A nearby long gamma-ray burst from a merger of compact objects


In a surprising twist of events, we found a kilonova associated with a long GRB! The conclusion is that mergers of compact objects can make both short and long GRBs, upending our common beliefs about these explosions.

Troja, E. et al. Nature 612, 228–231 (2022)

Read our article for The Conversation: Unusual, long-lasting gamma-rayburst challenges theories about these powerful cosmic explosions that make gold, uranium and other heavy metals

Castaway Gamma-Ray Bursts

A number of mysterious gamma-ray bursts appear as lonely flashes of intense energy far from any obvious galactic home, raising questions about their true origins and distances. Using data from some of the most powerful telescopes on Earth and in space, we may have finally found their true origins — a population of distant galaxies, some nearly 10 billion light-years away.

Graduate student Brendan O’Connor presented the results in A deep survey of short GRB host galaxies over z~0–2: implications for offsets, redshifts, and environments, Monthly Notices of the Royal Astronomical Society, 515, 4890

The BHianca project was awarded nearly 2 million euros by the European Research Council as part of the Consolidator grant scheme. BHianca, which stands for “Black Hole Interaction and Neutron stars Collisions Across the universe”, will study violent stellar mergers between neutron stars and black holes, and chase the spectacular cosmic fireworks that follow them: gamma-ray bursts, afterglows, kilonovae and more.

Do all stellar mergers launch relativistic jets? Are they the primary cosmic source of heavy metals? What are the properties of cold ultra-dense matter, and how do they affect the observed light? Can we use these mergers for precision cosmology? BHianca is uniquely positioned to timely address these central questions, and lead to seminal results in the nascent field of multi-messenger astronomy.

No light from the merger GW190814

During the last run of observations, the gravitational wave detectors discovered this amazing event GW190814, a merger between a massive black hole and a lighter object, which could either be a heavy neutron star or a very light black hole. This spurred the curiosity of hundreds of astronomers, who searched for any sort of electromagnetic radiation. Unfortunately we did not find any.

Graduate student Aish Thakur led the work summarizing all the observations, and showed that, even without any visible signal, we could still learn about this stellar collision and its aftermath.

Thakur A., Dichiara S., Troja E., A search for optical and near-infrared counterparts of the compact binary merger GW190814, Monthly Notices of the Royal Astronomical Society, Volume 499, Pages 3868–3883

Searching for the radio remnants of short GRBs

When two neutron stars collide, a large amount of fast-moving ejecta is expelled into the outer space . The interaction of these ejecta with the surrounding medium may produce a weak radio flash, detectable in relatively nearby events. Using two of the most sensitive radio facilities (ATCA and the VLA) we re-observed all the nearby short duration gamma-ray bursts to search for this signal. Unfortunately we did not find any yet, and suggest that most neutron star collisions result in a newborn black hole rather than a long-lived magnetar.

Our results were published in Ricci, R., Troja, E., et al., 2020, Monthly Notices of the Royal Astronomical Society, https://doi.org/10.1093/mnras/staa3241

A 1000 days of GW170817

X-ray light from the location of the neutron star merger GW170817 is seen to become brighter during the first 5 months and then rapidly fade away.

Nearly three years after the binary neutron star merger discovered by LIGO and Virgo on August 17 2017, the first (and only) electromagnetic counterpart of a gravitational wave source is still shining in X-rays. Our work provides possible explanations for X-rays that continue to radiate from the collision long after other radiation (radio, optical) faded and way past model predictions. 

Troja et al., 2020, Monthly Notices of the Royal Astronomical Society, 498, 5643

IC328, a galaxy in the nearby Universe possibly hosting the short gamma-ray burst GRB050906. Was this an analogue of GW170817?

Short gamma-ray bursts within 200 Mpc

Are gamma-ray flashes like GW170817 common?

Likely not, is the answer found by Simone Dichiara, a postdoctoral associate working with me at the University of Maryland. No more than a couple of similar events per year happen in our backyard, and finding one is a truly lucky shot. However, we could have missed a few of them in the past and now we plan to re-observe their locations at radio wavelengths and search for the possible remnant of the neutron star collision.

Our findings were published in Dichiara, S., Troja, E., et al., 2020, Monthly Notices of the Royal Astronomical Society, 492, 5011.

Chasing the first neutron star – black hole collision

Over the week-end we have been busy! We are scanning the sky every night hoping to catch a glimpse of light from a distant stellar collision.

Nearby galaxies observed with the Lowell Discovery Channel Telescope. One of them might have been hosting the NS – BH collision seen by LIGO on August 14, 2019.

On August 14, 2019 LIGO and Virgo spotted an unusual signal, dubbed S190814bv, and alerted us that they had detected gravitational waves from an exotic stellar encounter. Later on, the LIGO and Virgo collaboration suggested that it was a black hole swallowing a neutron star. If confirmed, this would be the first time such system is actually observed. We knew that these pairs of exotic objects may hang out together once in a while, but never had a direct confirmation that they existed.

Are neutron star-black hole (NS-BH) encounters as shiny as neutron star collisions? This is what astronomers are trying to understand, but thus far no fireworks were seen.

A candidate kilonova in GRB160821B

Three years ago we used some of the most powerful telescopes, including the Hubble Space Telescope and the Gran Telescopio de Canarias, to observe the short GRB 160821B, a distant explosion caused by the smash-up of two neutron stars over 2 billion light years far away from Earth. We were looking for the radioactive glow produced by the heavy metals forged in the explosion, the so-called kilonova, and were very disappointed not to see it shine.

Images of GRB160821B and its bright host galaxy taken with the Hubble Space Telescope.

After the incredible discovery of GW170817 and its kilonova AT2017gfo, we looked once again at our data wondering whether we had missed a similar signal in GRB160821B. The answer is likely YES.

Our ideas of what a kilonova looks like were not very clear at the time, and we were searching in the wrong places. The kilonova light in GRB160821B peaked at an earlier time than we expected, and was confused with the typical bright light produced by the explosion, the so-called afterglow. Without the example of AT2017gfo, we could not clearly tell the difference between afterglow and kilonova. Our new analysis of the data shows evidence that a red kilonova very similar to AT2017gfo, although fainter, had made its first appearance on August 21, 2016, a year before the LIGO discovery of GW170817.

Cosmic relatives: GW170817 and GRB150101B

A year ago we announced the groundbreaking discovery of GW170817, the first neutron star collision seen through gravitational waves and light. This explosion was nothing like we had ever seen before and we could not understand why. Was GW170817 an odd, once-in-a-lifetime event and we just got lucky to see it? Or was it a common phenomenon? In the past twelve months we played detectives and investigated over 10 years of Swift data to answer this question. The first results were published today on Nature Communications.

Here it s GRB150101B, an explosion very similar to GW170817 but much farther away: 1.7 billion light years!  Three years ago, when this GRB was found, we thought that it looked odd but could not understand why. However, when compared to GW170817, their cosmic DNA seems to match!


GRB150101B, seen by Chandra in X-rays (top two panels), and its home, seen by Hubble in optical and infrared (main panel). Credits: NASA/CXC/STScI/E. Troja

GW170817 is fading away


The X-ray signal from GW170817 compared with the predictions from different models (solid & dashed lines).

After eight months the X-ray signal is finally fading! The latest measurement by the Chandra X-ray Observatory reveal that the afterglow luminosity decreased by 30% in the last three months. However, no worries! The signal will still be observable for years to come. Future observations will be critical to distinguish between competing models and help us understanding this historical event.

The outflow of GW170817 from late time broadband observations

GW170817 continues to surprise! While the infrared glow from the kilonova quickly faded away, the X-ray light from the relativistic outflow continues to shine six months after the two neutron stars collided.
In our new paper we show that this behavior reveals a complex outflow structure, which we will be able to better understand with future observations.

Our work is published in MNRAS Letters.

Screen Shot 2018-05-03 at 11.57.42 PM.png

The X-ray light from GW170817 at the moment of its discovery (left) and 3 months after it (right). The X-ray signal from GW170817 became five times brighter. 


Award ceremony at the Farnesina in Rome

It was a great honor be awarded the Italian Bilateral Scientific Award (also known as the Farnesina prize) by the Minister of Foreign Affairs, Angelino Alfano, and the Minister of Public Education, Valeria Fedeli.

The award is given once a year to “an Italian eminent scientist who, in performing his/her research abroad, has made a remarkable contribution to the advancement of science and technology.”  Very happy to see my work appreciated in my home country.

Breakthrough of the Year 2017

The first multi-messenger observation of a neutron star merger was selected as Breakthrough of the Year 2017 by two prestigious scientific magazines, such as Science and Physics World. It was amazing to be part of this transformational discovery, and to see the excitement from the rest of the scientific community and the general public.  Here you can find the Science’s article describing how this epochal event impacted the life of three early-career scientists, including me.