The research interests of the group are focused on multi-wavelength observations and theoretical modelling of accreting sources, galaxies, supernova remnants, and the Ionised Intergalactic Medium.

We use data obtained with a wide suite of space observatories observing in X-ray, Ultraviolet, Optical and Infrared wavelengths. In addition we use data from ground-based optical and infrared facilities such as the Skinakas Observatory. The goal of these observations is: to understand the physical processes which produce the observed radiation, study the evolution of stellar systems and galaxies, and probe the baryonic content of the Universe.

The theoretical efforts of our group are focused on the radiation production mechanisms in accreting sources, the interaction between radiation and matter and the structure of galaxies.

Annual Reports:
A detailed description of the activities of the Group during 2018 can be found in our 2018 Annual report. Previous annual reports can be found here.

Recent science highlights from our group include:

The Musca molecular cloud: An interstellar symphony

Dr. Aris Tritsis and Prof. Konstantinos Tassis of the Department of Physics of the University of Crete have reported the first-ever discovery of normal modes in a molecular cloud, called Musca. This discovery has allowed the two researchers to precisely determine the physical dimensions of the cloud.

The Musca cloud appears as a luminous source when seen in infrared light. Data for this image are from the European Space Agency’s Herschel Space Observatory.

Molecular clouds can only be seen as two-dimensional projections on the plane of the sky with no way of knowing their true shapes. This is a fundamental constraint for studies of the interstellar medium, since the 3-dimensional (3D) structure of clouds encodes critical information regarding the physical processes that control the formation of stars and planets. Therefore, the determination of the 3D shape of clouds has been aggressively pursued through primarily statistical studies which however, have yielded contradictory results and do not provide information on a cloud-by-cloud basis.

Musca is an isolated cloud seen from the Southern hemisphere and it is one of the most ordered molecular clouds observed. The main dense structure of Musca, inside which stars will eventually form, is surrounded by hair-like structures called striations. In a previous study, also by Aris Tritsis and Konstantinos Tassis (Tritsis & Tassis 2016, MNRAS, 462, 3602), it was found that the only viable mechanism that could account for the formation of striations was through the excitation of magnetic pressure waves. However, in the case of the isolated Musca, the waves creating striations were trapped setting up normal modes.

Computer simulation of the Musca cloud, performed at the Metropolis HPC Facility of the Crete Center for Quantum Complexity and Nanotechnology (CCQCN) of the University of Crete. The colour scale denotes density of the gas in the cloud. Black lines threading the cloud almost at right angles to its plane are the magnetic field lines.



Because of its needle-like appearance on the plane of the sky, Musca was considered to be the poster-child of an interstellar filament against which many theoretical models were put to the test. However, in contradiction to conventional wisdom the normal-mode analysis of striations has shown that Musca is in fact a sheet seen edge-on.

A video presenting a summary of the results of the paper is available here.

Musca is the first cloud found to be oscillating as a whole, and the largest structure in the Galaxy to which a normal-mode analysis has been successfully applied to date.

Article:Magnetic Seismology of Interstellar Gas Clouds: Unveiling a Hidden Dimension", A. Tritsis, K. Tassis, Science 360, 6389, pp.635-638 (2018).








Warped disks during giant outbursts in Be/X-ray binaries: evidence from optical polarimetry

Drs Pablo Reig and Dmitry Blinov of the Institute of Astrophysics of the Foundation for Research & Technology-Hellas have found the first evidence of precessing warped disks from polarimetric observations of the Be/X-ray binary 4U 0115+65.

Artist impression of a BeX-ray binary

Be/X-ray binaries are accreting pulsars, in which a neutron star orbits a Be star. The neutron star accretes matter from its companion and generates hard X-rays. When, active these systems are among the brightest in the X-ray sky, with peak luminosities of LX~1038 erg/s during outbursts. The source of matter available for accretion is the circumstellar disk around the B star's equator. Although there is general consensus that the outbursts are caused by the mass transfer from the Be disk to the neutron star, the detailed mechanism is uncertain. In particular, it is still a matter of debate how the Be disk links to the X-ray activity. We observe giant X-ray outbursts in systems with large and small disks.  Likewise, systems with large disk show no X-ray activity.

Variation of the polarization angle (top) and polarization degree (bottom) during a giant X-ray outburst. The dotted red lines mark the beginning and end of the X-ray outburst, while the dashed-dotted black line corresponds to periastron.

The current idea is that the giant outbursts occur when the neutron star captures a large amount of gas from a warped, highly misaligned and eccentric Be disk. The models show that highly distorted disks result in enhanced mass accretion when the NS gets across the warped part.

We have found the first observational evidence for warped disks during an X-ray outburst using polarimetry. The light coming from the Be star is linearly polarized. Linear polarization results from Thomson scattering, when photons from the Be star scatter with electrons in the Be disk. The light becomes polarized perpendicularly to the scattering plane, which roughly coincides with the plane of the circumstellar disk. The polarization angle then gives information about the orientation of the disk.

Therefore, if X-ray outbursts In Be/X-ray binaries require tilted, misaligned, warped eccentric disks that precess, then the polarization angle should change.

Illustration 2 shows the variation of the polarization angle and polarization degree with time during the course of an outburst. The data were obtained using the RoboPol polarimeter at the 1.3-m telescope of the Skinakas Observatory.

We interpret this variability as evidence for a warped precessing disk, supporting models that predict highly perturbed disks as the origin of giant outbursts in BeXB.



Article: Warped disks during type II outbursts in Be/X-ray binaries: evidence from optical polarimetry, Reig P. & Blinov D. 2018, A&A 619, A19


Cosmic Magnifying Lens Reveals Inner Jets of Black Holes

An international team of astronomers, including Prof. Vasiliki Pavlidou of the Institute of Astrophysics and the Department of Physics of the University of Crete have discovered a unique lensing system in space, in which a 10,000-solar-mass lens is magnifying a much more distant galaxy containing a jet-spewing supermassive black hole. The discovery provides the best view yet of blobs of hot gas that are shot outward from supermassive black holes.

Schematic of the gravitational lens geometry (Credit: A. Readhead, Caltech).

Many supermassive black holes at the centers of galaxies blast out jets of gas traveling near the speed of light. The gravity of black holes pulls material toward them, but some of that material ends up ejected away from the black hole in jets. The jets are active for one to 10 million years—every few years, they spit out additional clumps of hot material. With the new gravitational lensing system, these clumps can be seen at scales about 100 times smaller than before – with a resolution of a millionth of a second of arc, which is equivalent to viewing a grain of salt on the moon from Earth. 

A critical element of this lensing system is the lens itself. If confirmed, this would be the first lens of "intermediate" mass—which means that it is bigger than previously observed "micro" lenses consisting of single stars and smaller than the well-studied massive lenses as big as galaxies. The lens, dubbed a "milli-lens," is thought to be about 10,000 solar masses. It is likely a cluster of stars, but it could also be a dark-matter agglomerate.
An advantage of the milli-sized lens is that it is small enough not to block the entire source, which allows the jet clumps to be magnified and viewed as they travel, one by one, behind the lens. What's more, the lens itself is of scientific interest because not much is known about objects of this intermediate-mass range.
The new observations are part of Caltech's Owens Valley Radio Observatory (OVRO) program to obtain twice-weekly images of 1,800 active supermassive black holes and their host galaxies, using OVRO's 40-meter telescope, which detects radio emissions from celestial objects. The program has been running since 2008 in support of NASA's Fermi mission, which observes the same galaxies in higher-energy gamma rays.

In 2010, the OVRO researchers noticed something unusual happening with an active galaxy called PKS 1413+ 135. Its radio emission had brightened, faded, and then brightened again in a very symmetrical fashion over the course of a year. The same type of event happened again in 2015. After a careful analysis that ruled out other scenarios, the researchers concluded that the overall brightening of the galaxy is most likely due to two successive high-speed clumps ejected by the galaxy's black hole a few years apart. The clumps traveled along the jet and became magnified when they passed behind the milli-lens.

 The variation on the radio emission of PKS 1413+ 135 as a function of time (Vedantham et al. 2017)

The international research team was led by Anthony Readhead (emeritus Professor at Caltech and a collaborator on many University of Crete key projects, including RoboPol and PASIPHAE, and it included Institute of Astrophysics affiliated researcher Vasiliki Pavlidou, who has been a member of the OVRO collaboration since 2008.
Other collaborators of this work include Harish Vedantham (the lead author), Timothy Pearson and Vikram Ravi of Caltech, Walter Max-Moerbeck and Anton Zensus of the Max Planck Institute for Radio Astronomy; Talvikki Hovatta of University of Turku and the Aalto University Metsähovi Radio Observatory; Anne Lähteenmäki and Merja Tornikoski of the Aalto University Metsähovi Radio Observatory; Mark Gurwell of the Smithsonian Astrophysical Observatory; Roger Blandford of Stanford University; and Rodrigo Reeves of the University of Concepción.

More information is also available in the original Caltech Press Release.

Articles: “Symmetric Achromatic Variability in Active Galaxies: A Powerful New Gravitational Lensing Probe?” by Vedantham et al. 2017, The Astrophysical Journal, 845, 89, and "The Peculiar Light Curve of J1415 + 1320: A Case Study in Extreme Scattering Events" by Vedantham et al. 2017, The Astrophysical Journal, 845, 90