Archival X-ray Observations Research Articles

The Radio Halo in PLCKESZ G171.94–40.65: Beacon of Merging Activity

We present the first multifrequency analysis of the candidate ultrasteep spectrum radio halo in the galaxy cluster PLCKESZ G171.94−40.65, using the upgraded Giant Metrewave Radio telescope (400 MHz), and Karl G. Jansky Very Large Array (1–2 GHz) observations. Our radio data have been complemented with archival Chandra X-ray observations to provide a crucial insight into the complex intracluster medium physics, happening at large scales. We detect the radio halo emission to the extent of ∼1.5 Mpc at 400 MHz, significantly larger than previously reported, along with five tailed galaxies in the central region. We also report the discovery of an unknown diffuse source “U,” at the cluster periphery, with an extent of 300 kpc. Using the available observations, we have found that the radio spectrum of the halo is well-fitted with a single power law, having a spectral index of −1.36 ± 0.05, indicating that it is not an ultrasteep spectrum radio halo. Our low-resolution (25″) resolved spectral map shows an overall uniform spectral index, with some patches of fluctuations. The X-ray and radio surface brightness are morphologically cospatial, with a slight extension along the northwest–southeast direction, seen in both maps. The radio and X-ray surface brightness indicates strong positive correlations, with sublinear correlation slopes (∼0.71). Multiple tailed galaxies and the radio halo indicate a high dynamical activity at the cluster central region.

AXES-2MRS: A new all-sky catalogue of extended X-ray galaxy groups

Context. Understanding baryonic physics at the galaxy-group level is a prerequisite for cosmological studies of the large-scale structure. One poorly understood aspect of galaxy groups is related to the properties of their hot intragroup medium. The well-studied X-ray groups have strong cool cores by which they were selected, so expanding the selection of groups is currently an important avenue in uncovering the diversity within the galaxy group population. Aims. We present a new all-sky catalogue of X-ray-detected groups (AXES-2MRS) based on the identification of large X-ray sources found in the ROSAT All-Sky Survey (RASS) with the Two Micron Redshift Survey (2MRS) Bayesian Group Catalogue. We studied the basic properties of these galaxy groups to gain insights into the effect of different group selections on the properties. Methods. In addition to X-ray luminosity from shallow survey data of RASS, we obtained detailed X-ray properties of the groups by matching the AXES-2MRS catalogue to archival X-ray observations by XMM-Newton and complemented this by adding the published XMM-Newton results on galaxy clusters in our catalogue. We analysed temperature and density to the lowest overdensity accessible by the data, obtaining hydrostatic mass estimates at a uniform overdensity of 10 000 times the critical, M10 000, and comparing them to the velocity dispersions of the groups. We explored the relationship between X-ray and optical properties of AXES-2MRS groups through the σv−LX, σv−kT, kT−LX, σv−M, and c200−LX scaling relations. Results. We find a large spread in the central mass ( M10 000), measured by XMM-Newton, to virial mass (M200), inferred by the velocity dispersion, ratios for galaxy groups. This can either indicate that large non-thermal pressure of galaxy groups affects our X-ray mass measurements or the effect of a diversity of halo concentrations on the X-ray properties of galaxy groups. Previous catalogues based on detecting the peak of the X-ray emission preferentially sample the high-concentration groups. In contrast, our new catalogue uncovered many low-concentration groups, completely revising our understanding of X-ray groups.

Unveiling the short and faint X-ray transient nature of IGR J17419-2802

We report new X-ray results from the INTErnational Gamma-Ray Astrophysics Laboratory ( and observations of the hitherto poorly studied unidentified X-ray transient We studied in detail the temporal, spectral, and energetic properties of three hard X-ray outbursts detected above 20 keV by They are all characterized by an average X-ray luminosity of 3times 1035 erg $ and a constrained duration of a few days. This marks a peculiarly short and faint X-ray transient nature for From archival unpublished soft X-ray observations, we found that the source spends most of the time undetected at very low X-ray fluxes (down to $<4.7 $ erg cm $^ $ s$^ $) for a dynamic range $>$2,000 when in outburst. We provided an accurate arcsecond-sized source error circle. Inside it, we pinpointed the best candidate near-infrared counterpart whose photometric properties are compatible with a late-type spectral nature. Based on our new findings, we suggest that is a new member of the very faint X-ray transients (VFXTs) class. Detailed investigations of VFXT outbursts above 20 keV are particularly rare. In this respect, our reported outbursts are among the best studied to date; in particular, their constrained duration of a few days is among the shortest ever measured for a VFXT. X-rays: binaries: individual: src

A Chandra search for periodic X-ray sources in the bulge of M31

ABSTRACT We present a systematic search for periodic X-ray sources in the bulge of M31, using ∼2 Ms of archival Chandra observations spanning a temporal baseline of 16 yr. Utilizing the Gregory–Loredo algorithm that is designed for photon-counting, phase-folded light curves, we detect seven periodic X-ray sources, among which four are newly discovered. Three of these sources are novae, the identified periods of which range between 1.3 and 2.0 h and are most likely the orbital period. The other four sources are low-mass X-ray binaries, the identified periods of which range between 0.13 and 19.3 h and are also likely orbital due to a clear eclipsing/dipping behaviour in the light curve. We address implications on the X-ray binaries population of the M31 bulge. Our study demonstrates the potential of using archival X-ray observations to systematically identify periodic X-ray sources in external galaxies, which would provide valuable information about the underlying exotic stellar populations.

An extreme ultra-compact X-ray binary in a globular cluster: multiwavelength observations of RZ 2109 explored in a triple system framework

ABSTRACT The globular cluster ultraluminous X-ray source, RZ 2109, is a complex and unique system that has been detected at X-ray, ultraviolet, and optical wavelengths. Based on almost 20 yr of Chandra and XMM–Newton observations, the X-ray luminosity exhibits order of magnitude variability, with the peak flux lasting on the order of a few hours. We perform robust time series analysis on the archival X-ray observations and find that this variability is periodic on a time-scale of 1.3 ± 0.04 d. The source also demonstrates broad [O iii] λ5007 emission, which has been observed since 2004, suggesting a white dwarf donor and therefore an ultra-compact X-ray binary. We present new spectra from 2020 and 2022, marking 18 yr of observed [O iii] emission from this source. Meanwhile, we find that the globular cluster counterpart is unusually bright in the NUV/UVW2 band. Finally, we discuss RZ 2109 in the context of the eccentric Kozai–Lidov mechanism and show that the observed 1.3 d periodicity can be used to place constraints on the tertiary configuration, ranging from 20 min (for a 0.1 M⊙ companion) to approximately 95 min (for a 1 M⊙ companion), if the eccentric Kozai–Lidov mechanism is at the origin of the periodic variability.

Monitoring the X-Ray Variability of Bright X-Ray Sources in M33

We present a new five-epoch Chandra X-ray Observatory monitoring survey of the nearby spiral galaxy M33 which probes X-ray variability with time sampling between two weeks and four months. We characterize the X-ray variability of 55 bright point sources outside of the nucleus, many of which are expected to be high-mass X-ray binaries (HMXBs). We detect eight new candidate transients not detected in previous X-ray catalogs of M33 and discuss their possible nature. The final catalog includes 26 known HMXB candidates identified in the literature. We extend the baseline of the X-ray light curves up to 21 yr by including archival X-ray observations of these sources. We compare the detection and nondetection epochs of the sources to suites of simulated source duty cycles and infer that most of our detected sources have duty cycles >30%. We find only four sources whose detection patterns are consistent with having duty cycles below 30%. This large fraction of sources with high duty cycles is unexpected for a population of HMXBs; thus more frequent X-ray monitoring will likely reveal many more low duty cycle HMXBs in M33.

Powerful Radio Sources in the Southern Sky. II. A Swift X-Ray Perspective

We recently constructed the G4Jy-3CRE, a catalog of extragalactic radio sources based on the GLEAM 4-Jy (G4Jy) sample, with the aim of increasing the number of powerful radio galaxies and quasars with similar selection criteria to those of the revised release of the Third Cambridge Catalog (3CR). The G4Jy-3CRE consists of a total of 264 radio sources mainly visible from the Southern Hemisphere. Here, we present an initial X-ray analysis of 89 G4Jy-3CRE radio sources with archival X-ray observations from the Neil Gehrels Swift Observatory. We reduced a total of 624 Swift observations, for about 0.9 Ms of integrated exposure time. We found X-ray counterparts for 59 radio sources belonging to the G4Jy-3CRE, nine of them showing extended X-ray emission. The remaining 30 sources do not show any X-ray emission associated with their radio cores. Our analysis demonstrates that X-ray snapshot observations, even if lacking uniform exposure times, as those carried out with Swift, allow us to (i) verify and/or refine the host galaxy identification; (ii) discover the extended X-ray emission around radio galaxies of the intracluster medium when harbored in galaxy clusters, as the case of G4Jy 1518 and G4Jy 1664; and (iii) detect X-ray radiation arising from their radio lobes, as for G4Jy 1863.

Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and τ Cet

During the first half of their main-sequence lifetimes, stars rapidly lose angular momentum to their magnetized winds, a process known as magnetic braking. Recent observations suggest a substantial decrease in the magnetic braking efficiency when stars reach a critical value of the Rossby number, the stellar rotation period normalized by the convective overturn timescale. Cooler stars have deeper convection zones with longer overturn times, reaching this critical Rossby number at slower rotation rates. The nature and timing of the transition to weakened magnetic braking have previously been constrained by several solar analogs and two slightly hotter stars. In this Letter, we derive the first direct constraints from stars cooler than the Sun. We present new spectropolarimetry of the old G8 dwarf τ Cet from the Large Binocular Telescope, and we reanalyze a published Zeeman Doppler image of the younger G8 star 61 UMa, yielding the large-scale magnetic field strengths and morphologies. We estimate mass-loss rates using archival X-ray observations and inferences from Lyα measurements, and we adopt other stellar properties from asteroseismology and spectral energy distribution fitting. The resulting calculations of the wind braking torque demonstrate that the rate of angular momentum loss drops by a factor of 300 between the ages of these two stars (1.4–9 Gyr), well above theoretical expectations. We summarize the available data to help constrain the value of the critical Rossby number, and we identify a new signature of the long-period detection edge in recent measurements from the Kepler mission.

The Arches cluster revisited – IV. Observational constraints on the binary properties of very massive stars

ABSTRACT Serving as the progenitors of electromagnetic and gravitational-wave transients, massive stars have received renewed interest in recent years. However, many aspects of their birth and evolution remain opaque, particularly in the context of binary interactions. The centre of our Galaxy hosts a rich cohort of very massive stars, which appear to play a prominent role in the ecology of the region. In this paper, we investigate the binary properties of the Arches cluster, which is thought to host a large number of very massive stars. A combination of multi-epoch near-IR spectroscopy and photometry was utilized to identify binaries. 13 from 36 cluster members meet our criteria to be classed as RV variable. Combining the spectroscopic data with archival radio and X-ray observations – to detect colliding wind systems – provides a lower limit to the binary fraction of ∼43 per cent; increasing to ≳50 per cent for the O-type hypergiants and WNLha. Dynamical and evolutionary masses reveal the primaries to be uniformly massive ($\gtrsim 50\, \mathrm{M}_{\odot }$). Where available, orbital analysis reveals a number of short period, highly eccentric binaries, which appear to be pre-interaction systems. Such systems are X-ray luminous, with 80 per cent above an empirical bound of (Lx/Lbol) ∼ 10−7 and their orbital configurations suggest formation and evolution via a single-star channel; however, we cannot exclude a binary formation channel for a subset. Qualitative comparison to surveys of lower mass OB-type stars confirms that the trend to an extreme binary fraction (≥60 per cent) extends to the most massive stars currently forming in the local Universe.

In the Trenches of the Solar–Stellar Connection. VI. Total EclipSS

The Ecliptic poles Stellar Survey (EclipSS; 2018–2019) collected Hubble Space Telescope/Cosmic Origins Spectrograph (COS) 1150–1420 Å spectra of 49 FGK dwarfs at high ecliptic latitudes. These regions are favored by the scanning programs of the Transiting Exoplanet Survey Satellite (TESS; high-precision optical light curves) and Extended Roentgen Survey with an Imaging Telescope Array (eROSITA; repeated X-ray visits over half a decade), which can provide vital ancillary support in the exploration of high-energy “stellar activity.” The present study is a follow-on, focused on new Chandra/High Resolution Camera 0.1–2.4 keV X-ray pointings, COS 1340–1720 Å spectra, and TESS photometry of the 10 most active EclipSS members, but also including archival X-ray and far-ultraviolet (FUV) observations of 13 representative solar neighborhood FGK dwarfs, together with high-energy irradiances of the Sun, for context. The EclipSS dK+dM binary HD 41004 proved exceptional. The unexpectedly short TESS period (1.3 days) appears to be associated with the red dwarf secondary, possibly about to engulf a close-orbiting brown-dwarf companion. The current study describes the emission-line profiles of a high-resolution subset of the full sample, finding the non-Gaussian line shapes (sharper peaks, broader wings) and redshifts of T ∼ 105 K features to be remarkably independent of activity; quantifies X-ray and FUV “variability bias” based on several multiple-epoch campaigns; builds enhanced flux–flux diagrams (e.g., X-rays versus C iv); and constructs an “XUV” proxy flux, representing the total coronal radiative loss, to test models of the convective turnover time, τ c. The associated Rossby number, Ro, is closely aligned to terrestrial and cosmic magnetic dynamo theories, and offers the prospect of a one-parameter description of stellar high-energy activity (relevant, for example, to host-star ionizing fluxes impacting exoplanets).

A Chandra survey of milky way globular clusters – III. Searching for X-ray signature of intermediate-mass black holes

ABSTRACT Globular clusters (GCs) are thought to harbor the long-sought population of intermediate-mass black holes (IMBHs). We present a systematic search for a putative IMBH in 81 Milky Way GCs, based on archival Chandra X-ray observations. We find in only six GCs a significant X-ray source positionally coincident with the cluster centre, which have 0.5–8 keV luminosities between ∼1 × 1030 erg s−1 and ∼4 × 1033 erg s−1. However, the spectral and temporal properties of these six sources can also be explained in terms of binary stars. The remaining 75 GCs do not have a detectable central source, most with 3σ upper limits ranging between 1029–32 erg s−1 over 0.5–8 keV, which are significantly lower than predicted for canonical Bondi accretion. To help understand the feeble X-ray signature, we perform hydrodynamic simulations of stellar wind accretion on to a 1000 M⊙ IMBH from the most-bound orbiting star, for stellar wind properties consistent with either a main-sequence (MS) star or an asymptotic giant branch (AGB) star. We find that the synthetic X-ray luminosity for the MS case ($\sim 10^{19}\rm ~erg \, s^{-1}$) is far below the current X-ray limits. The predicted X-ray luminosity for the AGB case ($\sim 10^{34}\rm ~erg \, s^{-1}$), on the other hand, is compatible with the detected central X-ray sources, in particular the ones in Terzan 5 and NGC 6652. However, the probability of having an AGB star as the most-bound star around the putative IMBH is very low. Our study strongly suggests that it is very challenging to detect the accretion-induced X-ray emission from IMBHs, even if they were prevalent in present-day GCs.

The Origin of Weakened Magnetic Braking in Old Solar Analogs

The rotation rates of main-sequence stars slow over time as they gradually lose angular momentum to their magnetized stellar winds. The rate of angular momentum loss depends on the strength and morphology of the magnetic field, the mass-loss rate, and the stellar rotation period, mass, and radius. Previous observations suggested a shift in magnetic morphology between two F-type stars with similar rotation rates but very different ages (88 Leo and ρ CrB). In this Letter, we identify a comparable transition in an evolutionary sequence of solar analogs with ages between 2–7 Gyr. We present new spectropolarimetry of 18 Sco and 16 Cyg A and B from the Large Binocular Telescope, and we reanalyze previously published Zeeman Doppler images of HD 76151 and 18 Sco, providing additional constraints on the nature and timing of this transition. We combine archival X-ray observations with updated distances from Gaia to estimate mass-loss rates, and we adopt precise stellar properties from asteroseismology and other sources. We then calculate the wind braking torque for each star in the evolutionary sequence, demonstrating that the rate of angular momentum loss drops by more than an order of magnitude between the ages of HD 76151 and 18 Sco (2.6–3.7 Gyr) and continues to decrease modestly to the age of 16 Cyg A and B (7 Gyr). We suggest that this magnetic transition may represent a disruption of the global dynamo arising from weaker differential rotation, and we outline plans to probe this phenomenon in additional stars spanning a wide range of spectral types.

Optical Studies of 10 Hard X-Ray-selected Cataclysmic Binaries

Abstract We conducted time-resolved optical spectroscopy and/or photometry of 10 cataclysmic binaries that were discovered in hard X-ray surveys, with the goal of measuring their orbital periods and searching for evidence that they are magnetic. Four of the objects in this study are new optical identifications: IGR J18017−3542, PBC J1841.1+0138, IGR J18434−0508, and Swift J1909.3+0124. A 311.8 s, coherent optical pulsation is detected from PBC J1841.1+0138, as well as eclipses with a period of 0.221909 days. A 152.49 s coherent period is detected from IGR J18434−0508. A probable period of 389 s is seen in IGR J18151−1052, in agreement with a known X-ray spin period. We also detect a period of 803.5 s in an archival X-ray observation of Swift J0717.8−2156. The last four objects are thus confirmed magnetic cataclysmic variables of the intermediate polar class. An optical period of 1554 s in AX J1832.3−0840 also confirms the known X-ray spin period, but a stronger signal at 2303 s is present whose interpretation is not obvious. We also studied the candidate intermediate polar Swift J0820.6−2805, which has low and high states differing by ≈4 mag and optical periods or quasi-periodic oscillations not in agreement with proposed X-ray periods. Of note is an unusually long 2.06-day orbital period for Swift J1909.3+0124, manifest in the radial velocity variation of photospheric absorption lines of an early K-type companion star. The star must be somewhat evolved if it is to fill its Roche lobe.

The Wolf–Rayet + Black Hole Binary NGC 300 X-1: What is the Mass of the Black Hole?

Abstract We present new X-ray and UV observations of the Wolf–Rayet + black hole (BH) binary system NGC 300 X-1 with the Chandra X-ray Observatory and the Hubble Space Telescope Cosmic Origins Spectrograph. When combined with archival X-ray observations, our X-ray and UV observations sample the entire binary orbit, providing clues to the system geometry and interaction between the BH accretion disk and the donor star wind. We measure a binary orbital period of 32.7921 ± 0.0003 hr, in agreement with previous studies, and perform phase-resolved spectroscopy using the X-ray data. The X-ray light curve reveals a deep eclipse, consistent with inclination angles of i = 60°–75°, and a pre-eclipse excess consistent with an accretion stream impacting the disk edge. We further measure radial velocity variations for several prominent far-UV spectral lines, most notably H ii λ1640 and C iv λ1550. We find that the He ii emission lines systematically lag the expected Wolf–Rayet star orbital motion by a phase difference of Δϕ ∼ 0.3, while C iv λ1550 matches the phase of the anticipated radial velocity curve of the Wolf–Rayet donor. We assume the C iv λ1550 emission line follows a sinusoidal radial velocity curve (semi-amplitude = 250 km s−1) and infer a BH mass of 17 ± 4 M ⊙. Our observations are consistent with the presence of a wind-Roche lobe overflow accretion disk, where an accretion stream forms from gravitationally focused wind material and impacts the edge of the BH accretion disk.

A Lack of Evidence for Global Ram-Pressure Induced Star Formation in the Merging Cluster Abell 3266

Interaction between the intracluster medium and the interstellar media of galaxies via ram-pressure stripping (RPS) has ample support from both observations and simulations of galaxies in clusters. Some, but not all of the observations and simulations show a phase of increased star formation compared to normal spirals. Examples of galaxies undergoing RPS induced star formation in clusters experiencing a merger have been identified in high resolution optical images supporting the existence of a star formation phase. We have selected Abell 3266 to search for ram-pressure induced star formation as a global property of a merging cluster. Abell 3266 (z = 0.0594) is a high mass cluster that features a high velocity dispersion, an infalling subcluster near to the line of sight, and a strong shock front. These phenomena should all contribute to making Abell 3266 an optimum cluster to see the global effects of RPS induced star formation. Using archival X-ray observations and published optical data, we cross-correlate optical spectral properties ([OII, Hβ]), indicative of starburst and post-starburst, respectively with ram-pressure, ρv2, calculated from the X-ray and optical data. We find that post-starburst galaxies, classified as E + A, occur at a higher frequency in this merging cluster than in the Coma cluster and at a comparable rate to intermediate redshift clusters. This is consistent with increased star formation due to the merger. However, both starburst and post-starburst galaxies are equally likely to be in a low or high ram pressure environment. From this result, we infer that the duration of the starburst phase must be very brief so that: 1) at any time only a small fraction of the galaxies in a high ram pressure environment show this effect, and 2) most post-starburst galaxies are in an environment of low ram pressure due to their continued orbital motion in the cluster.

A possible nonthermal X-ray emission from γ Cas analogues stars

Abstract We analyze the archival XMM-Newton X-ray observations of 15 γ Cas analogue stars and two candidates for such objects. The EPIC spectra of the considered stars in the range of 0.2-8 keV were extracted and fitted by different models. Our estimates show that assuming the X-ray emission from γ Cas analogues to be totally thermal, their model plasma temperatures can reach anomalously high values. However including an additional power components to the model spectra leads to significant decreasing of the plasma temperatures. The spectral index of the power component is about 1.5, and the fraction of this in the total model flux is rather large (50-90%). Moreover, it decreases with expanding temperature of the X-ray emitting plasma as compared to typical OB stars. We conclude that γ Cas analogues can produce nonthermal X-ray emission within the framework of the Chen & White (1991) model, while if the nonthermal X-rays from typical OB stars exists, they should be generated by different processes.

The great Kite in the sky: A LOFAR observation of the radio source in Abell 2626

Context.The radio source at the center of the galaxy cluster Abell 2626, also known as the Kite, stands out for its unique morphology composed of four symmetric arcs. Previous studies have probed the properties of this source at different frequencies and its interplay with the surrounding thermal plasma, but the puzzle of its origin is still unsolved.Aims.We use a new LOw-Frequency ARray (LOFAR) observation from the LOFAR Two-meter Sky Survey at 144 MHz to investigate the origin of the Kite.Methods.We present a detailed analysis of the new radio data, which we combined with archival radio and X-ray observations. We produced a new, resolved spectral index map of the source with a resolution of 7″ and we studied the spatial correlation of radio and X-ray emission to investigate the interplay between thermal and nonthermal plasma.Results.The new LOFAR data changed our view of the Kite because we discovered two steep-spectrum (α < −1.5) plumes of emission connected to the arcs. The spectral analysis shows, for the first time, a spatial trend of the spectrum along the arcs with evidence of curved synchrotron spectra and a spatial correlation with the X-ray surface brightness. On the basis of our results, we propose that the Kite was originally an X-shaped radio galaxy whose fossil radio plasma, after the end of the activity of the central active galactic nucleus, has been compressed as a consequence of motions of the thermal plasma encompassing the galaxy. The interplay between the compression and advection of the fossil plasma, with the restarting of the nuclear activity of the central galaxy, could have enhanced the radio emission of the fossil plasma producing the arcs of the Kite. We also present the first, low-frequency observation of a jellyfish galaxy in the same field, in which we detect extended, low-frequency emission without a counterpart at higher frequencies.

Search for Continuous Gravitational Waves from the Central Compact Objects in Supernova Remnants Cassiopeia A, Vela Jr., and G347.3–0.5

Abstract We perform a sub-threshold follow-up search for continuous nearly monochromatic gravitational waves from the central compact objects associated with the supernova remnants (SNRs) Vela Jr., Cassiopeia A, and SNR G347.3−0.5. Across the three targets, we investigate the most promising ≈10,000 combinations of gravitational-wave frequency and frequency derivative values, based on the results from an Einstein@Home search of the LIGO O1 observing run data, dedicated to these objects. The selection threshold is set so that a signal could be confirmed using the newly released O2 run LIGO data. In order to achieve best sensitivity we perform two separate follow-up searches, on two distinct stretches of the O2 data. Only one candidate survives the first O2 follow-up investigation, associated with the central compact object in SNR G347.3−0.5, but it is not conclusively confirmed. In order to assess a possible astrophysical origin we use archival X-ray observations and search for amplitude modulations of a pulsed signal at the putative rotation frequency of the neutron star and its harmonics. This is the first extensive electromagnetic follow-up of a continuous gravitational-wave candidate performed to date. No significant associated signal is identified. New X-ray observations contemporaneous with the LIGO O3 run will enable a more sensitive search for an electromagnetic counterpart. A focused gravitational-wave search in O3 data based on the parameters provided here should be easily able to shed light on the nature of this outlier. Noise investigations on the LIGO instruments could also reveal the presence of a coherent contamination.

Broad-band spectral energy distribution of the X-ray transient Swift J1745−26 from outburst to quiescence

Aims. We aim to analyse our study of the X-ray transient Swift J1745−26, using observations obtained from its outburst in September 2012, up to its decay towards quiescence in March 2013. Methods. We obtained optical and infrared observations, through override programme at ESO/VLT with FORS2 and ISAAC instruments, and added archival optical (VLT/VIRCAM), radio and X-ray (Swift) observations, to build the light curve and the broad-band spectral energy distribution (SED) of Swift J1745−26. Results. We show that, during its outburst and also during its decay towards quiescence, Swift J1745−26 SED can be adjusted, from infrared up to X-rays, by the sum of both a viscous irradiated multi-colour black body emitted by an accretion disc, and a synchrotron power law at high energy. In the radio domain, the SED arises from synchrotron emission from the jet. While our SED fitting confirms that the source remained in the low/hard state during its outburst, we determine an X-ray spectral break at frequency 3.1 ≤ νbreak ≤ 3.4 × 1014 Hz, and a radio spectral break at 1012 Hz ≤ νbreak ≤ 1013 Hz. We also show that the system is compatible with an absorption AV of ∼7.69 mag, lies within a distance interval of D ∼ [2.6 − 4.8] kpc with an upper limit of orbital period Porb = 11.3 h, and that the companion star is a late spectral type in the range K0–M0 V, confirming that the system is a low-mass X-ray binary. We finally plot the position of Swift J1745−26 on an optical-infrared – X-ray luminosity diagram: its localisation on this diagram is consistent with the source staying in the low-hard state during outburst and decay phases. Conclusions. By using new observations obtained at ESO/VLT with FORS2 and ISAAC, and adding archival optical (VLT/VIRCAM), radio and X-ray (Swift) observations, we built the light curve and the broad-band SED of Swift J1745−26, and we plotted its position on an optical-infrared – X-ray luminosity diagram. By fitting the SED, we characterized the emission of the source from infrared, via optical, up to X-ray domain, we determined the position of both the radio and X-ray spectral breaks, we confirmed that it remained in the low-hard state during outburst and decay phases, and we derived its absorption, distance interval, orbital period upper limit, and the late-type nature of companion star, confirming Swift J1745−26 is a low-mass X-ray binary.

A New Likely Redback Millisecond Pulsar Binary with a Massive Neutron Star: 4FGL J2333.1–5527

Abstract We present the discovery of a likely new redback millisecond pulsar (MSP) binary associated with the Fermi γ-ray source 4FGL J2333.1–5527. Using optical photometric and spectroscopic observations from the Southern Astrophysical Research telescope, we identify a low-mass, main-sequence-like companion in a 6.9 hr, highly inclined orbit around a suspected massive neutron star primary. Archival XMM-Newton X-ray observations show this system has a hard power-law spectrum Γ = 1.6 ± 0.3 and L X ∼ 5 × 1031 erg s−1, consistent with redback MSP binaries. Our data suggest that for secondary masses typical of redbacks, the mass of the neutron star is likely well in excess of ∼1.4 M ⊙, but future timing of the radio pulsar is necessary to bolster this tentative conclusion. This work shows that a bevy of nearby compact binaries still await discovery, and that unusually massive neutron stars continue to be common in redbacks.