Pulsars, transients and relativistic astrophysics

Abstract:

We present BVRIK images and spectroscopic observations of the z = 0.17 host galaxy of the compact steep-spectrum (CSS) radio source MRC B1221 -423. This is a young (∼105 yr) radio source with double lobes lying well within the visible galaxy. The host galaxy is undergoing tidal interaction with a nearby companion, with shells, tidal tails and knotty star-forming regions all visible. We analyse the images of the galaxy and its companion pixel-by-pixel, first using colour-magnitude diagrams and then fitting stellar population models to the spectral energy distributions (SEDs) of each pixel. We also present medium-resolution spectroscopy of the system. The pixels separate cleanly in colour-magnitude diagrams, with pixels of different colours occupying distinct regions of the host galaxy and its companion. Fitting stellar population models to these colours, we have estimated the age of each population. We find three distinct groups of ages: an old population (τ ∼ 15 Gyr) in the outskirts of the host galaxy; an intermediate-age population (τ ∼ 300 Myr) around the nucleus and tidal tail, and a young population (τ 10 Myr) in the nucleus and blue knots. The spectrum of the nucleus shows numerous strong emission lines, including [O I] λ6300, [O II] λ3727, [S II] λλ6716, 6731, Hα and [N II] λλ6548, 6583, characteristic of a low-ionization nuclear emission-line region (LINER) spectrum. The companion galaxy shows much narrower emission lines with very different line ratios, characteristic of a starburst galaxy. We have evidence for three distinct episodes of star formation in B1221-423. The correlation of age with position suggests the two most recent episodes were triggered by tidal interactions with the companion galaxy. The evidence points to the active galactic nucleus (AGN) in the centre of B1221-423 having been caught in the act of ignition. However, none of the components we have identified is as young as the radio source, implying that the delay between the interaction and the triggering of the AGN is at least 3 × 108 yr.

Abstract:

We present the X-ray images of all the available Chandra observations of the galactic jet source SS 433. We have studied the morphology of the X-ray images and inspected the evolution of the arcsec X-ray jets, recently found to be manifestations of in situ reheating of the relativistic gas downstream in the jets. The Chandra images reveal that the arcsec X-ray jets are not steady long term structures; the structure varies, indicating that the reheating processes have no preference for a particular precession phase or distance from the binary core. Three observations made within about five days in May 2001, and a 60 ks observation made in July 2003 show that the variability of the jets can be very rapid, from timescales of days to (possibly) hours. The three May 2001 images show two resolved knots in the east jet getting brighter one after the other, suggesting that a common phenomenon might be at the origin of the sequential reheatings of the knots. We discuss possible scenarios and propose a model to interpret these brightenings in terms of a propagating shock wave, revealing a second, faster outflow in the jet.