Near IR Astrometry of Magnetars

We report on the progress of our five-year program for astrometric monitoring of magnetars using high-resolution NIR observations using the laser guide star adaptive optics (LGS-AO) supported NIRC2 camera on the 10-meter Keck telescope. We have measured the proper motion of two of the youngest magnetars, SGR 1806–20 and SGR 1900+14, which have counterparts with K ~21 mag, and have placed a preliminary upper limit on the motion of the young AXP 1E 1841–045. The precision of the proper motion measurement is at the milliarcsecond per year level. Our proper motion measurements now provide evidence to link SGR 1806–20 and SGR 1900+14 with neighboring young star clusters. At the distances of these magnetars, their proper motion corresponds to transverse space velocities of 350 ± 100 km s^(−1) and 130 ± 30 km s^(−1) respectively. The upper limit on the proper motion of AXP 1E 1841–045 is 160 km s^(−1). With the sample of proper motions available, we conclude that the kinematics of the magnetar family are not distinct from that of pulsars

Fast Rotation and Trailing Fragments of the Active Asteroid P/2012 F5 (Gibbs)

While having a comet-like appearance, P/2012 F5 (Gibbs) has an orbit native to the Main Asteroid Belt, and physically is a km-sized asteroid which recently (mid 2011) experienced an impulsive mass ejection event. Here we report new observations of this object obtained with the Keck II telescope on UT 2014 August 26. The data show previously undetected 200-m scale fragments of the main nucleus, and reveal a rapid nucleus spin with a rotation period of 3.24 $\pm$ 0.01 hr. The existence of large fragments and the fast nucleus spin are both consistent with rotational instability and partial disruption of the object. To date, many fast rotators have been identified among the minor bodies, which, however, do not eject detectable fragments at the present-day epoch, and also fragmentation events have been observed, but with no rotation period measured. P/2012 F5 is unique in that for the first time we detected fragments and quantified the rotation rate of one and the same object. The rapid spin rate of P/2012 F5 is very close to the spin rates of two other active asteroids in the Main Belt, 133P/Elst-Pizarro and (62412), confirming the existence of a population of fast rotators among these objects. But while P/2012 F5 shows impulsive ejection of dust and fragments, the mass loss from 133P is prolonged and recurrent. We believe that these two types of activity observed in the rapidly rotating active asteroids have a common origin in the rotational instability of the nucleus.Comment: To appear in the 2015 March 20 issue of ApJ Letter

Proper Motions and Origins of SGR 1806–20 and SGR 1900+14

We present results from high-resolution infrared observations of magnetars SGR 1806–20 and SGR 1900+14 over 5 years using laser-supported adaptive optics at the 10 m Keck Observatory. Our measurements of the proper motions of these magnetars provide robust links between magnetars and their progenitors and provide age estimates for magnetars. At the measured distances of their putative associations, we measure the linear transverse velocity of SGR 1806–20 to be 350 ± 100 km s^(–1) and of SGR 1900+14 to be 130 ± 30 km s^(–1). The transverse velocity vectors for both magnetars point away from the clusters of massive stars, solidifying their proposed associations. Assuming that the magnetars were born in the clusters, we can estimate the braking index to be ~1.8 for SGR 1806–20 and ~1.2 for SGR 1900+14. This is significantly lower than the canonical value of n = 3 predicted by the magnetic dipole spin-down suggesting an alternative source of dissipation such as twisted magnetospheres or particle winds

Proper Motions and Origins of AXP 1E 2259+586 and AXP 4U 0142+61

Using high-resolution NIR images supported by laser guide star adaptive optics from the Keck II telescope from 2005 to 2012, we have measured the proper motions of two anomalous X-ray pulsars, AXP 1E 2259+586 and AXP 4U 0142+61. The proper motion of AXP 1E 2259+586 in the sky frame is (μ_α, μ_δ) = (– 6.4 ± 0.6, –2.3 ± 0.6) mas yr^(–1) and that of AXP 4U 0142+61 is (μ_α, μ_δ) = (– 4.1 ± 1, 1.9 ± 1) mas yr^(–1). After correcting for the velocity of the progenitors, we calculate the tangential ejection velocities of the magnetars to be 157 ± 17 km s^(–1) and 102 ± 26 km s^(–1) respectively. The proper motion vector of AXP 1E 2259+586 is directed away from the putative center of the supernova remnant CTB 109 that has long been proposed to be associated with AXP 1E 2259+586. This is significant evidence for linking the pulsar with CTB 109. We comment on the possible movement of CTB 109 after the explosion. We narrow the search cone for the birthsite or remnant of AXP 4U 0142+61 to an opening angle of 24°. However, we are unable to find any suitable association

Probabilistic Association of Transients to their Hosts (PATH)

We introduce a new method to estimate the probability that an extragalactic transient source is associated with a candidate host galaxy. This approach relies solely on simple observables: sky coordinates and their uncertainties, galaxy fluxes and angular sizes. The formalism invokes Bayes' rule to calculate the posterior probability P(O_i|x) from the galaxy prior P(O), observables x, and an assumed model for the true distribution of transients in/around their host galaxies. Using simulated transients placed in the well-studied COSMOS field, we consider several agnostic and physically motivated priors and offset distributions to explore the method sensitivity. We then apply the methodology to the set of 13~fast radio bursts (FRBs) localized with an uncertainty of several arcseconds. Our methodology finds nine of these are securely associated to a single host galaxy, P(O_i|x)>0.95. We examine the observed and intrinsic properties of these secure FRB hosts, recovering similar distributions as previous works. Furthermore, we find a strong correlation between the apparent magnitude of the securely identified host galaxies and the estimated cosmic dispersion measures of the corresponding FRBs, which results from the Macquart relation. Future work with FRBs will leverage this relation and other measures from the secure hosts as priors for future associations. The methodology is generic to transient type, localization error, and image quality. We encourage its application to other transients where host galaxy associations are critical to the science, e.g. gravitational wave events, gamma-ray bursts, and supernovae. We have encoded the technique in Python on GitHub: https://github.com/FRBs/astropath.Comment: In press, ApJ; comments still welcome; Visit https://github.com/FRBs/astropath to use and build PAT

High-efficiency Autonomous Laser Adaptive Optics

As new large-scale astronomical surveys greatly increase the number of objects targeted and discoveries made, the requirement for efficient follow-up observations is crucial. Adaptive optics imaging, which compensates for the image-blurring effects of Earth's turbulent atmosphere, is essential for these surveys, but the scarcity, complexity and high demand of current systems limits their availability for following up large numbers of targets. To address this need, we have engineered and implemented Robo-AO, a fully autonomous laser adaptive optics and imaging system that routinely images over 200 objects per night with an acuity 10 times sharper at visible wavelengths than typically possible from the ground. By greatly improving the angular resolution, sensitivity, and efficiency of 1-3 m class telescopes, we have eliminated a major obstacle in the follow-up of the discoveries from current and future large astronomical surveys.Comment: Published in ApJL. 6 pages, 4 figures, and 1 tabl

PTF10iya: a short-lived, luminous flare from the nuclear region of a star-forming galaxy

We present the discovery and characterization of PTF10iya, a short-lived (Δt≈ 10 d, with an optical decay rate of ∼0.3 mag d^(−1)), luminous (M_(g') ≈ -21) transient source found by the Palomar Transient Factory. The ultraviolet/optical spectral energy distribution is reasonably well fitted by a blackbody with T≈ (1–2) × 10^4 K and peak bolometric luminosity LBB≈ (1–5) × 10^(44) erg s^(−1) (depending on the details of the extinction correction). A comparable amount of energy is radiated in the X-ray band that appears to result from a distinct physical process. The location of PTF10iya is consistent with the nucleus of a star-forming galaxy (z= 0.224 05 ± 0.000 06) to within 350 mas (99.7 per cent confidence radius), or a projected distance of less than 1.2 kpc. At first glance, these properties appear reminiscent of the characteristic ‘big blue bump’ seen in the near-ultraviolet spectra of many active galactic nuclei (AGNs). However, emission-line diagnostics of the host galaxy, along with a historical light curve extending back to 2007, show no evidence for AGN-like activity. We therefore consider whether the tidal disruption of a star by an otherwise quiescent supermassive black hole may account for our observations. Though with limited temporal information, PTF10iya appears broadly consistent with the predictions for the early ‘super-Eddington’ phase of a solar-type star being disrupted by a ∼10^7 M_⊙ black hole. Regardless of the precise physical origin of the accreting material, the large luminosity and short duration suggest that otherwise quiescent galaxies can transition extremely rapidly to radiate near the Eddington limit; many such outbursts may have been missed by previous surveys lacking sufficient cadence