We present the first optical spectroscopy of five confirmed (or strong candidate) redback millisecond pulsar binaries, obtaining complete radial velocity curves for each companion star. The properties of these millisecond pulsar binaries with low-mass, hydrogen-rich companions are discussed in the context of the 14 confirmed and 10 candidate field redbacks. We find that the neutron stars in redbacks have a median mass of 1.78 ± 0.09M with a dispersion of σ = 0.21 ± 0.09. Neutron stars with masses in excess of 2M are consistent with, but not firmly demanded by, current observations. Redback companions have median masses of 0.36 ± 0.04M with a scatter of σ = 0.15 ± 0.04M , and a tail possibly extending up to 0.7-0.9M . Candidate redbacks tend to have higher companion masses than confirmed redbacks, suggesting a possible selection bias against the detection of radio pulsations in these more massive candidate systems. The distribution of companion masses between redbacks and the less massive black widows continues to be strongly bimodal, which is an important constraint on evolutionary models for these systems. Among redbacks, the median efficiency of converting the pulsar spindown energy to γ-ray luminosity is ∼ 10%.
Globular clusters host a variety of lower-luminosity (L X < 1035 erg s−1) X-ray sources, including accreting neutron stars (NSs) and black holes (BHs), millisecond pulsars (MSPs), cataclysmic variables, and chromospherically active binaries. In this paper, we provide a comprehensive catalog of more than 1100 X-ray sources in 38 Galactic globular clusters (GCs) observed by the Chandra X-ray Observatory’s Chandra/ACIS detector. The targets are selected to complement the MAVERIC survey’s deep radio continuum maps of Galactic GCs. We perform photometry and spectral analysis for each source, determine a best-fit model, and assess the possibility of it being a foreground or background source based on its spectral properties and location in the cluster. We also provide basic assessments of variability. We discuss the distribution of X-ray binaries in GCs and their X-ray luminosity function, and we carefully analyze systems with L X > 1033 erg s−1. Among these moderately bright systems, we discover a new source in NGC 6539 that may be a candidate accreting stellar-mass BH or a transitional MSP. We show that quiescent NS low-mass X-ray binaries in GCs may spend ∼2% of their lifetimes as transitional MSPs in their active (L X > 1033 erg s−1) state. Finally, we identify a substantial underabundance of bright (L X > 1033 erg s−1) intermediate polars in GCs compared to the Galactic field, in contrast with the literature of the past two decades.
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We present the discovery and characterization of a radio-bright binary in the Galactic globular cluster M10. First identified in deep radio continuum data from the Karl G. Jansky Very Large Array, M10-VLA1 has a flux density of 27 ± 4 µJy at 7.4 GHz and a flat to inverted radio spectrum. Chandra imaging shows an X-ray source with L X ≈ 10 31 erg s −1 matching the location of the radio source. This places M10-VLA1 within the scatter of the radio-X-ray luminosity correlation for quiescent stellarmass black holes, and a black hole X-ray binary is a viable explanation for this system. The radio and X-ray properties of the source disfavor, though do not rule out, identification as an accreting neutron star or white dwarf system. Optical imaging from the Hubble Space Telescope and spectroscopy from the SOAR telescope show the system has an orbital period of 3.339 d and an unusual "red straggler" component: an evolved star found redward of M10's red giant branch. These data also show UV/optical variability and double-peaked Hα emission characteristic of an accretion disk. However, SOAR spectroscopic monitoring reveals that the velocity semi-amplitude of the red straggler is low. We conclude that M10-VLA1 is most likely either a quiescent black hole X-ray binary with a rather face-on (i < 4 • ) orientation or an unusual flaring RS CVn-type active binary, and discuss future observations that could distinguish between these possibilities.
We report the discovery of an eclipsing low-mass X-ray binary at the center of the 3FGL error ellipse of the unassociated Fermi /Large Area Telescope γ-ray source 3FGL J0427.9-6704. Photometry from OGLE and the SMARTS 1.3-m telescope and spectroscopy from the SOAR telescope have allowed us to classify the system as an eclipsing low-mass X-ray binary (P = 8.8 hr) with a main sequence donor and a neutron star accretor. Broad double-peaked H and He emission lines suggest the ongoing presence of an accretion disk. Remarkably, the system shows shows separate sets of absorption lines associated with the accretion disk and the secondary, and we use their radial velocities to find evidence for a massive (∼ 1.8-1.9 M ) neutron star primary. In addition to a total X-ray eclipse of duration ∼ 2200 s observed with NuSTAR, the X-ray light curve also shows properties similar to those observed among known transitional millisecond pulsars: short-term variability, a hard power-law spectrum (Γ ∼ 1.7), and a comparable 0.5-10 keV luminosity (∼ 2.4 × 10 33 erg s −1 ). We find tentative evidence for a partial (∼ 60%) γ-ray eclipse at the same phase as the X-ray eclipse, suggesting the γ-ray emission may not be confined to the immediate region of the compact object. The favorable inclination of this binary is promising for future efforts to determine the origin of γ-rays among accreting neutron stars.
We report the first detection of hard (>10 keV) X-ray emission simultaneous with gamma rays in a nova eruption. Observations of the nova V5855 Sgr carried out with the NuSTAR satellite on Day 12 of the eruption revealed faint, highly absorbed thermal X-rays. The extreme equivalent hydrogen column density towards the X-ray emitting region (∼3 × 10 24 cm −2 ) indicates that the shock producing the X-rays was deeply embedded within the nova ejecta. The slope of the X-ray spectrum favors a thermal origin for the bulk of the emission, and the constraints of the temperature in the shocked region suggest a shock velocity compatible with the ejecta velocities inferred from optical spectroscopy. While we do not claim the detection of non-thermal X-rays, the data do not allow us to rule out an additional, fainter component dominating at energy above 20 keV, for which we obtained upper limits. The inferred luminosity of the thermal X-rays is too low to be consistent with the gamma-ray luminosities if both are powered by the same shock under standard assumptions regarding the efficiency of nonthermal particle acceleration and the temperature distribution of the shocked gas.
We present a multi-wavelength study of the unidentified Fermi object, 3FGL J0212.1+5320. Within the 95% error ellipse, Chandra detects a bright X-ray source (i.e., F 0.5−7keV = 1.4 × 10 −12 erg cm −2 s −1 ), which has a low-mass optical counterpart (M 0.4M ⊙ and T ∼ 6000K). A clear ellipsoidal modulation is shown in optical/infrared at 20.87 hours. The gamma-ray properties of 3FGL J0212.1+5320 are all consistent with that of a millisecond pulsar, suggesting that it is a γ-ray redback millisecond pulsar binary with a low-mass companion filling 64% of the Roche-lobe. If confirmed, it will be a redback binary with one of the longest orbital periods known. Spectroscopic data taken in 2015 from the Lijiang observatory show no evidence of strong emission lines, revealing that the accretion is currently inactive (the rotation-powered pulsar state). This is consistent with the low X-ray luminosities (L X ≈ 10 32 erg s −1 ) and the possible X-ray modulation seen by Chandra and Swift. Considering that the X-ray luminosity and the high X-ray-to-γ-ray flux ratio (8%) are both comparable to that of the two known γ-ray transitional millisecond pulsars, we suspect that 3FGL J0212.1+5320 could be a potential target to search for future transition to the accretion active state.
The population of millisecond pulsars (MSPs) has been expanded considerably in the last decade. Not only is their number increasing, but also various classes of them have been revealed. Among different classes of MSPs, the behaviours of black widows and redbacks are particularly interesting. These systems consist of an MSP and a low-mass companion star in compact binaries with an orbital period of less than a day. In this article, we give an overview of the high energy nature of these two classes of MSPs. Updated catalogues of black widows and redbacks are presented and their X-ray/γ-ray properties are reviewed. Besides the overview, using the most updated eight-year Fermi Large Area Telescope point source catalog, we have compared the γ-ray properties of these two MSP classes. The results suggest that the X-rays and γ-rays observed from these MSPs originate from different mechanisms. Lastly, we will also mention the future prospects of studying these spider pulsars with the novel methodologies as well as upcoming observing facilities. What Are Millisecond Pulsars?Rotation-powered pulsars, which act as unipolar inductors by coupling the strong magnetic field and fast rotation, radiate at the expense of their rotational energy. As a result, the rotation of a pulsar gradually slows down as it ages. When the rotation becomes too slow to sustain the particles' acceleration in their magnetospheres, the radiation beam shuts down. In such a case, we say a pulsar is "dead". While their magnetospheric particle accelerators have been turned off, other emission mechanisms (e.g., accretion) can still possibly work in these dead pulsars.By the time of writing, there are 2702 pulsars in total, including radio pulsars, radio-quiet γ-ray pulsars and magnetars [1]. In Figure 1, we show the distribution of their period P and period derivativė P. Applying a clustering analysis in this 2D parameter space with k-means partitioning, the whole population can be divided into two parts. The largest one is displayed as black dots in Figure 1. This partition spans a range of P ∼ 0.02 − 23.5 s andṖ ∼ 5 × 10 −18 − 5 × 10 −10 s/s. This group includes canonical pulsars as well as magnetars. Assuming the surface magnetic field is dipolar and the rotational energy of the pulsar goes entirely to the dipolar radiation, one is able to estimate their surface magnetic field strength B s and their characteristic age τ as:
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