Archive for October, 2017

The first neutron star merger

Posted October 16, 2017 By grigoris

A new era, of multi-messenger astronomy (combining gravitational waves, particles, photons), has been born today. LIGO/VIRGO plus 70 teams around the world announced the first detection of colliding neutron stars, which means that a gravitational wave detection (GW170817) triggered the observations using almost everything available (from gamma rays to radio wavelengths) that identify and follow its electromagnetic counterpart.

I feel very fortunate that I participated in the above exciting event, along with Thodoris Bitsakis and Carlson Adams, by obtaining spectroscopic and photometric observations from the Clay telescope (at Las Campanas Observatory), a few days after the event. These are included in the following papers published in Science:

Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis

M. R. Drout, A. L. Piro, B. J. Shappee, C. D. Kilpatrick, J. D. Simon, C. Contreras, D. A. Coulter, R. J. Foley, M. R. Siebert, N. Morrell, K. Boutsia, F. Di Mille, T. W.-S. Holoien, D. Kasen, J. A. Kollmeier, B. F. Madore, A. J. Monson, A. Murguia-Berthier, Y.-C. Pan, J. X. Prochaska, E. Ramirez-Ruiz, A. Rest, C. Adams, K. Alatalo, E. Bañados, J. Baughman, T. C. Beers, R. A. Bernstein, T. Bitsakis, A. Campillay, T. T. Hansen, C. R. Higgs, A. P. Ji, G. Maravelias, J. L. Marshall, C. Moni Bidin, J. L. Prieto, K. C. Rasmussen, C. Rojas-Bravo, A. L. Strom, N. Ulloa, J. Vargas-González, Z. Wan, D. D. Whitten

On 17 August 2017, gravitational waves were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical, and infrared light curves of SSS17a extending from 10.9 hours to 18 days post-merger. We constrain the radioactively powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in r-process nucleosynthesis in the universe.

2017, Sci, 358, 1570 / NASA/ADS / Science 16 Oct 2017, eaaq0049

Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger

B. J. Shappee, J. D. Simon, M. R. Drout, A. L. Piro, N. Morrell, J. L. Prieto, D. Kasen, T. W.-S. Holoien, J. A. Kollmeier, D. D. Kelson, D. A. Coulter, R. J. Foley, C. D. Kilpatrick, M. R. Siebert, B. F. Madore, A. Murguia-Berthier, Y.-C. Pan, J. X. Prochaska, E. Ramirez-Ruiz, A. Rest, C. Adams, K. Alatalo, E. Bañados, J. Baughman, R. A. Bernstein, T. Bitsakis, K. Boutsia, J. R. Bravo, F. Di Mille, C. R. Higgs, A. P. Ji, G. Maravelias, J. L. Marshall, V. M. Placco, G. Prieto, Z. Wan

On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after merger. Over the first hour of observations the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measure the photosphere cooling from 11000(+3400,-900) Kto 9300(+300,-300) K, and determine a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a begin displaying broad features after 1.46 days, and evolve qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high velocity, lanthanide-free material.

2017, Sci, 358, 1574 / NASA/ADS / Science 16 Oct 2017, eaaq0186

[1] About GW170817

The last day at Valparaiso

Posted October 3, 2017 By grigoris

I think it had everything:

– waking up early to go to the bank,
– then go to the university and finalize some aspects of my talk (as a final contribution),
– go for lunch with the whole group,
– return to the university,
– to say goodbye to people,
– do the presentation,
– materialize a long-postponed discussion,
– show/discuss a script,
– buy some small gifts on the way home,
– where I just dropped my things,
– to go to an outreach talk (in spanish),
– to continue for the last drinks with all the friends from the university,
– say farewell to all,
– return home to take a shower
– and rest a bit,
– to finally think of today
– and write this post.

Definitely not the most typical day, but it isn’t after all. It is strange as I remember my first days in Valpo, almost 9 months before. It was short but nevertheless a very full experience and I loved it all. I have only a couple of days to spend at Santiago before the final departure on Thursday, to head to Greece. A process ends but another starts.

For lunch with the group (clockwise from left): Ignacio Araya, Michel Cure, Lydia Cidale, Maxi Haucke, Catalina Arcos, Alex Gormaz, and me (I do not smile because I really couldn’t keep my eyes open due to the Sun!)
[Photo by Ignacio]

A memory from Valparaiso.
Gracias a tod@s por todo!
[Photo by Michel]

A short talk for IFA-UV Monday meetings

Posted October 2, 2017 By grigoris

Every Monday the IFA-UV organizes a meeting that starts with a short (or longer) presentation of 20 mins. The whole process requires the active participation by the students to present the speaker, control, the discussion, while there is also a feedback given by the audience to the speaker. This is a great opportunity for the students to help them gain experience especially when they have to present. However, the topics (and the stage) is open to everyone. And this Monday was my turn to present a short talk on:

“Searching for Hα counterparts of Be/X-ray binaries in the Small Magellanic Cloud”

Abstract: The Small Magellanic Cloud (SMC) hosts a large number of high-mass X-ray binaries, and in particular of Be/X-ray Binaries (BeXRBs; neutron stars orbiting OBe-type stars), offering a unique laboratory to address the effect of metalicity. One key property of their optical companion is Hα in emission, which makes them bright sources when observed through a narrow-band Hα filter. We performed a survey of the SMC Bar and Wing regions using wide-field cameras (WFI@MPG/ESO and MOSAIC@CTIO/Blanco) in order to identify the counterparts of the sources detected in our XMM-Newton survey of the same area. We obtained broad-band R and narrow-band Hα photometry, and identified ~10000 Hα emission sources down to a sensitivity limit of 18.7 mag (equivalent to ~B8 type Main Sequence stars). We find the fraction of OBe/OB stars to be 13% down to this limit, and by investigating this fraction as a function of the brightness of the stars we deduce that Hα excess peaks at the O9-B2 spectral range. Using the most up-to-date numbers of SMC BeXRBs we find their fraction over their parent population to be ~0.002-0.025 BeXRBs/OBe, a direct measurement of their formation rate.

After the talks it comes another important step which is the wine and cheese ceremony, accompanying the informal discussions.