Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fail to escape, it may fall back onto the neutron star 1 or it may possess sufficient angular momentum to form a disk. 2 Such 'fallback' is both a general prediction of current supernova models 3 and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation. 4 Fallback disks could dramatically affect the early evolution of pulsars, 2,5 yet there are few observational constraints on whether significant fallback occurs or even the existence of such disks. Here we report the discovery of mid-infrared emission from a cool disk around an isolated young X-ray pulsar. The disk does not power the pulsar's X-ray emission but is passively illuminated by these X-rays. The estimated mass of the disk is of the order of 10 Earth masses, and its lifetime ( > ∼ 10 6 yr) significantly exceeds the spin-down age of the pulsar, supporting a supernova fallback origin. The disk resembles protoplanetary disks seen around ordinary young stars, 6 suggesting the possibility of planet formation around young neutron stars.The so-called anomalous X-ray pulsars 7 (AXPs) are a group of young ( < ∼ 10 5 yr) neutron stars with spin periods falling in a narrow range (5-12 s), no evidence for binary companions, and whose X-ray luminosities (∼ 10 36 erg s −1 ) greatly exceed their rates of rotational kinetic energy loss (∼ 10 33 erg s −1 ). AXPs are generally believed to be 'magnetars', 8 which are isolated neutron stars with exceptionally strong ( > ∼ 10 14 G) surface magnetic field strengths and whose magnetic energy ultimately powers their X-ray emission. An alternative explanation for AXPs attributes their X-ray emission to accretion from a residual debris disk, 9-11 but this model has had difficulties explaining observations. 12 The brightest known AXP is the 8.7 s pulsar 4U 0142+61, at a distance of 3.9 kpc (ref. 13). Besides its X-ray emission, the pulsar also has known optical 14 and near-infrared 12 (near-IR) counterparts.As part of a systematic search for fallback disks around young neutron stars, we observed the field around 4U 0142+61 in the 4.5 µm and 8.0 µm bands with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope to look for the infrared excess predicted by models for an X-ray heated fallback disk. 15 We found a candidate mid-IR counterpart at the pulsar's position in both bands (Fig. 1) that has very unusual IR colours (Fig. 2). Based on the position coincidence and colours, we conclude that we have identified the mid-IR counterpart of 4U 0142+61.We can reconstruct the observed low-energy spectral energy distribution of 4U 0142+61 by combining our Spitzer data with existing IR and optical data 12,16 (Fig. 3). We may then infer the intrinsic spectrum by correcting for interstellar reddening. Although this reddening correction has little effect on the mid-IR data, it significantly affects the optical...

We report the discovery of the orbital period of the ultracompact low-mass X-ray binary (LMXB) 4U 1543−624 using time-resolved optical photometry taken with the 6.5-m Clay (Magellan II) telescope in Chile. The light curve in the Sloan r ′ band clearly shows a periodic, sinusoidal modulation at 18.2±0.1 min with a fractional semiamplitude of 8%, which we identify as the binary period. This is the second shortest orbital period among all the known LMXBs, and it verifies the earlier suggestion of 4U 1543−624 as an ultracompact binary based on X-ray spectroscopic properties. The sinusoidal shape of the optical modulation suggests that it arises from X-ray heating of the mass donor in a relatively low-inclination binary, although it could also be a superhump oscillation in which case the orbital period is slightly shorter. If the donor is a C-O white dwarf as previously suggested, its likely mass and radius are around 0.03 M ⊙ and 0.03 R ⊙ , respectively. For conservative mass transfer onto a 1.4 M ⊙ neutron star and driven by gravitational radiation, this implies an X-ray luminosity of 6.5×10 36 erg s −1 and a source distance of ≈7 kpc. We also discuss optical photometry of another LMXB, the candidate ultracompact binary 4U 1822−000. We detected significant optical variability on a time scale of about 90 min, but it is not yet clear whether this was due to a periodic modulation.

In a tunnel at the base of sub-polar Urumqi Glacier No. 1, China, three new mechanisms of glacier flow at sub-freezing temperatures have been observed.Taken individually or in combination, these modes of flow can account for nearly all (60-80%) of the overall glacier motion and, yet, they act only within the lowermost 1-2% of the effective glacier thickness. These mechanisms are:(I) enhanced deformation of the frozen and ice-laden subglacial drift; (2) motion across discrete shear planes or shear bands within the frozen drift or at the ice-drift interface; and (3) basal sliding at an ice-rock interface at a temperature of nearly -5°C. The ice-laden drift has an effective viscosity of more than one hundred times less than that measured in the overlying ice, thus allowing very rapid shear deformation. The observed rate of basal sliding at the ice-rock interface agrees favorably with that predicted by the recent work of Shreve (1984) if proper account is taken of the measured surface roughness and reduced ice viscosity.

Carbon-supported low-Pt ordered intermetallic nanoparticulate catalysts (PtM 3 , M = Fe, Co, and Ni) are explored in order to enhance the oxygen reduction reaction (ORR) activity while achieving a high stability compared to previously reported Pt-richer ordered intermetallics (Pt 3 M and PtM) and low-Pt disordered alloy catalysts. Upon high-temperature thermal annealing, ordered PtCo 3 intermetallic nanoparticles are successfully prepared with minimum particle sintering. In contrast, the PtFe 3 catalyst, despite the formation of ordered structure, suffers from obvious particle sintering and detrimental metal-support interaction, while the PtNi 3 catalyst shows no structural ordering transition at all but significant particle sintering. The ordered PtCo 3 catalyst exhibits durably thin Pt shells with a uniform thickness below 0.6 nm (corresponding to 2-3 Pt atomic layers) and a high Co content inside the nanoparticles after 10 000 potential cycling, leading to a durably compressive Pt surface and thereby both high activity (fivefold vs a commercial Pt catalyst and 1.7-fold vs an ordered PtCo intermetallic catalyst) and high durability (5 mV loss in half-wave potential and 9% drop in mass activity). These results provide a new strategy toward highly active and durable ORR electrocatalysts by rational development of low-Pt ordered intermetallics.

The Sloan Digital Sky Survey (SDSS) source J102347.6+003841 was recently revealed to be a binary 1.69 millisecond radio pulsar with a 4.75 hr orbital period and a ∼0.2 M ⊙ companion. Here we analyze the SDSS spectrum of the source in detail. The spectrum was taken on 2001 February 1, when the source was in a bright state and showed broad, double-peaked hydrogen and helium linesdramatically different from the G-type absorption spectrum seen from 2002 May onward. The lines are consistent with emission from a disk around the compact primary. We derive properties of the disk by fitting the SDSS continuum with a simple disk model, and find a temperature range of 2000-34000 K from the outer to inner edge of the disk. The disk inner and outer radii were approximately 10 9 and 5.7×10 10 cm, respectively. These results further emphasize the unique feature of the source: it is a system likely at the end of its transition from an X-ray binary to a recycled radio pulsar. The disk mass is estimated to have been ∼ 10 23 g, most of which would have been lost due to pulsar wind ablation (or due to the propeller effect if the disk had extended inside the light cylinder of the pulsar) before the final disk disruption event. The system could undergo repeated episodes of disk formation. Close monitoring of the source is needed to catch the system in its bright state again, so that this unusual example of a pulsar-disk interaction can be studied in much finer detail.

We report the precise optical and X-ray localization of the 3.2 ms accretion-powered X-ray pulsar XTE J1814−338 with data from the Chandra X-Ray Observatory as well as optical observations conducted during the 2003 June discovery outburst. Optical imaging of the field during the outburst of this soft X-ray transient reveals an R = 18 star at the X-ray position. This star is absent (R > 20) from an archival 1989 image of the field and brightened during the 2003 outburst, and we therefore identify it as the optical counterpart of XTE J1814−338. The best source position derived from optical astrometry is R.A. = 18 h 13 m 39. s 04, Dec.= −33 • 46 ′ 22. ′′ 3 (J2000). The featureless X-ray spectrum of the pulsar in outburst is best fit by an absorbed power-law (with photon index γ = 1.41 ± 0.06) plus blackbody (with kT = 0.95 ± 0.13 keV) model, where the blackbody component contributes approximately 10% of the source flux. The optical broad-band spectrum shows evidence for an excess of infrared emission with respect to an X-ray heated accretion disk model, suggesting a significant contribution from the secondary or from a synchrotron-emitting region. A follow-up observation performed when XTE J1814−338 was in quiescence reveals no counterpart to a limiting magnitude of R = 23.3. This suggests that the secondary is an M3 V or later-type star, and therefore very unlikely to be responsible for the soft excess, making synchroton emission a more reasonable candidate.

Since 2016 October, the active galaxy PKS 2247−131 has undergone a γ-ray outburst, which we studied using data obtained with the Fermi Gamma-ray Space Telescope. The emission arises from a relativistic jet in PKS 2247−131, as an optical spectrum only shows a few weak absorption lines, typical of the BL Lacertae sub-class of the blazar class of active galactic nuclei. Here we report a ≃34.5 day quasi-periodic oscillation (QPO) in the emission after the initial flux peak of the outburst. Compared to one-year time-scale QPOs, previously identified in blazars in Fermi energies, PKS 2247−131 exhibits the first clear case of a relatively short, month-like oscillation. We show that this QPO can be explained in terms of a helical structure in the jet, where the viewing angle to the dominant emission region in the jet undergoes periodic changes. The time scale of the QPO suggests the presence of binary supermassive black holes in PKS 2247−131.

We describe observations of the 2003 outburst of the hard-spectrum X-ray transient 4U 1901+03 with the Rossi X-ray Timing Explorer. The outburst was first detected in 2003 February by the All-Sky Monitor, and reached a peak 2.5-25 keV flux of 8 × 10 −9 ergs cm −2 s −1 (around 240 mCrab). The only other known outburst occurred 32.2 yr earlier, likely the longest presently known recurrence time for any X-ray transient. Proportional Counter Array (PCA) observations over the 5-month duration of the 2003 outburst revealed a 2.763 s pulsar in a 22.58 d orbit. The detection of pulsations down to a flux of 3 × 10 −11 ergs cm −2 s −1 (2.5-25 keV), along with the inferred long-term accretion rate of 8.1 × 10 −11 M ⊙ yr −1 (assuming a distance of 10 kpc) suggests that the surface magnetic field strength is below ∼ 5 × 10 11 G. The corresponding cyclotron energy is thus below 4 keV, consistent with the non-detection of resonance features at high energies. Although we could not unambiguously identify the optical counterpart, the lack of a bright IR candidate within the 1 ′ RXTE error circle rules out a supergiant mass donor. The neutron star in 4U 1901+03 probably accretes from the wind of a main-sequence O-B star, like most other high-mass binary X-ray pulsars. The almost circular orbit (e = 0.036) confirms the system's membership in a growing class of wide, low-eccentricity systems in which the neutron stars may have received much smaller kicks as a result of their natal supernova explosions.