Marscher A.,Institute for Astrophysical Research |
Jorstad S.G.,Institute for Astrophysical Research |
Jorstad S.G.,Astronomical Institute |
Larionov V.M.,Astronomical Institute |
And 2 more authors.
Journal of Astrophysics and Astronomy | Year: 2011
We are leading a comprehensive multi-waveband monitoring program of 34 γ-ray bright blazars designed to locate the emission regions of blazars from radio to γ-ray frequencies. The 'maps' are anchored by sequences of images in both total and polarized intensity obtained with the VLBA at an angular resolution of ~0. 1 milliarcseconds. The time-variable linear polarization at radio to optical wavelengths and radio to γ-ray light curves allow us to specify the locations of flares relative to bright stationary features seen in the images and to infer the geometry of the magnetic field in different regions of the jet. Our data reveal that some flares occur simultaneously at different wavebands and others are only seen at some of the frequencies. The flares are often triggered by a superluminal knot passing through the stationary 'core' on the VLBA images. Other flares occur upstream or even parsecs downstream of the core. © 2011 Indian Academy of Sciences.
News Article | August 30, 2016
Within the astronomical community, Kipping is best known for his work with exomoons. But his research also extends to the study and characterization of exoplanets, which he pursues with his colleagues at the Cool Worlds Laboratory at Columbia University. And what has interested him most in recent years is finding exoplanets around our Sun's closest neighbor – Proxima Centauri. Kipping describes himself as a "modeler", combining novel theoretical modeling with modern statistical data analysis techniques applied to observations. He is also the Principal Investigator (PI) of The Hunt for Exomoons with Kepler (HEK) project and a fellow at the Harvard College Observatory. For the past few years, he and his team have been taking the hunt for exoplanets to the local stellar neighborhood. The inspiration for this search goes back to 2012, when Kipping was at a conference and was heard the news about a series exoplanets being discovery around Kepler 42 (aka. KOI-961). Using data from the Kepler mission, a team from the California Institute of Technology discovered three exoplanets orbiting this red dwarf star, which is located about 126 light yeas from Earth. At the time, Kipping recalled how the author of the study – Professor Philip Steven Muirhead, now an associate professor at the Institute for Astrophysical Research at Boston University – commented that this star system looked a lot like our nearest red dwarf stars – Barnard's Star and Proxima Centauri. In addition, Kepler 42's planets were easy to spot, given that their proximity to the star meant that they completed an orbital period in about a day. Since they pass regularly in front of their star, the odds of catching sight of them using the Transit Method were good. As Prof. Kipping told Universe Today via email, this was the "ah-ha moment" that would inspire him to look at Proxima Centauri to see if it too had a system of planets: "We were inspired by the discovery of planets transiting KOI-961 by Phil Muirhead and his team using the Kepler data. The star is very similar to Proxima, a late M-dwarf harboring three sub-Earth sized planets very close to the star. It made me realize that if that system was around Proxima, the transit probability would be 10% and the star's small size would lead to quite detectable signals." In essence, Kipping realized that if such a planetary system also existed around Proxima Centauri, a star with similar characteristics, then they would very easy to detect. After that, he and his team began attempting to book time with a space telescope. And by 2014-15, they had been given permission to use the Canadian Space Agency's Microvariability and Oscillation of Stars (MOST) satellite. Roughly the same size as a suitcase, the MOST satellite weighs only 54 kg and is equipped with an ultra-high definition telescope that measures just 15 cm in diameter. It is the first Canadian scientific satellite to be placed in orbit in 33 years, and was the first space telescope to be entirely designed and built in Canada. Despite its size, MOST is ten times more sensitive than the Hubble Space Telescope. In addition, Kipping and his team knew that a mission to look for transiting exoplanets around Proxima Centauri would be too high-risk for something like Hubble. In fact, the CSA initially rejected their applications for this same reason. "MOST initially denied us because they wanted to look at Alpha Centauri following the announcement by Dumusque et al. of a planet there," said Kipping. "So understandably Proxima, for which no planets were known at the time, was not as high priority as Alpha Cen. We never even tried for Hubble time, it would be a huge ask to stare HST at a single star for months on end with just a a 10% chance for success." By 2014 and 2015, they secured permission to use MOST and observed Proxima Centauri twice – in May of both years. From this, they acquired a month and half's-worth of space-based photometry, which they are currently processing to look for transits. As Kipping explained, this was rather challenging, since Proxima Centauri is a very active star – subject to star flares. "The star flares very frequently and prominently in our data," he said. "Correcting for this effect has been one the major obstacles in our analysis. On the plus side. the rotational activity is fairly subdued. The other issue we have is that MOST orbits the Earth once every 100 minutes, so we get data gaps every time MOST goes behind the Earth." Their efforts to find exoplanets around Proxima Centauri are especially significant in light of the European Southern Observatory's recent announcement about the discovery of a terrestrial exoplanet within Proxima Centauri's habitable zone (Proxima b). But compared to the ESO's Pale Red Dot project, Kipping and his team were relying on different methods. As Kipping explained, this came down to the difference between the Transit Method and the Radial Velocity Method: "Essentially, we seek planets which have the right alignment to transit (or eclipse) across the face of the star, whereas radial velocities look for the wobbling motion of a star in response to the gravitational influence of an orbiting planet. Transits are always less likely to succeed for a given star, because we require the alignment to be just right. However, the payoff is that we can learn way more about the planet, including things like it's size, density, atmosphere and presence of moons and rings." In the coming months and years, Kipping and his team may be called upon to follow up on the success of the ESO's discovery. Having detected Proxima b using the Radial Velocity method, it now lies to astronomers to confirm the existence of this planet using another detection method. In addition, much can be learned about a planet through the Transit Method, which would be helpful considering all the things we still don't know about Proxima b. This includes information about its atmosphere, which the Transit Method is often able to reveal through spectroscopic measurements. Suffice it to say, Kipping and his colleagues are quite excited by the announcement of Proxima b. As he put it: "This is perhaps the most important exoplanet discovery in the last decade. It would be bitterly disappointing if Proxima b does not transit though, a planet which is paradoxically so close yet so far in terms of our ability to learn more about it. For us, transits would not just be the icing on the cake, serving merely as a confirmation signal – rather, transits open the door to learning the intimate secrets of Proxima, changing Proxima b from a single, anonymous data point to a rich world where each month we would hear about new discoveries of her nature and character." This coming September, Kipping will be joining the faculty at Columbia University, where he will continue in his hunt for exoplanets. One can only hope that those he and his colleagues find are also within reach! Explore further: Hubble's new shot of Proxima Centauri, our nearest neighbor
Pineda J.E.,Harvard - Smithsonian Center for Astrophysics |
Pineda J.E.,University of Manchester |
Arce H.G.,Yale University |
Schnee S.,U.S. National Radio Astronomy Observatory |
And 11 more authors.
Astrophysical Journal | Year: 2011
We present the detection of a dust continuum source at 3mm (CARMA) and 1.3mm (Submillimeter Array, SMA), and 12CO (2-1) emission (SMA) toward the L1451-mm dense core. These detections suggest a compact object and an outflow where no point source at mid-infrared wavelengths is detected using Spitzer. An upper limit for the dense core bolometric luminosity of 0.05L is obtained. By modeling the broadband spectral energy distribution and the continuum interferometric visibilities simultaneously, we confirm that a central source of heating is needed to explain the observations. This modeling also shows that the data can be well fitted by a dense core with a young stellar object (YSO) and a disk, or by a dense core with a central first hydrostatic core (FHSC). Unfortunately, we are not able to decide between these two models, which produce similar fits. We also detect 12CO (2-1) emission with redshifted and blueshifted emission suggesting the presence of a slow and poorly collimated outflow, in opposition to what is usually found toward YSOs but in agreement with prediction from simulations of an FHSC. This presents the best candidate, so far, for an FHSC, an object that has been identified in simulations of collapsing dense cores. Whatever the true nature of the central object in L1451-mm, this core presents an excellent laboratory to study the earliest phases of low-mass star formation. © 2011 The American Astronomical Society. All rights reserved.
News Article | February 3, 2016
Paradoxically, the most luminous things in the cosmos are actually invisible to the naked eye. They are "blazars," mysterious objects that glow not just with visible light—the kind our eyes can see—but with every kind of radiation, from radio waves to gamma rays. At the Boston University Blazar Lab, astronomers Alan Marscher and Svetlana Jorstad and their students are trying to understand how blazars work and where they get their tremendous energy. They think that blazars are powered by supermassive black holes containing the mass of hundreds of millions of suns. But how do black holes—where gravity is so strong that nothing, not even light, can escape—power the brightest objects in the cosmos? That is the puzzle that Marscher, a professor of astronomy in BU's Institute for Astrophysical Research (IAR), and Jorstad, an IAR senior research scientist, are trying to resolve. Add up all the light that comes from blazars and they are the most luminous objects in the universe. But most of this light isn't in a form that we can see. It is spread out through the entire electromagnetic spectrum—the true rainbow that extends far beyond the colors that our eyes can detect and includes radio waves, X-rays, gamma rays, and more. Though some fluke astrophysical phenomena may shine brighter than a blazar for a few minutes or less, blazars keep up the light show for the long haul. Today, astrophysicists have catalogued thousands of blazars. In fact, say Jorstad and Marscher, if we could see the cosmos with gamma-ray eyes, blazars would dominate the night sky. But what are they, and how do they sustain such powerful cosmic fireworks? When the first blazar was discovered in 1962, astronomers were stumped: they did not know what it was and had never seen anything like it. But time and technology, like NASA's Hubble Space Telescope, have yielded some clues. First, astronomers tracked blazars to ancient galaxies located hundreds of millions, or even billions, of light years from Earth. Each of these galaxies, like our own Milky Way, is centered on a supermassive black hole that's engulfed millions of suns' worth of matter. Somehow, researchers think, those behemoth black holes must be firing up the blazars. But even though nearly every galaxy has a supermassive black hole, only a small fraction of galaxies—about one in ten—is an "active" galaxy, radiating a huge amount of energy. And fewer than one in a thousand active galaxies is a blazar. What makes them different? It all starts with the black hole's diet. Black holes gobble up anything that gets too close. When a black hole is "well fed," says Marscher, matter on its way down the gullet will congeal in a pancake-shaped disk centered on the black hole. Friction in the disk heats it up and makes it glow and flicker with ultraviolet and visible light. That explains one part of the mystery—why some galaxies are "active" when others aren't—but something more seems to happen to transform an ordinary active galaxy into a blazar capable of firing off high-energy gamma rays and X-rays. Astronomers think that "something" is a jet: a fire hose of charged particles, magnetic fields, and radiation that shoots out from the top and bottom of the rotating disk. When one of these jets is pointed directly at Earth, our telescopes pick it up as a blazar. "The black hole sucks nearly everything in from its surroundings," says Marscher, "but it creates so much havoc as everything falls in that somehow jets get shot out." When fast-moving electrons near the black hole meet the strong magnetic field inside the jet, they give off a broad spectrum of radiation, from low-frequency radio waves all the way up to high-energy X-rays. Meanwhile, those electrons can also ram into particles of light, called photons, and give them the extra boost of energy to make gamma rays. Which might leave you asking: What, exactly, kicks the electrons up to such high speeds? Astrophysicists are still debating, but many think that the electrons are whorled through a corkscrew-shaped magnetic field that shoots them out at blinding velocity. Marscher compares the effect to cleaning a pipe out with a snake. "If you keep twisting it around, then it will propel in the forward direction," says Marscher. "If the black hole's rotation can wind up the magnetic field enough, that's what propels the jets out at nearly the speed of light." If that hypothesis is right, the twisty magnetic field should leave a characteristic imprint, called polarization, on light coming out of the jet. But isolating that signature is not easy. To do it, Marscher, Jorstad, and a team of international collaborators had to wait for a blazar to discharge a flare—temporary, concentrated emission—that would give them a chance to trace out the shape of the magnetic field. The team started searching for the polarization signal in 2004, and in 2005, they found just what they were seeking: while peering nearly straight down the barrel of the jet of a powerful, flaring blazar called BL Lacertae, they caught the polarization within the flare rotating by one-and-a-half turns, mapping out exactly the spiral shape astronomers had predicted. They presented their results in Nature in 2008. Flares like this one represent "nature doing its most extreme thing," says Marscher, but flares are rare, and catching them in the act requires long-term, dedicated telescope time. Thanks to a partnership between BU and the Lowell Observatory, Marscher and Jorstad have near-continuous coverage of more than three dozen blazars on the Perkins telescope, a 1.8-meter optical telescope near Flagstaff, Arizona, where Jorstad spends about one week each month. When a flare erupts, she quickly notifies the managers of NASA's Swift satellite, which can be rapidly pointed toward the flare source to capture ultraviolet and X-ray readings, and taps into publicly available data from the Fermi Gamma-ray Space Telescope. By scrutinizing differences in the shape and timing of the flare at different wavelengths, she, Marscher, and their colleagues can deduce the physics behind the flare. Marscher and Jorstad also enlist a network of radio telescopes, called the Very Long Baseline Array (VLBA), to zoom in on the flare and take pictures of it as it moves and changes. Because the telescopes that make up the VLBA are located on opposite sides of the Earth, the VLBA can pick out, or "resolve," fine details about 1,000 times better than the Hubble Space Telescope. In fact, even though the jets are enormous—many light years long, in some cases—they are so far from Earth that the VLBA is the only instrument in the world that can actually see bright spots (technical term: "blobs") moving through the jets. Now, the researchers in BU's Blazar Lab are trying to understand the source of blazars' most energetic gamma ray flares, the blazar equivalent of a baseball pitcher's 100-mile-an-hour fastball. Astrophysicists expected that the gamma rays should all be coming from very close to the black hole at the center of the blazar. But, to everyone's surprise, the BU team found that a major fraction of the gamma rays is coming from a point, light years away. How does such an extreme burst of energy happen so far from the blazar's central engine? The BU blazar team is testing out a variety of ideas using computer models, and they hope to put them to a real-world test soon with the Discovery Channel Telescope, a 4.3-meter optical telescope at Lowell Observatory. Yet the "eureka" moments often belong to the undergraduate and graduate students in blazar lab, who are the first link in the lab's data analysis chain. "They are often the first to tell us when an event—a flare in brightness, change in polarization, or a new 'blob'—appears in the data," says Marscher. "They are in fact directly participating in the exploration of cosmic phenomena." More information: Alan P. Marscher et al. The inner jet of an active galactic nucleus as revealed by a radio-to-γ-ray outburst, Nature (2008). DOI: 10.1038/nature06895
Pineda J.E.,Harvard - Smithsonian Center for Astrophysics |
Goodman A.A.,Harvard - Smithsonian Center for Astrophysics |
Arce H.G.,Yale University |
Caselli P.,University of Leeds |
And 4 more authors.
Astrophysical Journal Letters | Year: 2010
We present NH3 observations of the B5 region in Perseus obtained with the Green Bank Telescope. The map covers a region large enough (11′×14′) that it contains the entire dense core observed in previous dust continuum surveys. The dense gas traced by NH3(1,1) covers a much larger area than the dust continuum features found in bolometer observations. The velocity dispersion in the central region of the core is small, presenting subsonic non-thermal motions which are independent of scale. However, it is because of the coverage and high sensitivity of the observations that we present the detection, for the first time, of the transition between the coherent core and the dense but more turbulent gas surrounding it. This transition is sharp, increasing the velocity dispersion by a factor of 2 in less than 0.04 pc (the 31″ beam size at the distance of Perseus, 250 pc). The change in velocity dispersion at the transition is 3 km s-1 pc -1. The existence of the transition provides a natural definition of dense core: the region with nearly constant subsonic non-thermal velocity dispersion. From the analysis presented here, we can neither confirm nor rule out a corresponding sharp density transition. © 2010. The American Astronomical Society. All rights reserved.