Where are all of the nebulae ionized by supersoft X-ray sources?

February 01, 2016

The ultimate fate of low-mass stars, like our own Sun, is to exhaust the nuclear furnace in their cores, expel their extended atmospheres, and leave behind a hot remnant called a white dwarf. Left to their own devices, these objects will simply cool slowly over billions of years. However, if a white dwarf comes to accrete material from some stellar companion, it can become an incredibly luminous source of extreme UV and soft X-ray emission, a “supersoft X-ray source” or SSS. Such radiation is readily absorbed by any surrounding interstellar gas, producing emission line nebulae. Therefore, we would expect such nebulae to be found accompanying all supersoft X-ray sources. However, of all SSSs found in the past three decades, only one has been observed to have such a nebula. Clearly, something is amiss in our understanding of these incredible objects. Now, scientists at MPA and the Monash Centre for Astrophysics have pieced together the puzzle.

Artist's depiction of an accreting white dwarf

Under the right conditions, a white dwarf accreting hydrogen-rich matter from a binary companion can process all of this material through nuclear burning at its surface, with luminosities and temperatures of thousands of times that of our Sun (1038 erg/s and 105K-106K, respectively). First discovered more than 30 years ago by NASA's Einstein observatory, these close binary supersoft X-ray sources soon became favoured candidates for the progenitors of type Ia supernovae: as white dwarfs accrete material, they may grow to reach the Chandrasekhar mass limit and explode. However, testing this hypothesis by trying to find the true number of such objects has been complicated by the great ease with which the emitted extreme UV and soft X-ray photons are completely absorbed by even a modest amount of intervening interstellar matter.

Therefore, an alternative approach is to use this absorption and search for nebular emission lines in interstellar matter that is ionized by these hot, luminous sources [1,2]. However, narrow-band observations of the vicinity of supersoft X-ray sources in the Magellanic Clouds revealed only one such nebula [3]. This led to a vexing question: is there something very wrong in our understanding of the nature of these sources? Or is there something special about the interstellar environment of CAL 83, where the nebula was found, ­ and not every other SSS? This dilemma put emission line studies of supersoft X-ray sources largely on hold for the next two decades.

In their recent work [4], Tyrone Woods (formerly at MPA, now a research fellow at the Monash Centre for Astrophysics) and Marat Gilfanov (MPA) noted that the gaseous nebula surrounding CAL 83 is at least ten-fold overdense relative to the gas densities found in most of the volume of typical star-forming galaxies. The high surface brightness nebula of CAL 83 thus appears to be the result of a chance encounter of the accreting white dwarf with a region of initially cold, dense interstellar matter. Additional analysis of the size and distribution of cold dense clouds in galaxies (with the aid of mathematics borrowed from the study of concrete porosity) provided further support for this interpretation.

ISM density (vertical axis) required to produce a detectable nebula ionized by a accreting white dwarf (2x10^5 K, with bolometric luminosity L, horizontal axis). The three lines denote a signal-to-noise ratio of 50, i.e. clear detection, for 150, 1500, and 9000 seconds total integration times using the Magellan Baade telescope. For reference, the inferred density and time-averaged luminosity of CAL 83 is also shown.

Most supersoft X-ray sources are likely to lie in much lower density media, with correspondingly lower surface brightness nebulae, which extend to larger radii (up to more than 100 parsecs, compared with 10 parsecs for CAL 83). Even though such nebulae are below the detection threshold of past observations, they are detectable given modest integration times with large modern telescopes such as Magellan or the VLT (see fig. 2).

This not only re-opens a channel for the study of close binary supersoft X-ray sources; given that the decay time for any SSS nebula will be on the order of ten thousand to a hundred thousand years, one may also search for “fossil” nebulae surrounding the sites of SSSs, which have long since stopped accreting. In particular, this includes those that may have exploded as type Ia supernovae in the recent past and in our cosmic neighbourhood. This means that one should be able to resolve the surrounding nebula and inner supernova remnant separately. A deep narrow-band search using the Magellan Baade telescope is already underway, and we may soon measure (or tightly constrain) the temperatures and luminosities of the progenitors of nearby type Ia supernova remnants.

Woods, T. E., & Gilfanov, M.

 

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