) deg\(^{-1}\) at sensitivities of 56.4 mJy, 19.2 mJy, and 3.9 mJy for the respective time scales. We find one possible candidate that could be associated with a stellar flare, that was rejected due to strict image quality control. Further short time-scale radio observations of this candidate could give definite results to its origin.
Many transient and variable sources detected at multiple wavelengths are also observed to vary at radio frequencies. However, these samples are typically biased towards sources that are initially ...detected in wide-field optical, X-ray or gamma-ray surveys. Many sources that are insufficiently bright at higher frequencies are therefore missed, leading to potential gaps in our knowledge of these sources and missing populations that are not detectable in optical, X-rays or gamma-rays. Taking advantage of new state-of-the-art radio facilities that provide high quality wide-field images with fast survey speeds, we can now conduct unbiased surveys for transient and variable sources at radio frequencies. In this paper, we present an unbiased survey using observations obtained by MeerKAT, a mid-frequency (\(\sim\)1.4 GHz) radio array in South Africa's Karoo Desert. The observations used were obtained as part of a weekly monitoring campaign for X-ray binaries (XRBs) and we focus on the field of MAXI J1820+070. We develop methods to optimally filter transient and variable candidates that can be directly applied to other datasets. In addition to MAXI J1820+070, we identify four likely active galactic nuclei, one source that could be a Galactic source (pulsar or quiescent X-ray binary) or an AGN, and one variable pulsar. No transient sources, defined as being undetected in deep images, were identified leading to a transient surface density of \(<3.7\times10^{-2}\) deg\(^{-2}\) at a sensitivity of 1 mJy on timescales of one week at 1.4 GHz.
