Abstract Fast X-ray Transients (FXTs) are extragalactic bursts of soft X-rays first identified ≳10 yr ago. Since then, nearly 40 events have been discovered, although almost all of these have been ...recovered from archival Chandra and XMM-Newton data. To date, optical sky surveys and follow-up searches have not revealed any multiwavelength counterparts. The Einstein Probe, launched in 2024 January, has started surveying the sky in the soft X-ray regime (0.5–4 keV) and will rapidly increase the sample of FXTs discovered in real time. Here we report the first discovery of both an optical and radio counterpart to a distant FXT, the fourth source publicly released by the Einstein Probe. We discovered a fast-fading optical transient within the 3′ localization radius of EP 240315a with the all-sky optical survey ATLAS, and our follow-up Gemini spectrum provides a redshift, z = 4.859 ± 0.002. Furthermore, we uncovered a radio counterpart in the S band (3.0 GHz) with the MeerKAT radio interferometer. The optical (rest-frame UV) and radio luminosities indicate that the FXT most likely originates from either a long gamma-ray burst or a relativistic tidal disruption event. This may be a fortuitous early mission detection by the Einstein Probe or may signpost a mode of discovery for high-redshift, high-energy transients through soft X-ray surveys, combined with locating multiwavelength counterparts.
Abstract
We present the results of a photometric and spectroscopic analysis of the Galactic Bulge Survey X-ray source CXOGBS J174954.5−294335 (hereafter, referred to as CX19). CX19 is a long period, ...eclipsing intermediate polar-type cataclysmic variable with broad, single-peaked Balmer and Paschen emission lines along with He ii λ4686 and Bowen blend emission features. With coverage of one full and two partial eclipses and archival photometry, we determine the ephemeris for CX19 to be HJD(eclipse) = 2455691.8581(5) + 0.358704(2) × N. We also recovered the white dwarf spin period of P
spin = 503.32(3) s that gives a P
spin/P
orb = 0.016(6), comparable to several confirmed, long-period intermediate polars. CX19 also shows a clear X-ray eclipse in the 0.3–8.0 keV range observed with Chandra. Two optical outbursts were observed lasting between 6 and 8 h (lower limits) reaching ∼1.3 mag in amplitude. The outbursts, both in duration and magnitude, the accretion disc-dominated spectra and hard X-ray emission are reminiscent of the intermediate polar V1223 Sgr sharing many of the same characteristics. If we assume a main-sequence companion, we estimate the donor to be an early G-type star and find a minimum distance of d ≈ 2.1 kpc and a 0.5–10.0 keV X-ray luminosity upper limit of 2.0 × 1033 erg s−1. Such an X-ray luminosity is consistent with a white dwarf accretor in a magnetic cataclysmic variable system. To date, CX19 is only the second deeply eclipsing intermediate polar with X-ray eclipses and the first that is optically accessible.
ABSTRACT
We present the results of a multiwavelength follow-up campaign for the luminous nuclear transient Gaia16aax, which was first identified in 2016 January. The transient is spatially consistent ...with the nucleus of an active galaxy at z = 0.25, hosting a black hole of mass ${\sim }6\times 10^8\, \mathrm{M}_\odot$. The nucleus brightened by more than 1 mag in the Gaia G band over a time-scale of less than 1 yr, before fading back to its pre-outburst state over the following 3 yr. The optical spectra of the source show broad Balmer lines similar to the ones present in a pre-outburst spectrum. During the outburst, the H α and H β emission lines develop a secondary peak. We also report on the discovery of two transients with similar light-curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by a microlensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disc. We consider variability in the accretion flow in the inner part of the disc, or a tidal disruption event of a star ${\ge } 1 \, \mathrm{M}_{\odot }$ by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria.
Optical/near-infrared (optical/NIR, OIR) light from low-mass neutron star X-ray binaries (NSXBs) in outburst is traditionally thought to be thermal emission from the accretion disc. Here we present a ...comprehensive collection of quasi-simultaneous OIR and X-ray data from 19 low magnetic field NSXBs, including new observations of three sources: 4U 0614+09, LMC X−2 and GX 349+2. The average radio–OIR spectrum for NSXBs is α≈+ 0.2 (where Lν∝να) at least at high luminosities when the radio jet is detected. This is comparable to, but slightly more inverted than the α≈ 0.0 found for black hole X-ray binaries. The OIR spectra and relations between OIR and X-ray fluxes are compared to those expected if the OIR emission is dominated by thermal emission from an X-ray or viscously heated disc, or synchrotron emission from the inner regions of the jets. We find that thermal emission due to X-ray reprocessing can explain all the data except at high luminosities for some NSXBs, namely, the atolls and millisecond X-ray pulsars. Optically thin synchrotron emission from the jets (with an observed OIR spectral index of αthin < 0) dominate the NIR light above and the optical above in these systems. For NSXB Z-sources, the OIR observations can be explained by X-ray reprocessing alone, although synchrotron emission may make a low-level contribution to the NIR, and could dominate the OIR in one or two cases.
A short year 2005 review of ultra-compact binaries, focused on ultra-compact X-ray binaries, is followed by a discussion of results of our VLT campaign to obtain optical spectra of (candidate) ...ultra-compact X-ray binaries. We find evidence for carbon/oxygen as well as helium/nitrogen discs and no evidence for (traces) of hydrogen. This suggests that the donors in the observed systems are white dwarfs. However, we also find large differences between the two C/O discs of which we have good spectra, which highlights the need for a better understanding of the optical spectra.
ABSTRACT We present the results of a high angular resolution (1.1 arcsec) and sensitivity (maximum of ∼0.1 mJy) radio survey at 1–2 GHz in the Galactic Bulge. This complements the X-ray Chandra ...Galactic Bulge Survey, and investigates the full radio source population in this dense Galactic region. Radio counterparts to sources at other wavelengths can aid in classification, as there are relatively few types of objects that are reasonably detectable in radio at kiloparsec distances, and even fewer that are detected in both X-rays and radio. This survey covers about 3 sq deg of the Galactic Bulge Survey area (spanning the Galactic coordinate range of −3° < l < +3° and +1.6° < b < +2.1°) as a first look into this region of the Galaxy with this combination of frequency, resolution, and sensitivity. Spectral indices within the observed band of 1–2 GHz were calculated for each source to assist in determining its emission mechanism. We find 1617 unique sources in the survey, 25 of which are radio counterparts to X-ray sources, and about 100 of which are steep-spectrum (α ≲ −1.4) point sources that are viable pulsar candidates. Four radio sources are of particular interest: a compact binary; an infrared transient with an inverted radio spectrum; a potential transitional millisecond pulsar candidate; and a very steep spectrum radio source with an X-ray and bright infrared counterpart. We discuss other notable sources, including possible radio transients, potential new planetary nebulae, and active galactic nuclei.
Thermal X-ray radiation from neutron star soft X-ray transients in quiescence provides the strongest constraints on the cooling rates of neutron stars and thus on the interior composition and ...properties of matter in the cores of neutron stars. We analyze new (2006) and archival (2001) XMM-Newton observations of the accreting millisecond pulsar SAX J1808.4-3658 in quiescence, which provide the most stringent constraints to date. The X-ray spectrum of SAX J1808.4-3658 in the 2006 observation is consistent with a power law of photon index 1.83 plus or minus 0.17, without requiring the presence of a blackbody-like component from a neutron star atmosphere. Our 2006 observation shows a slightly lower 0.5-10 keV X-ray luminosity, at a level of unk of that inferred from the 2001 observation. Simultaneous fitting of all available XMM-Newton data allows a constraint on the quiescent neutron star (0.01-10 keV) luminosity of L sub(NS) < 1.1 x 10 super(30) ergs s super(-1). This limit excludes some current models of neutrino emission mediated by pion condensates and provides further evidence of additional cooling processes, such as neutrino emission via direct Urca processes involving nucleons and/or hyperons, in the cores of massive neutron stars.
Abstract
In outburst, neutron star X-ray binaries produce less powerful jets than black holes at a given X-ray luminosity. This has made them more difficult to study as they fade towards quiescence. ...To explore whether neutron stars power jets at low accretion rates (L
X ≲ 1036 erg s−1), we investigate the radio and X-ray properties of three accreting millisecond X-ray pulsars (IGR J17511−3057, SAX J1808.4−3658 and IGR J00291+5934) during their outbursts in 2015, and of the non-pulsing neutron star Cen X−4 in quiescence (2015) and in outburst (1979). We did not detect the radio counterpart of IGR J17511−3057 in outburst or of Cen X−4 in quiescence, but did detect IGR J00291+5934 and SAX J1808.4−3658, showing that at least some neutron stars launch jets at low accretion rates. While the radio and X-ray emission in IGR J00291+5934 seem to be tightly correlated, the relationship in SAX J1808.4−3658 is more complicated. We find that SAX J1808.4−3658 produces jets during the reflaring tail, and we explore a toy model to ascertain whether the radio emission could be attributed to the onset of a strong propeller. The lack of a universal radio/X-ray correlation, with different behaviours in different neutron star systems (with various radio/X-ray correlations; some being radio faint and others not), points at distinct disc–jet interactions in individual sources, while always being fainter in the radio band than black holes at the same X-ray luminosity.
ABSTRACT
The electromagnetic counterparts to gravitational wave (GW) merger events are highly sought after, but difficult to find owing to large localization regions. In this study, we present a ...strategy to search for compact object merger radio counterparts in wide-field data collected by the Low-Frequency Array (LOFAR). In particular, we use multi-epoch LOFAR observations centred at 144 MHz spanning roughly 300 deg2 at optimum sensitivity of a since retracted neutron star–black hole merger candidate detected during O2, the second Advanced Ligo–Virgo GW observing run. The minimum sensitivity of the entire (overlapping) 1809 deg2 field searched is 50 mJy and the false negative rate is 0.1 per cent above 200 mJy. We do not find any transients and thus place an upper limit at 95 per cent confidence of 0.02 transients per square degree above 20 mJy on one, two, and three month time-scales, which are the most sensitive limits available to date. Finally, we discuss the prospects of observing GW events with LOFAR in the upcoming GW observing run and show that a single multibeam LOFAR observation can probe the full projected median localization area of binary neutron star mergers down to a median sensitivity of at least 8 mJy.
ABSTRACT
We present two contiguous nights of simultaneous time-resolved Gran Telescopio Canarias spectroscopy and William Herschel Telescope photometry of the black hole X-ray transient XTE ...J1859+226, obtained in 2017 July during quiescence. Cross-correlation of the individual spectra against a late K-type spectral template enabled us to constrain the orbital period to 0.276 ± 0.003 d and the radial velocity semi-amplitude of the donor star to K2 = 550 ± 59 km s−1. An ellipsoidal modulation is detected in the photometric r- and i-band light curves, although it is strongly contaminated by flickering activity. By exploiting correlations between the properties of the double-peaked H α emission-line profile and the binary parameters, we derived an orbital inclination of 66.6 ± 4.3 deg, a refined K2 = 562 ± 40 km s−1 and mass ratio q = M2/M1 = 0.07 ± 0.01. From these values, we obtained an updated black hole mass of M1 = 7.8 ± 1.9 M⊙. An independent mass estimate based on X-ray timing agrees well with our value, which gives further support for the outburst quasi-periodic oscillation triplet being explained by the relativistic precession model. We also obtained a companion star mass M2 = 0.55 ± 0.16 M⊙, which is consistent with its K5-K7 V spectral type.
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