R. W. Romani

Ultraviolet, X-Ray, and Optical Radiation from the Geminga Pulsar*

We observed the � -ray pulsar Geminga with the FUV-MAMA and NUV-MAMA detectors of the Space Telescope Imaging Spectrometer to measure Geminga’s spectrum and pulsations in the ultraviolet. The slope of the far-ultraviolet (FUV) spectrum is close to that of a Rayleigh-Jeans spectrum, suggesting that the FUV radiation is dominated by thermal emissionfromthe neutron star(NS) surface.ThemeasuredFUVflux,FFUV ¼ (3:7 � 0:2) ; 10 � 15 ergscm � 2 s � 1 inthe1155–17028band,correspondstoabrightnesstemperatureTRJ � (0:3–0:4)(d200/R13) 2 MK,dependingon the interstellarextinction(d ¼ 200d200 pcandR ¼ 13R13 kmarethedistanceandtheNSradius,respectively).Thesoft thermal component of Geminga’s X-ray spectrum measured with the XMM-Newton observatory corresponds to a temperature Ts ¼ 0:49 � 0:01 MK and radius Rs ¼ (12:9 � 1:0)d200 km. Contrary to other NSs detected in the UV-optical,forwhich theextrapolation ofthe X-ray thermalcomponentinto theopticalunderpredicts the observed flux of thermal radiation, the FUV spectrum of Geminga lies slightly below the extrapolation of the soft thermal component,which might beassociatedwith Geminga’s very low temperature.Surprisingly, thethermalFUVradiation is strongly pulsed, showing a narrow dip at a phase close to that of a broader minimum of the soft X-ray light curve. The strong pulsations might be attributed to partial occultations of the thermal UV radiation by regions of the magnetosphere filled with electron/positron plasma. In contrast to the FUV spectrum, the near-infrared (NIR) through near-ultraviolet (NUV) spectrum of Geminga is clearly nonthermal. It can be described by a power-law model, F� / � � � þ1 , with a photon index � ¼ 1:43 � 0:15, close to the slope � ¼ 1:56 � 0:24 of the hard X-ray (E > 2:5 keV) magnetospheric component. The extrapolation of the X-ray magnetospheric spectrum into the optical is marginally consistent with (or perhaps lies slightly above) the observed NIR-optical-NUV spectrum. The NUV pulsations, however, do not show a clear correlation with the hard X-ray pulsations. Subject headingg pulsars: individual (Geminga) — stars: neutron — ultraviolet: stars

Characteristics of Gamma-Ray Loud Blazars in the VLBA Imaging and Polarimetry Survey

The radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been observed as part of the VLBA Imaging and Polarimetry Survey. This large, flux-limited sample of active galactic nuclei (AGNs) provides insights into the mechanism that produces strong γ-ray emission. At lower flux levels, radio flux density does not directly correlate with γ-ray flux. We find that the LAT-detected BL Lac objects tend to be similar to the non-LAT BL Lac objects, but that the LAT-detected FSRQs are often significantly different from the non-LAT FSRQs. The differences between the γ-ray loud and quiet FSRQs can be explained by Doppler boosting; these objects appear to require larger Doppler factors than those of the BL Lac objects. It is possible that the γ-ray loud FSRQs are fundamentally different from the γ-ray quiet FSRQs. Strong polarization at the base of the jet appears to be a signature for γ-ray loud AGNs.

Assessing the Significance of Apparent Correlations between Radio and Gamma-Ray Blazar Fluxes

Whether or not a correlation exists between the radio and gamma-ray flux densities of blazars is a long-standing question, and one that is difficult to answer confidently because of various observational biases, which may either dilute or apparently enhance any intrinsic correlation between radio and gamma-ray luminosities. We introduce a novel method of data randomization to evaluate quantitatively the effect of these biases and to assess the intrinsic significance of an apparent correlation between radio and gamma-ray flux densities of blazars. The novelty of the method lies in a combination of data randomization in luminosity space (to ensure that the randomized data are intrinsically, and not just apparently, uncorrelated) and significance assessment in flux space (to explicitly avoid Malmquist bias and automatically account for the limited dynamical range in both frequencies). The method is applicable even to small samples that are not selected with strict statistical criteria. For larger samples we describe a variation of the method in which the sample is split in redshift bins, and the randomization is applied in each bin individually; this variation is designed to yield the equivalent to luminosity-function sampling of the underlying population in the limit of very large, statistically complete samples. We show that for a smaller number of redshift bins, the method yields a worse significance, and in this way it is conservative: although it may fail to confirm an existing intrinsic correlation in a small sample that cannot be split into many redshift bins, it will not assign a stronger, artificially enhanced significance. We demonstrate how our test performs as a function of number of sources, strength of correlation, and number of redshift bins used, and we show that while our test is robust against common-distance biases and associated false positives for uncorrelated data, it retains the power of other methods in rejecting the null hypothesis of no correlation for correlated data.

GEMINGA’S PUZZLING PULSAR WIND NEBULA

We report on six new Chandra observations of the Geminga pulsar wind nebula (PWN). The PWN consists of three distinct elongated structures - two

A search for the radio counterpart of the unidentified γ-ray source 3EG J1410-6147

\approx 0.2 d_{250}

Compact symmetric objects and supermassive binary black holes in the VLBA Imaging and Polarimetry Survey

pc long lateral tails and a segmented axial tail of

J16021+3326: New Multi-Frequency Observations Of a Complex Source

\approx 0.05 d_{250}

15 GHz Monitoring of Gamma-ray Blazars with the OVRO 40 Meter Telescope in Support of Fermi

pc length, where

Imaging Compact Supermassive Binary Black Holes with Very Long Baseline Interferometry

d_{250}=d/(250 {\rm pc})

Identifying Compact Symmetric Objects from the VLBA Imaging and Polarization Survey

. The photon indices of the power law spectra of the lateral tails,