Shadows of spherically symmetric black holes and naked singularities [HEAP]

We compare shadows cast by Schwarzschild black holes with those produced by two classes of naked singularities that result from gravitational collapse of spherically symmetric matter. The latter models consist of an interior naked singularity spacetime restricted to radii $r\leq R_b$, matched to Schwarzschild spacetime outside the boundary radius $R_b$. While a black hole always has a photon sphere and always casts a shadow, we find that the naked singularity models have photon spheres only if a certain parameter $M_0$ that characterizes these models satisfies $M_0\geq 2/3$, or equivalently, if $R_b\leq 3M$, where $M$ is the total mass of the object. Such models do produce shadows. However, models with $M_0<2/3$ (or $R_b>3M$) have no photon sphere and do not produce a shadow. Instead, they produce an interesting “full-moon” image. These results imply that the presence of a shadow does not by itself prove that a compact object is necessarily a black hole. The object could be a naked singularity with $M_0\geq 2/3$, and we will need other observational clues to distinguish the two possibilities. On the other hand, the presence of a full-moon image would certainly rule out a black hole and might suggest a naked singularity with $M_0<2/3$. It would be worthwhile to generalize the present study, which is restricted to spherically symmetric models, to rotating black holes and naked singularities.

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R. Shaikh, P. Kocherlakota, R. Narayan, et. al.
Fri, 23 Feb 18

Comments: 24 pages, 9 figures

Broad absorption line symbiotic stars: highly ionized species in the fast outflow from MWC 560 [HEAP]

In symbiotic binaries, jets and disk winds may be integral to the physics of accretion onto white dwarfs from cool giants. The persistent outflow from symbiotic star MWC 560 (=V694 Mon) is known to manifest as low-ionization broad absorption lines (BALs), most prominently at the Balmer transitions, and as high-ionization BALs from metastable He I*. We report the detection of higher-ionization BALs from C IV, Si IV, N V, and He II in International Ultraviolet Explorer spectra obtained on 1990 April 29-30, when an optical outburst temporarily erased the obscuring ‘iron curtain’ of absorption troughs from Fe II and similar ions. The C IV and Si IV BALs reached maximum radial velocities at least 1000 km/s faster than contemporaneous Mg II and He II BALs; the same behaviors occur in the winds of quasars and cataclysmic variables. An iron curtain lifts to unveil high-ionization BALs during the P Cygni phase observed in some novae, suggesting by analogy a temporary switch in MWC 560 from persistent outflow to discrete mass ejection. At least three more symbiotic stars exhibit broad absorption with blue edges faster than 1500 km/s; exclusively high-ionization BALs have been reported in AS 304 (=V4018 Sgr), while instead transient Balmer BALs have been reported in Z And and CH Cyg. These BAL-producing fast outflows can have wider opening angles than has been previously supposed. BAL symbiotics are short-timescale laboratories for their giga-scale analogs, broad absorption line quasars (BALQSOs), which display a similarly wide range of ionization states in their winds.

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A. Lucy, C. Knigge and J. Sokoloski
Fri, 23 Feb 18

Comments: submitted to MNRAS; comments welcome; 8 pages, 3 figures, 2 tables, centered on A4 paper [for most 8.5×11 printers, set scaling to None/100% and leave auto-centering on]

The PoGO+ view on Crab off-pulse hard X-ray polarisation [HEAP]

The linear polarisation fraction and angle of the hard X-ray emission from the Crab provide unique insight into high energy radiation mechanisms, complementing the usual imaging, timing and spectroscopic approaches. Results have recently been presented by two missions operating in partially overlapping energy bands, PoGO+ (18-160 keV) and AstroSat CZTI (100-380 keV). We previously reported PoGO+ results on the polarisation parameters integrated across the light-curve and for the entire nebula-dominated off-pulse region. We now introduce finer phase binning, in light of the AstroSat CZTI claim that the polarisation fraction varies across the off-pulse region. Since both missions are operating in a regime where errors on the reconstructed polarisation parameters are non-Gaussian, we adopt a Bayesian approach to compare results from each mission. We find no statistically significant variation in off-pulse polarisation parameters, neither when considering the mission data separately nor when they are combined. This supports expectations from standard high-energy emission models.

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M. Chauvin, H. Floren, M. Friis, et. al.
Fri, 23 Feb 18

Comments: 6 pages, 3 figures, 5 tables. Accepted for publication in MNRAS Letters

Effects of Fall-Back Accretion on Proto-Magnetar Outflows in Gamma-Ray Bursts and Superluminous Supernovae [HEAP]

Rapidly spinning, strongly magnetized proto-neutron stars (“millisecond proto-magnetars”) are candidate central engines of long-duration gamma-ray bursts (GRB), superluminous supernovae (SLSNe), and binary neutron star mergers. Magnetar birth may be accompanied by the fall-back of stellar debris, lasting for seconds or longer following the explosion. Accretion alters the magnetar evolution by (1) providing an additional source of rotational energy (or a potential sink, if the propeller mechanism operates); (2) enhancing the spin-down luminosity above the dipole rate by compressing the magnetosphere and expanding the polar cap region of open magnetic field lines; (3) supplying an additional accretion-powered neutrino luminosity that sustains the wind baryon-loading, even after the magnetar’s internal neutrino luminosity has subsided. The more complex evolution of the jet power and magnetization of an accreting magnetar more readily accounts for the high 56Ni yields GRB SNe and irregular time evolution of some GRB light curves (e.g.~bursts with precursors followed by a long quiescent interval before the main emission episode). Additional baryon-loading from accretion-powered neutrino irradiation of the magnetar polar cap lengthens the timeframe over which the jet magnetization is in the requisite range sigma ~< 1e3 for efficient gamma-ray emission, thereby accommodating GRBs with ultra-long durations. Though accretion does not significantly raise the maximum energy budget from the limit of <~ few 1e52 ergs for an isolated magnetar, it greatly expands the range of magnetic field strengths and birth spin periods capable of powering GRB jets, reducing the differences between the magnetar properties normally invoked to explain GRBs versus SLSNe.

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B. Metzger, P. Beniamini and D. Giannios
Fri, 23 Feb 18

Comments: 18 pages, 13 figures, submitted to ApJ

Is the macronova in GW170817 powered by the central engine? [HEAP]

The gravitational wave event GW170817 from a binary neutron star (NS) merger is accompanied by electromagnetic counterparts, and the optical and near-infared emission is called a macronova (or kilonova). Although the radioactivity of synthesized r-process elements is widely discussed as an energy source, its decisive evidence is not clearly shown yet. We discuss a macronova powered by the central engine activities such as jet activities and X-rays from the matter fallback, and show that the engine model allows much broader parameter spaces, in particular smaller ejecta mass ($\sim10^{-4}-0.01\,\Msun$) than the r-process model. The blue and red macronovae are naturally explained by various combinations of the ejecta such as a cocoon and merger ejecta with the energy sources of jets and X-rays. The required energy injection is very similar to the X-ray excess observed in GRB 130603B with the power-law slope of $\sim-1.3$. The required lanthanoid fraction for the opacity can be also consistent with the Galactic one. Early or late multi-wavelength observations are crucial for revealing the central engine of short gamma-ray bursts and the r-process nucleosynthesis.

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T. Matsumoto, K. Ioka, S. Kisaka, et. al.
Fri, 23 Feb 18

Comments: 12 pages, 6 figures, submitted to ApJ, comments welcome!

In-depth study of long-term variability in the X-ray emission of the Be/X-ray binary system AX J0049.4-7323 [HEAP]

AX J0049.4-7323 is a Be/X-ray binary in the Small Magellanic Cloud hosting a ~750 s pulsar which has been observed over the last ~17 years by several X-ray telescopes. Despite numerous observations, little is known about its X-ray behaviour. Therefore, we coherently analysed archival Swift, Chandra, XMM-Newton, RXTE, and INTEGRAL data, and we compared them with already published ASCA data, to study its X-ray long-term spectral and flux variability. AX J0049.4-7323 shows a high X-ray variability, spanning more than three orders of magnitudes, from L ~ 1.6E37 erg/s (0.3-8 keV, d=62 kpc) down to L ~ 8E33 erg/s. RXTE, Chandra, Swift, and ASCA observed, in addition to the expected enhancement of X-ray luminosity at periastron, flux variations by a factor of ~ 270 with peak luminosities of ~2.1E36 erg/s far from periastron. These properties are difficult to reconcile with the typical long-term variability of Be/XRBs, traditionally interpreted in terms of type I and type II outbursts. The study of AX J0049.4-7323 is complemented with a spectral analysis of Swift, Chandra, and XMM-Newton data which showed a softening trend when the emission becomes fainter, and an analysis of optical/UV data collected by the UVOT telescope on board Swift. In addition, we measured a secular spin-up rate of $\dot{P}=(-3.00\pm0.12)\times 10^{-3}$ s day$^{-1}$, which suggests that the pulsar has not yet achieved its equilibrium period. Assuming spherical accretion, we estimated an upper limit for the magnetic field strength of the pulsar of ~3E12 G.

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L. Ducci, P. Romano, C. Malacaria, et. al.
Fri, 23 Feb 18

Comments: Accepted for publication in Astronomy & Astrophysics

Spectra of Hydrogen-Poor Superluminous Supernovae from the Palomar Transient Factory [HEAP]

Most Type I superluminous supernovae (SLSNe-I) reported to date have been identified by their high peak luminosities and spectra lacking obvious signs of hydrogen. We demonstrate that these events can be distinguished from normal-luminosity SNe (including Type Ic events) solely from their spectra over a wide range of light-curve phases. We use this distinction to select 19 SLSNe-I and 4 possible SLSNe-I from the Palomar Transient Factory archive (including 7 previously published objects). We present 127 new spectra of these objects and combine these with 39 previously published spectra, and we use these to discuss the average spectral properties of SLSNe-I at different spectral phases. We find that Mn II most probably contributes to the ultraviolet spectral features after maximum light, and we give a detailed study of the O II features that often characterize the early-time optical spectra of SLSNe-I. We discuss the velocity distribution of O II, finding that for some SLSNe-I this can be confined to a narrow range compared to relatively large systematic velocity shifts. Mg II and Fe II favor higher velocities than O II and C II, and we briefly discuss how this may constrain power-source models. We tentatively group objects by how well they match either SN 2011ke or PTF12dam and discuss the possibility that physically distinct events may have been previously grouped together under the SLSN-I label.

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R. Quimby, A. Cia, A. Gal-Yam, et. al.
Fri, 23 Feb 18

Comments: 70 pages, 41 figures, 7 tables, accepted for publication in ApJ