This week on HST

HST Programs: July 14 - July 20, 2014

Selected highlights

GO 12903: The Evolutionary Link Between Type 2 and Type 1 Quasars

Artist's impression of the black hole and surrounding torus in an AGN

This year (2013) is the fiftieth anniversary of the recognition that QSOs (quasars, quasi-stellar objects) were extremely luminous objects lying at substantial redshifts. The central engine powering these luminous objects is now recognised as a supermassive black hole, marking the central regions of a galaxy. As such, QSOs are clearly related to (and more luminous than) active galactic nuclei (AGN). Like AGNs, QSOs have been segregated into two categories based on their spectral properties: systems with broad lines are characterised as Type 1 QSOs; systems with narrow lines are classed as Type 2. As with AGN, the underlying cause of these differences is generally believed to reside more in our perspective than on the sources themselves: heavily obscured systems, where the central accretion disk lies behind a thick veil of dust, are observed as Type 2 systems; they are expected to evolve to form Type 1 systems as the dust is ablated and destroyed. The present SNAP program aims to test this scenario by coupling mid-infrared Herschel observations, probing the dust environment, with HST near-infrared WFC3 imaging of targets drawn from two matched samples of Type 1 and Type 2 QSOs.

GO 13284: Unravelling the evolution and accretion morphology of an extraordinary black hole X-ray binary

An artist's representation of a low-mass X-ray binary

Low-mass X-ray binaries are generally believed to be binary systems where a compact, high-mass component, either a black hole or a neutron star, is accreting mass from a lower-mass component that is overflowing its Roche lobe. These systems are extremely luminous at short wavelengths, and are readily detected at X-ray wavelengths in both the Milky Way and nearby galaxies. The present program targets a Galactic LMXB, system Swift J1357.2-0933. Discovered during an outburst in January 2011, extensive observations have revealed a 2.8 hour periodicity that is likely associated with the orbital period of the M-dwarf companion. The present program aims to use COS to obtain far-UV spectra, probing the morphology of the gas accreting onto the black hole.

GO 13332: A SNAPSHOT Survey of the Local Interstellar Medium: New NUV Observations of Stars with Archived FUV Observation

A map of the Local Stellar Neighbourhood

Understanding the nature and structure of gas within the interstellar medium is a key step towards understanding how material is recycled and how energetic processes, such as stellar winds and outflows, feed energy into the overall system. UV spectroscopy plays a key role in probing these effects: hot, background objects that produce relatively few intrinsic absorption features serve to map the the velocities and temperatures within the intervening gas along the line of sight. Observations of quasars are used to probe galaxy halos at moderate and high redshift; observations of hot stars provide similar information for gas in the Milky Way. The present program is using STIS to target stars within 100 parsecs of the Sun, studying the nearby interstellar medium. All of these stars have prior observations at far-UV wavelengths; the STIS data will cover the near-UV, surveying Fe II and Mg II absorption.This program builds on observations spanning 36 targets from Cycle 17.

GO 13463: Detecting Isolated Black Holes and Neurton Stars through Astrometric Microlensing

A rather spectacular version of black hole lensing.

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The expected astrometric signal from a black hole lens is > 1.4 millarcseconds, just measureable with HST. This program aims to capitalise on this fact by searching for lensing by black holes and neutron stars in the Galactic field. Over the past three cycles, the program has targeted a number of sources that exhibit long-duration lensing events in the Galactic Bulge. The present set of Cycle 21 observations will complete the search for astrometric signals due to massive foreground objects.