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My main research interests include time-domain astronomy, observational cosmology, and multi-messenger astronomy. I am interested in some of the greatest questions:
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  • How the matter is distributed? What is the dark matter?

    • Project 1: Measuring velocity/density fields using Type Ia supernovae.

  • What is the age of the Universe? Is the difference between the local and early Universe values real?

    • Project 2: Measuring the Hubble constant using Type II supernovae.​

  • What is the future of the Universe? What is the dark energy?

    • Project 3: Using Type Ia supernovae to measure the dark energy nature.

    • Project 4: Using Type II supernovae to measure the matter and dark energy densities.

  • What are the supernovae? How do they explose?

    • Projects 5,6,7,8: Studying photometric (colours) and spectroscopic (velocities)  properties of large sample and weird objects.

  • What is the dominant particle population responsible for the high-energy emission? What could be the natural accelerators that generate neutrinos?

    • Projects 9,10: γ-ray blazar flare correlations: understanding the high-energy emission process using ASAS-SN and Fermi light curves. Search for gravitational waves and neutrinos optical counterpart.

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To answer those questions, I use the Hubble diagram (distance vs redshift) as a cosmological test and the supernovae as extragalactic distance indicators. I also obtain multi-messenger information from optical surveys (ASAS-SN, ATLAS, Pan-STARRS, ZTF), gamma-ray photons (Fermi), neutrinos (IceCube), and gravitational waves (LIGO/Virgo) to characterise gravitational wave and neutrino sources or to understand better the physics of jet and the mechanism of acceleration of relativistic particles.
 
My research deeply impacts cosmology (dark energy, dark matter, Hubble-Lemaitre constant), galaxy evolution, supernova physics, black holes, and jet acceleration processes.

My projects:

Please, find below publications with more details about those questions. In all those papers, I am

first author or at least 2nd/3rd co-authors. Note that the projects are not ranked by importance or date, they are ranked randomly.

Project | 01

Publications:

Map the dark matter distribution using peculiar velocities and supernovae:
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We use Type Ia supernovae to measure galaxy peculiar motions. Peculiar velocities are deviation from the Hubble expansion due to overdensity in the local Universe. In linear perturbation theory peculiar velocity field v(x) is related to the matter density fluctuations δ(x) by ∇·v(x) = -f H0 δ(x). f directly depends on the gravity models and can serve as a test of gravity

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  • Hawaii SN flows: using near-infrared observations of nearby SNe Ia to obtain systematics-limited (∼3%) distances. We will measure the growth rate of cosmic structure parameterized by fσ8.

  • Democratic Samples of Supernovae: Compilation of 775 low-redshift Type Ia and II supernovae. We obtained fσ8=0.390 ± 0.022 and an external bulk flow velocity of 195 ± 22 km/s in the direction (ℓ,b)=(292±7,−6±5) deg. Our constraint on fσ8 -- the tightest derived from SNe to date

Project | 02

Publications:

Hubble constant tension:
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The most stringent local measurement of the Hubble-Lemaı̂tre constant from Cepheid-calibrated Type Ia supernovae (SNe Ia) differs from the value inferred via the cosmic microwave background radiation (Planck+ΛCDM) by ∼ 5σ. This so-called “Hubble tension” has been confirmed by other independent methods, and thus does not appear to be a possible consequence of systematic errors. Here, we continue upon our prior work of using Type II supernovae to provide another, largely-independent
method to measure the Hubble-Lemaı̂tre constant. From 13 SNe II with geometric,
Cepheid, or tip of the red giant branch (TRGB) host-galaxy distance measurements,

we derive H0 = 75.4 +3.8−3.7 km/s/Mpc  consistent with the local but in disagreement by ∼ 2.0σ with the Planck+ΛCDM value. Using only Cepheids (N = 7), we find H0 = 77.6 +5.2−4.8 km/s/Mpc , while using only TRGB (N = 5), we derive H0 = 73.1 +5.7−5.3 km/s/Mpc . Via 13 variants of our dataset, we de-
rive a systematic uncertainty estimate of 1.5 km/s/Mpc . Because we only replace SNe Ia with SNe II — and we do not find statistically
significant difference between the Cepheid and TRGB H0 measurements — our work
reveals no indication that SNe Ia or Cepheids could be the sources of the “H 0 tension.”
We caution, however, that our conclusions rest upon a modest calibrator sample; as this sample grows in the future, our results should be verified.

 

 

Project | 03

Publications:

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Type Ia Supernova cosmology
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The success of Type Ia supernova (SN Ia) distance standardization for cosmology relies on a single global linear relationship between their peak luminosity and colour, the β parameter. To standardize SNe Ia, we measure photometric parameters using empirical models constructed from both spectroscopic and photometric observations.

 

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  • In the current SN light-curve fitter the training phase is deliberately kept distinct from the distance inference process. It complicates significantly the propagation of model-related uncertainties. We are working on a new model called NaCl model. The complete training process, encompassing both the model and the error model, can be implemented as a single log-likelihood minimisation which results in a significantly lighter training procedure. This simplification ensures accurate uncertainty propagation of model uncertainties. Though untested on real data, a dataset of 2,500 low-z SNe Ia with Zwicky Transient Factory data offers a unique opportunity to test NaCl and develop a reliable light-curve fitting for the future projects.

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  • The effects of varying colour–luminosity relations on Type Ia supernova science: we find that the SN data favour non-universal distributions of β when split according to SN colour and/or host-galaxy mass. For galaxy mass, we obtain a β-step relation in which low β values occur in more massive galaxies, a trend that can be explained by differing dust reddening laws for two types of environments. For colour, we find that bluer/redder SNe Ia are consistent with a lower/larger β. This trend is explained with β being a combination of a low intrinsic colour–luminosity relation dominant in bluer SNe and a higher extrinsic reddening relation dominant at redder colours.

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Project | 04

Publications:

Type II Supernova cosmology
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We study the usefulness of Type II supernovae as extragalactic distance indicators. We will develop a new method to correct and to standardise the SNe II solely from photometry (PCM), with no input of spectral information. This new method will be an asset in the coming era of large photometric wide-field surveys (LSST, DES) for which spectroscopic follow-up will be impossible for all the SNeII. We also compare our method with the Standard Candle Method (SCM)

Applying the PCM to 90 SNeII (z=0.01-0.5), we derive an intrinsic dispersion of 0.39 mag. A comparison with the SCM using 70 SNe II is also performed and an intrinsic dispersion in the Hubble diagram of 0.27 mag is derived. Assuming a flat Universe and using the PCM or the SCM, we derive a Universe's matter density: Omega_m=0.32 +0.30/-0.21 providing a new independent evidence for dark energy at the level of two sigma.

 

 

Project | 05

Publications:

UC Berkeley data release
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BVRI light curves of 55 Type II supernovae the Lick Observatory Supernova Search obtained with the Katzman Automatic Imaging Telescope and the 1 m Nickel telescope from 2006 to 2018 are presented. Additionally, more than 150 spectra gathered with the 3 m Shane telescope are published. We conduct an analyse of the peak absolute magnitudes, decline rates, and time durations of different phases of the light and colour curves.  For each band, the plateau slope correlates with the plateau length and the absolute peak magnitude: SNe II with steeper decline have shorter plateau duration and are brighter. Nebular spectra are in good agreement with theoretical models using a progenitor star having a mass <16M⊙. All the data are available to the community and will help to understand SN II diversity better, and therefore to improve their utility as cosmological distance indicators.

 

 

Project | 06

Publications:​​​

Observed Type II supernova colours from the Carnegie Supernova Project-I
 

We present an extensive study of the observed Type II supernova colours using optical/near-infrared photometry data from the Carnegie Supernovae Project I (CSP-I). For the first time, a such complete study is achieved and allow us to find correlations between the observed colours and some photometric/spectroscopic parameters.

 

  • The colour evolution depends on the initial conditions. Bluer SNe II at 15d after explosion are also bluer at 70d after explosion.

  • SNe II form a continuous population in observed colours, consistent with other recent light-curve analyses finding an absence of a clear separation of these events into distinct classes

  • Redder SNe II have fainter absolute magnitude at maximum. This correlation seems to originate from intrinsic colours, not from dust effects. Progenitor radius differences, together with the presence or absence of CSM close to progenitor stars, are the probable causes of this relation.

 

Project | 07

Publications:

SN 2016esw: a luminous Type II supernova observed within the first day after explosion ​

 

Photometric and spectroscopic data on SN 2016esw, together with a study of its host galaxy. Thanks to our rapid follow-up observations from less than one day after the explosion up to 120 days later. 

  • Luminous: Mv =-18.36 mag maybe due to a contribution from CSM-ejecta interaction.

  • Ejecta-CSM interaction and mostly identical spectroscopic evolution with SN 2007pk,

  • The early-time spectra do not exhibit strong high-ionisation emission lines as seen in SNe~II having progenitors closely surrounded by dense CSM 

  • Unusually long, flat, plateau slope relative to its luminosity

  • Analytical models lead to a small progenitor radius (200Rsol)

  • A high-metallicity progenitor (Z ~1.5 Zsol)
     

Project | 08

Publications:​​​

Interacting SN/impostor: SN 2011A
A detailed study of SN 2011A which a number of interesting and unusual properties:
 
  • Double plateau in light curve likely due to CSM composed of two shells.
  • Low luminosity, -15.10<Mv
  • Low P-Cygni Halpha velocity <1200 km/s
  • Low velocity absorption close to NaID <1100 km/s
  • More likely an impostor

Project | 09

Publications:

γ-ray blazar flare correlations: understanding the high-energy emission process using ASAS-SN and Fermi light curves

 

Using blazar light curves from the optical All-Sky Automated Survey for Supernovae (ASAS-SN) and the γ-ray Fermi-LAT telescope, we performed the most extensive statistical correlation study between both bands, using a sample of 1,180 blazars. This is almost an order of magnitude larger than other recent studies. Blazars represent more than 98% of the AGNs detected by Fermi-LAT and are the brightest γ-ray sources in the extragalactic sky. They are essential for studying the physical properties of astrophysical jets from central black holes. However, their γ-ray flare mechanism is not fully understood. Multi-wavelength correlations help constrain the dominant mechanisms of blazar variability. We search for temporal relationships between optical and γ-ray bands. Using a Bayesian Block Decomposition, we detect 1414 optical and 510 γ-ray flares, we find a strong correlation between both bands. Among all the flares, we find 321 correlated flares from 133 blazars, and derive an average rest-frame time delay of only 1.1+7.1−8.5 days, with no difference between the flat-spectrum radio quasars, BL Lacertae-like objects or low, intermediate, and high-synchrotron peaked blazar classes. Our time-delay limit rules out the hadronic proton-synchrotron model as the driver for non-orphan flares and suggests a leptonic single-zone model. Limiting our search to well-defined light curves and removing 976 potential but unclear ``orphan'' flares, we find 191 (13%) and 115 (22%) clear ``orphan'' optical and γ-ray flares. The presence of ``orphan'' flares in both bands challenges the standard one-zone blazar flare leptonic model and suggests multi-zone synchrotron sites or a hadronic model for some blazars.

Project | 08

Publications:​​​

Search for gravitational waves and neutrinos optical counterparts using ASAS-SN
 
  • Optical counterparts of gravitational-wave events from the third observing run of Advanced LIGO/Virgo: No optical counterparts associated with a gravitational-wave event were found. However, thanks to its network of telescopes, the average area visible to at least one ASAS-SN site during the first 10 h after the trigger contained ~30 per cent of the integrated source location probability. With its observing strategy, five sites around the world, and a large field of view, ASAS-SN will be one of the leading surveys to optically search for nearby neutron star mergers during LVC fourth observation run (O4)
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  • Search for optical counterparts to IceCube neutrino alerts released between 2016 April and 2021 August with ASAS-SN: Through a combination of normal survey and triggered target-of-opportunity observations, ASAS-SN obtained images within 1 h of the neutrino detection for 20 per cent (11) of all observable IceCube alerts and within one day for another 57 per cent (32). For all observable alerts, we obtained images within at least two weeks from the neutrino alert. ASAS-SN provides the only optical follow-up for about 17 per cent of IceCube's neutrino alerts. We recover the two previously claimed counterparts to neutrino alerts, the flaring-blazar TXS 0506 + 056 and the tidal disruption event AT2019dsg
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