Discovery of a single faint AGN in a large sample of z > 5 Lyman break galaxies

As part of a large spectroscopic survey of   z > 5  Lyman break galaxies (LBGs), we have identified a single source which is clearly hosting an active galactic nucleus (AGN). Out of a sample of more than 50 spectroscopically confirmed R -band dropout galaxies at   z ∼ 5  and above, only J104048.6−115550.2 at   z = 5.44  shows evidence for a high ionization potential emission line indicating the presence of a hard ionizing continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Lyα line. Uniquely within this sample of LBGs, emission from N  v is also detected, a clear signature of AGN photoionization. The object is spatially resolved in Hubble Space Telescope ( HST ) imaging. This, and the comparatively high Lyα/N  v flux ratio indicates that the majority of the Lyα (and the UV continuum longward of it) originates from stellar photoionization, a product of the ongoing starburst in the LBG. Even without the AGN emission, this object would have been photometrically selected and spectroscopically confirmed as a Lyman break in our survey. The measured optical flux  ( I _AB= 26.1)  is therefore an upper limit to that from the AGN and is of order 100 times fainter than the majority of known quasars at these redshifts. The detection of a single object in our survey volume is consistent with the best current models of high redshift AGN luminosity function, providing a substantial fraction of such AGN is found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.     

Ionization freeze-out and hydrogen excitation in the SN IIP atmosphere

We study the nonstationary recombination of hydrogen in the atmosphere of SN 1987A by taking into account ion-molecular processes. The hydrogen excitation due to nonstationary recombination is shown to be enough to explain the observed hydrogen lines in a time interval until day 30 in the absence of additional excitation mechanisms. Thus, the problem of a deficit in the hydrogen excitation that has recently been found in modeling the hydrogen spectrum of SN 1987A at an early photospheric stage by assuming statistical ionization equilibrium is resolved. The mass of the radioactive ⁵⁶Ni with a spherically symmetric distribution in the outer layers is shown to be close to 10^?6 M _⊙. Our model predicts the appearance of a blue peak in the Hα profile between days 20 and 30. This peak bears a close similarity to the observed peak known as the Bochum event. The presence of this peak in the model is attributable to nonstationary recombination and to a substantial contribution of hydrogen neutralization involving H^? and H₂.     

The origin of the high-velocity circumstellar gas around SN 1998S

Modelling of high-resolution Balmer line profiles in the early-time spectra of SN 1998S shows that the inferred fast (≈400 km s⁻¹) circumstellar (CS) gas on days 23 and 42 post-explosion is confined to a narrow, negative velocity gradient shell just above the photosphere. This gas may be identified with a slow  (v<40 km s⁻¹)  progenitor wind accelerated at the ejecta–wind interface. In this scenario, the photosphere coincides with a cool dense shell formed in the reverse shock. Acceleration by radiation from the supernova or by a shock-accelerated relativistic particle precursor are both possible explanations for the observed fast CS gas. An alternative, equally plausible scenario is that the fast CS gas is accelerated within shocked clouds engulfed by the outer shock, as it propagates through the intercloud wind.     

Optical observations of the Be/X‐ray transient system KS 1947+300

ROTSE‐IIId observations of the Be/X‐ray transient system KS 1947+300 obtained between September 2004 and December 2005 make it possible to study the correlation between optical and X‐ray activity. The optical outburst of 0.1 mag was accompanied by an increase of the X‐ray flux in the 2004 observations. A strong correlation between the optical and X‐ray light curves suggests that the neutron star directly accretes from the outflowing material of the Be star. The nearly zero time lag between X‐ray and optical light curves suggests a heating of the disk of Be star by X‐rays. No optical brightening and X‐ray enhancement was seen in the 2005 observations. There is no indication of an orbital modulation in the optical light curve. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)