Chandra News Issue 23 Summer 2016 X-ray Dust Tomography: The New Frontier in Galactic Exploration by Sebastian Heinz and Lia Corrales Contents X-ray Dust Tomography: the New Frontier in Galactic Exploration . 1 Stephen S. Murray (1944-2015) ��. 8 Daniel E. Harris (1934 - 2015) . 13 Director’s Log, Chandra date: 577065605 ��. 15 Useful Chandra Web Addresses ��. 16 Project Scientist’s Report . 17 Project Manager’s Report . 17 Chandra Source Catalog . 19 ACIS. 20 Ralph Kraft Appointed as New HRC Principal Investigator. 21 HRC . 21 HETG . 22 LETG . 26 CIAO in 2015-16 ��. 29 Brainstorming The Universe in High-resolution X-ray Spectra . 34 Recent Updates to Chandra Calibration . 35 Results of the Cycle 17 Peer Review . 36 Chandra Users’ Committee Membership List . 40 Einstein Postdoctoral Fellowship Program . 41 The Group Formerly Known as EPO . 42 Chandra-Related Meetings and Important Dates ��. 44 2015 Press Releases . 45 The Chandra Newsletter appears once a year and is edited by Rodolfo Montez Jr., with editorial assistance and layout by Tara Gokas. We welcome contributions from readers. Comments on the newsletter, or corrections and additions to the hardcopy mailing list should be sent to: [email protected]. Follow the Chandra Director's Office on Facebook, Google+ and Twitter (@ChandraCDO). Summer 2016 1 X-ray Dust Tomography: the New curves spanning the infrared to ultraviolet, compari- sons of gas phase metal line absorption to solar abun- Frontier in Galactic Exploration dance (depletion), and some infrared absorption and Sebastian Heinz & Lia Corrales emission lines, many of which come from unidenti- fied molecular species (Draine 2003b). X-ray light can magine a dusty basement with bright sunlight provide crucial insight into many outstanding dust Istreaming through a narrow window: The rays of questions through the phenomenon of absorption and light will reflect off tiny dust particles floating through scattering. the air, creating a silver stream of light. This is dust The prospect of scattering of X-rays by interstellar scattering in action, an illustration of a process hap- dust (Overbeck 1965) was realized almost immedi- pening throughout the universe. ately following the discovery of the first X-ray point When astronomers discuss dust, they refer to some- sources outside of the solar system (Giacconi 1962) thing much less macroscopic than the kind of dust you and has been a mainstay of studies of interstellar gas might wipe up from your window sill or the dust-bun- ever since. Surveys of X-ray scattering halos have been ny you find in a hard-to-reach corner under your bed. used to measure relationships between optical extinc- Astronomical dust is anything bigger than a molecule tion, gas column, and distance (Predehl & Schmitt (a typical dust grain spans anywhere from a few hun- 1995, Valencic & Smith 2015). dredths to a few tens of microns, orders of magnitude With the launch of the current generation of flag- smaller than the width of a human hair) and is typi- ship X-ray imaging telescopes, Chandra, XMM-New- cally the only kind of solid matter found in interstellar ton, and Swift, a new diagnostic has become available space. Around 90-100% of interstellar silicon, magne- to X-ray astronomers: High-resolution dust tomogra- sium, and iron—the same materials that comprise the phy from X-ray scattering studies, that is, the use of Earth’s crust—are locked in solid form. dust scattering echoes to measure distances, to map This is no coincidence: Dust serves as an interstel- the distribution of matter throughout the galaxy, and lar transport for these materials. Interstellar dust also to determine the properties of interstellar dust grains. plays a vital role in forming new stars—and planets— Dust Echoes and Halos Explained by shielding molecular gas from destructive ultraviolet X-ray scattering has long been used as a laborato- light so that it can form the dense, cold, clouds that ry tool to study the structure of solid matter through are the nurseries of stars. Dust grains also serve as a Bragg-crystallography. Like solid matter in the lab, in- catalyst by providing a surface for chemical reactions terstellar dust grains can scatter X-ray photons. They on which to form complex molecules and ices in mo- can also absorb X-rays, and whether a photon is ab- lecular clouds and protostellar disks. sorbed or scattered by a dust grain (or whether it just It is probably fair to say that the typical astronomer flies by unaffected) is set by the dust scattering cross will have an underdeveloped appreciation for interstel- section—the probability of a photon being scattered in lar dust. That’s because dust gets in the way of observa- a particular direction by a dust grain—which depends tions at visible and shorter wavelengths and makes life very strongly on the angle at which the photon inter- harder for astronomers by absorbing and scattering acts with the dust grain and the outgoing angle of the photons, making images darker and, at X-ray wave- scattered photon. lengths, blurrier. Scattering is restricted to very small angles, a de- But dust deserves a second look not just because it is gree or smaller, and all X-ray dust scattering is for- a key ingredient in the formation of planets and stars: ward-scattering. Dust scattering cross sections are so Dust can also be an important observational tool. For small that typically only a few percent of the X-ray example, infrared astronomers have long used the light gets scattered. In order to see meaningful surface emission from cool dust grains at long wavelengths to brightness from scattering, one needs both a bright search for concentrations of dense gas that are invisi- X-ray source and significant amount of dust between ble at other wavelengths to find heavily obscured black the observer and the source. holes in the centers of distant galaxies. At any given time the Galaxy is illuminated by doz- Much of our knowledge of interstellar dust is indi- ens of bright point sources, all powered by compact rectly inferred from meteoritic materials, extinction objects: Neutron stars and black holes. These sources 2 CXC Newsletter The front cover of this newsletter shows the 2014 echo from the neutron star Cir- cinus X-1, with four colorful rings, cen- tered on the source. To understand why we see multiple rings and what kind of in- formation is encoded in images like this, it is instructive to go through the signal step by step, illustrated by the cartoon in Figure 2. Suppose a neutron star emits a flash of X-rays. Without any dust between us and the neutron star, all we would see is a Figure 1: X-ray image of the entire sky from several years of observation, taken single flash of X-rays from the position of by the MAXI instrument aboard the International Space Station (Matsuoka et al. 2009), showing dozens of sources in the plane of the Milky Way. the source. How does dust alter the signal? Dust make excellent beacons around which we can study grains can scatter some of the X-ray photons initially the interaction of X-ray light with interstellar dust. emitted by the neutron star in (slightly) different di- Figure 1 shows an image of the X-ray sky, taken by the rections towards our telescope. We will see these scat- MAXI instrument onboard the International Space tered X-rays arriving from a different direction in the Station, dotted with bright point sources. sky with a time delay compared to the flash of X-rays Most bright X-ray sources lie in the plane of the arriving directly from the black hole. Because of the Milky Way, which is good news for the study of dust change in direction, the path the scattered photons because most of the Galaxy’s dust is found right along take is longer than the direct path from the source— that plane. How, then, does interstellar dust interact typically a few days. with the X-rays emanating from a distant source, and X-rays arriving at the telescope some time after the what do we see with Chandra as a result? Observation- initial flash must have been scattered on the surface of ally, we can categorize the signal into three groups, an ellipsoid, with the telescope and the X-ray source based on the appearance and information content that at the two focal points. That is because an ellipsoid is can be extracted: the surface of constant path length between the two Constant dust scattering halos are formed by per- focal points and any point on the surface, so any pho- sistently bright X-ray sources behind a thick veil of ton scattered on the surface of that ellipsoid will have dust. These fuzzy hazes of X-ray light were first dis- traveled the same distance to the observer. The longer covered by the Einstein X-ray Telescope (Rolf 1983) the time delay (the later the observation) the larger the and have been studied for decades. They are the easiest ellipsoid. Because of the large distance involved and to observe—just point your X-ray telescope at a bright the small angles that scattering is restricted to, dust X-ray point source in the plane of the Galaxy—but scattering ellipsoids are extremely long—about a thou- hardest to extract information from because the inter- sand times longer than they are wide, so the scattering play between scattering angle and dust location is hard ellipsoid in the cartoon is not to scale. to disentangle. While dust can be found throughout interstel- Variable dust scattering halos are formed by bright, lar space, the largest concentrations along any given but variable X-ray sources behind a thick veil of dust.