Monsters in the Deep Cosmos

There are many enduring mysteries in astrophysics that challenge our theoretical understanding of different phenomena, are understood through careful observation or simulation, then become deep mysteries again when new data conspires to reveal some heretofore ignored or misunderstood aspect of their character. One of the most prominent examples of this in the modern era are black holes in the early Universe, recently reviewed by Volonteri and Billovary (Rep. Prog. Phys. 75, 124901 (2012), arxiv:1209.2243).

The Report poses several questions which are still much debated and the focus of ongoing research: What physical mechanisms lead to the formation of the first massive black holes? How massive were the initial massive black hole seeds? When and where did they form? How is the growth of black holes linked to that of their host galaxy? These questions, and many others, can be considered with gravitational wave observations from future space-based detectors like LISA through the study of massive black hole binaries.

(LEFT) Composite of 3C75 in X-ray (blue) and radio (pink) [X-ray: NASA/CXC/AIfA/D.Hudson & T.Reiprich et al.; Radio: NRAO/VLA/NRL]. (RIGHT) NGC 326 from VLA [NRAO].

There is ample observational evidence to infer the existence of massive black hole binaries.  Consider the images to the right, showing the famed double source in 3C 75 (suggesting two, active, accreting black holes; see Chandra’s page, or 3C75’s SIMBAD record), and the “kinked jet” source in NGC 326 (which might be due to a jet orientation change after the merger of two spinning black holes; NGC 326’s SIMBAD record).  These kinds of “x-shaped sources” were first cataloged by Leahy and Parma [1992].

The figure below shows the last two hours of a simulated gravitational waveform (courtesy, John Baker [GSFC])from a 10^5 + 10^5 Msun black hole binary at z = 15, including the expected instrumental and astrophysical noise for an observatory like LISA. The waveform is easily detectable! Heuristically, gravitational wave astronomers think about binary black holes in three distinct phases of their evolution.

Gravitational waveform for the last two hours before merger of a 10^5 + 10^5 massive black hole binary at z=15. The signal is superposed with noise appropriate to a LISA style detector. [J. Baker, GSFC]

1. Inspiral. This phase lasts much longer than the short bit shown in this two-hour segment of the waveform. The long slow inspiral of two massive black holes will be visible to a spaceborn gravitational wave observatory for a year or more. During this time, the localization by a detector in a LISA-like orbit will slowly pare down the source’s location on the sky. Information gleaned from waveform characterization in this time will also help determine the initial spins of the black holes, the total mass of the system, and the luminosity distance (one of the most important, classic results that will emerge from gravitational wave astronomy: Schutz [1986]).

2. Merger. Long regarded as one of the holy grails of numerical simulation, the last decade has seen tremendous strides in our understanding of the late inspiral and merger of black hole systems. Current gravitational wave research in this area is focused on producing accurate gravitational waveforms, and predicting the astrophysical outcome of the mergers, particularly as they depend on spin and mass ratio, and whether they result in significant “kicks” to the final remnant (see a recent review by Komossa, arxiv: 1202.1977).

3. Ringdown. The remnant that forms from the merger of two black holes will itself be a black hole. How big is it? How is it spinning? How do the distortions from merger settle down into a well behaved black hole described by general relativity? The answers to these questions are encoded in the ringdown phase, when the black hole is excited much like a struck bell, radiating the distortions and energy away in gravitational waves. This is the phase where searches for electromagnetic counterparts are expected to begin in earnest, depending on whether there is prompt or delayed emission associated with the merger event and the interaction of the remnant with the surrounding astrophysical environment.

The biggest unknown with regard to these sources is the number of them that will merge as a function of time. That uncertainty, in and of itself, is one of the outstanding questions in this field, deeply connected to formation and growth scenarios for massive black holes. Recent provocative ideas from McWilliams et al (arxiv: 1211.5377) and Sesana (arxiv:  1211.5375) have examined models for massive black hole merger scenarios (particularly with an eye toward another gravitational wave detection technique, known as puslar timing arrays). Their results suggest the number of detectable systems may be much higher than previously estimated, underscoring how poorly constrained our knowledge of the massive black hole distribution functions are using current observational techniques. Gravitational wave detections of binary black hole systems will be a direct measurement of the merger rate that will allow us to significantly constrain the models currently in vogue.

Measuring the inspiral of double white dwarfs

For almost 40 years, the de facto indirect evidence for the existence of gravitational waves has been precision measurements of the slow orbital inspiral of the Hulse-Taylor binary pulsar (PSR B1913+16). The slow loss of orbital energy and resultant shrinking of the orbit precisely matches the energy loss due to gravitational radiation emission predicted by general relativity.

The measured inspiral of PSR B1913+56 (red dots) with the predicted inspiral due to the emission of gravitational radiation (line).

Searches for more binary neutron star systems like PSR B1913+16 have only turned up a handful, including the Double Pulsar PSR J0737-3039 (for a discussion of the demographics of neutron star systems, including binaries, see the excellent review by Duncan Lorimer).

PSR B1913+16 has an orbital period of 7.75 hours, putting the peak frequency of its emitted gravitational waves below the expected observation window of most space-based gravitational wave detector concepts.  One might imagine that other pulsar systems might exist in tighter orbits, making them prime candidates for a mission like eLISA or SGO.  However, our expectations from population synthesis studies of the galaxy suggest that only a handful of NS-NS binaries might exist in a galaxy like the Milky Way.  By contrast, there are expected to be some 10 million + binaries that are comprised of white dwarfs and a companion.

Many such systems have been discovered by electromagnetic telescopes, and are expected to be strong gravitational radiators in the millihertz band.  These known binaries are called verification binaries (for a detailed listing, see Gijs Neleman’s verification binary wiki).

Recently, astronomers have replicated the binary pulsar feat with a close, white dwarf binary: they have measured the slow inspiral of the orbit in a WD-WD system due to the emission of gravitational radiation.  As with the Hulse-Taylor pulsar, this is an indirect measurement of gravitational waves, but provides definitive confirmation that predicted sources for future gravitational wave observatories do exist and are behaving as general relativity predicts they will.

Artists conception of two close white dwarfs, slowly spiraling together.

The system is known as SDSS J0651+2844, and contains two white dwarfs in a 12.75 minute orbit (arxiv:1208.5051 | ApJ).  What makes J0651+2844 such a useful system is it is eclipsing— the orbital plane of the binary is oriented on the sky such that from the vantage point of earth, the stars pass in front of each other once every six minutes.  This regular clockwork of eclipses makes precision timing with the light curve possible.  Long term monitoring of the orbital period shows a slow but steady decrease in the orbital period; the two stars are shortening their orbital period by about 0.31 ms/yr.  This slow inspiral is consistent with the loss of energy due to the emission of gravitational radiation.  Given the orbital period, distance and physical character of this system, it should be a very loud source for space-based missions like eLISA or SGO.

We’ve known for a long time that there are ultracompact binaries in the galaxy; indeed there is a long history of simulating the ultra-compact binary population based on observations of known systems (see, for instance, the seminal work by Hils, Bender and Webbink [1990]).  The discovery of J0651+2844 is important because it validates our expectations from theory.  Our theoretical expectations from general relativity are supported by a still-limited number of observations of systems where gravitational wave emission is important to the dynamics.  In addition, the ultracompact binary population of the galaxy has been one of the principal science pillars for space-based gravitational wave observatories. They provide a detailed measure of the fossil record of Sun-like stars throughout the entire volume of the Milky Way, they provide a method to probe the interaction and evolutionary scenarios for doubly degenerate systems, and they are a progenitor population for most of the popular models that are being considered as viable candidates for Type Ia supernovae.  Moreover, J0651+2844 is the first system that has been observed to be inspiralling due to the emission of gravitational radiation that will also be visible to a space-based gravitational wave observatory.

Welcome!

Welcome to Gravity is Talking.  Gravitational wave astronomy is still a fledgling observational science, and like all new branches of scientific exploration grows in bursts and non-linear jumps. This has particularly been the case over the past two years with the low-frequency gravitational wave community that had collected around the LISA mission concept.

A robust community grew around the joint NASA/ESA LISA mission concept, including astronomers, astrophysicists, general relativity theorists, data analysis experts, spacecraft designers, and experimental physicists; sometimes members of this community wear these different hats at the same time.  Although that mission concept is no longer being pursued, the community has forged to ahead, examining the low-frequency gravitational wave science that can be done and the feasibility of new and creative mission concepts.

In Europe, those efforts have led to the creation of the eLISA consortium (http://elisa-ngo.org), which is currently working toward competing in Europe’s upcoming competition for the expected “L2” mission.  In the United States efforts have focused on the Gravitational Wave Mission Concept Study, which fielded some 20 different mission concepts, studied several in extreme detail, and produced a final report summarizing the findings.

All the while, new and interesting astrophysical results have emerged that continue to support our basic premise here at Gravity is Talking: low-frequency gravitational wave astronomy will be an important tool for probing the Cosmos, supplementing what we can learn from traditional astronomical observations, as well as expanding the scope of our understanding by probing high-energy astrophysical systems in ways that have never been possible before.

Our primary interest here at Gravity is Talking is the low-frequency part of the gravitational wave spectrum, covered by space-based observatory concepts like the long-standing LISA concept.  Here, we will feature the latest news about the evolution of mission concepts, both in Europe and the United States, as well as news from the broader scientific community that is engaged in research related to astrophysical sources, data analysis, and technology. We will also feature news, as it emerges, from our colleagues working on high-frequency ground-based efforts like LIGO and Virgo, as well as the pulsar-timing-array efforts around the world.

“We” are currently a small group of low-frequency gravitational wave scientists and astrophysicists.  Our team, the principal instigators and contributors to this forum, includes Scott Hughes (MIT), Shane Larson (Utah State University), and Michele Vallisneri (JPL).  Over time, we expect this cabal to grow, and welcome interest and inquiries.

In the meantime, if you have interesting information to share, or new scientific results that you would like to broadcast to our larger community, please contact us!