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Tanja Petrushevska

Doktorand

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Arbetar vid Fysikum
E-post tanja.petrushevska@fysik.su.se
Besöksadress Roslagstullsbacken 21 A, plan 5
Rum A5:1051
Postadress Stockholms universitet, Fysikum 106 91 Stockholm

Forskning

Supernovor är mycket sällsynta företeelser i universum och deras övergående natur har gjort dem svåra att hitta under lång tid. Så det är inte förvånande att omkring två supernovor per månad upptäcktes för 30 år sedan. Idag kan vi hitta supernovor dagligen, men de flesta av dessa är alla relativt närliggande, eftersom dagens teleskop inte är känsliga för de mer avlägsna.

Supernova rates, eller mätningen av frekvensen av supernovaexplosioner per volymenhet, är viktigt av flera skäl. De kan till exempel användas för att spåra historien om stjärnbildning i galaxerna. Genom att mäta frekvensen av kärn-kollaps supernovor vilket är kollapsen av massiva och kortlivade stjär- nor per volumenhet. Supernovor är en av de största tillverkarna av tunga grundämnen i universum, så mätning av supernova rates informerar oss om kemisk anrikning av galaxer över tid.

Mätning av supernova rates i det avlägsna universum är svårt att upp- skatta. Även om supernovor är en av de ljusstarkaste explosionerna som finns, på avstånd större än fyra miljarder ljusår, är de svåra att hitta ef- tersom de blir för ljussvaga. Detta har varit särskilt problematiskt vid studier av kärn-kollaps explosioner eftersom de i genomsnitt är de ljussvagaste ob- jekten i supernova familjen och ofta inbäddade i stoftrika miljöer. Dessutom, på grund av universums expansion, är det synliga ljuset från alla avlägsna objekt skiftade till längre våglängder. Från marken, är dock nära-infraröda observationer särskilt utmanande på grund av ljusstyrkan och variabilitet av atmosfären vid dessa våglängder.

I stället för att vänta på mer kraftfulla teleskop för att komma online, använde vi befintliga anläggningar och förstoringskraften i galaxhopar som gravitationsteleskop för att studera supernovor i det tidiga universum. Galaxhopar är de mest massiva gravitationsbundna objekt i universum, som fungerar som gravitationslinser. Gravitationslinser förstorar ljuset av bak- grundsobjekt, och kan därigenom öka djupet för undersökningen, dvs hur långt borta vi kan se supernovor. På detta sätt ökar förmågan att hitta mycket avlägsna supernovor. Det var Fritz Zwicky som föreslog användningen av gravitations teleskop för nästan 80 år sedan, men det är först nyligen som systematiska supernova sökningar har utförts i bakgrunden av galaxer bakom kluster. Supernova gruppen i Stockholm har lett den här typen av sökning från markbaserade anläggningar sedan 2003. Som en fortsättning på detta arbete under 2008-2014, undersökte vi galaxhoparna Abell 1689 och A370, som är ett av de mest kraftfulla gra- vitationsteleskop som naturen erbjuder. Vi använde det nära infraröda instrument på Very Large Telescope i Chile, med medföljande optiska data från Nordic Optical Telescope på La Palma. Vår undersökning resulterade i upptäckten av fem mycket avlägsna och förstorade supernovor. I synnerhet upptäckte vi en supernova som ligger nästan 10 miljarder ljusår bort och som förstorats fyra gånger av en galaxhop, vilket gör det bland den mest avlägsna kärnakollaps-supernova som observerats vid den tidpunkten. Med hjälp av dessa upptäckter, mätte vi supernova rates upp till den tid då universum bara var två miljarder år gammal, utan att det krävdes kostsamma rymdbaserade uppföljningar.

Supernovor typ Ia har visat sig vara ett mycket användbart verktyg som avståndsindikatorer, som kan kartlägga den accelererande expansionen av universum. Spektroskopiska observationer av linsade supernovor Ia vid stora avstånd är särskilt intressanta. Dessa kan användas för att testa utvecklingen i sina egna inneboende egenskaper. 

Starka gravitationslinser förstorar inte bara, men kan också ge flera bilder av bakgrundsobjektet. Vi upptäckte den första upplösta starkt linsade supernovan Ia, iPTF16geu. Med hjälp av högupplösta bilder kunde vi samtidigt observera fyra bilder av samma supernova. 

Publikationer

I urval från Stockholms universitets publikationsdatabas
  • Tanja Petrushevska (et al.). Astronomy and Astrophysics 594

    Aims. We present a ground-based, near-infrared search for lensed supernovae behind the massive cluster Abell 1689 at z = 0.18, which is one of the most powerful gravitational telescopes that nature provides. Methods. Our survey was based on multi-epoch J-band observations with the HAWK-I instrument on VLT, with supporting optical data from the Nordic Optical Telescope. Results. Our search resulted in the discovery of five photometrically classified, core-collapse supernovae with high redshifts of 0.671 < z < 1.703 and magnifications in the range Delta m = -0.31 to -1.58 mag, as calculated from lensing models in the literature. Owing to the power of the lensing cluster, the survey had the sensitivity to detect supernovae up to very high redshifts, z similar to 3, albeit for a limited region of space. We present a study of the core-collapse supernova rates for 0.4 < z < 2.9, and find good agreement with previous estimates and predictions from star formation history. During our survey, we also discovered two Type Ia supernovae in A 1689 cluster members, which allowed us to determine the cluster Ia rate to be 0.14(-0.09)(+0.19) SNuB h(2) (SNuB 10(-12) SNe L-circle dot,B(-1) yr(-1)), where the error bars indicate 1 sigma confidence intervals, statistical and systematic, respectively. The cluster rate normalized by the stellar mass is 0.10(-0.06)(+0.13) +/- 0.02 in SNuM h(2) (SNuM = 10(-12) SNe M-1 yr(-1)). Furthermore, we explore the optimal future survey for improving the core-collapse supernova rate measurements at z greater than or similar to 2 using gravitational telescopes, and for detections with multiply lensed images, and we find that the planned WFIRST space mission has excellent prospects. Conclusions. Massive clusters can be used as gravitational telescopes to significantly expand the survey range of supernova searches, with important implications for the study of the high-z transient Universe.

  • Tanja Petrushevska (et al.).
  • Tanja Petrushevska (et al.).
  • Ariel Goobar (et al.). Science 356 (6335), 291-295

    We report the discovery of a multiply imaged, gravitationally lensed type Ia supernova, iPTF16geu (SN 2016geu), at redshift z = 0.409. This phenomenon was identified because the light from the stellar explosion was magnified more than 50 times by the curvature of space around matter in an intervening galaxy. We used high-spatial-resolution observations to resolve four images of the lensed supernova, approximately 0.3 arc seconds from the center of the foreground galaxy. The observations probe a physical scale of ~1 kiloparsec, smaller than is typical in other studies of extragalactic gravitational lensing. The large magnification and symmetric image configuration imply close alignment between the lines of sight to the supernova and to the lens. The relative magnifications of the four images provide evidence for substructures in the lensing galaxy.

  • Raphael Ferretti (et al.). Astronomy and Astrophysics 592

    Context. Temporal variability of narrow absorption lines in high-resolution spectra of Type Ia supernovae (SNe Ia) is studied to search for circumstellar matter. Time series which resolve the profiles of absorption lines such as Na I D or Ca II H&K are expected to reveal variations due to photoionisation and subsequent recombination of the gases. The presence, composition, and geometry of circumstellar matter may hint at the elusive progenitor system of SNe Ia and could also affect the observed reddening law. Aims. To date, there are few known cases of time-varying Na I D absorption in SNe Ia, all of which occurred during relatively late phases of the supernova (SN) evolution. Photoionisation, however, is predicted to occur during the early phases of SNe Ia, when the supernovae peak in the ultraviolet. We attempt, therefore, to observe early-time absorption-line variations by obtaining high-resolution spectra of SNe before maximum light. Methods. We have obtained photometry and high-resolution spectroscopy of SNe Ia 2013gh and iPTF 13dge, to search for absorption-line variations. Furthermore, we study interstellar absorption features in relation to the observed photometric colours of the SNe. Results. Both SNe display deep Na I D and Ca II H&K absorption features. Furthermore, small but significant variations are detected in a feature of the Na I D profile of SN 2013gh. The variations are consistent with either geometric effects of rapidly moving or patchy gas clouds or photoionisation of Na I gas at R approximate to 10(19) cm from the explosion. Conclusions. Our analysis indicates that it is necessary to focus on early phases to detect photoionisation effects of gases in the circumstellar medium of SNe Ia. Different absorbers such as Na I and Ca II can be used to probe for matter at different distances from the SNe. The nondetection of variations during early phases makes it possible to put limits on the abundance of the species at those distances.

  • Rahman Amanullah (et al.). Monthly notices of the Royal Astronomical Society 453 (3), 3300-3328

    We present ultraviolet (UV) observations of six nearby Type Ia supernovae (SNe Ia) obtained with the Hubble Space Telescope, three of which were also observed in the near-IR (NIR) with Wide-Field Camera 3. UV observations with the Swift satellite, as well as ground-based optical and NIR data provide complementary information. The combined data set covers the wavelength range 0.2-2 mu m. By also including archival data of SN 2014J, we analyse a sample spanning observed colour excesses up to E(B - V) = 1.4 mag. We study the wavelength-dependent extinction of each individual SN and find a diversity of reddening laws when characterized by the total-to-selective extinction R-V. In particular, we note that for the two SNe with E(B - V) greater than or similar to 1 mag, for which the colour excess is dominated by dust extinction, we find R-V = 1.4 +/- 0.1 and R-V = 2.8 +/- 0.1. Adding UV photometry reduces the uncertainty of fitted R-V by similar to 50 per cent allowing us to also measure R-V of individual low-extinction objects which point to a similar diversity, currently not accounted for in the analyses when SNe Ia are used for studying the expansion history of the Universe.

  • Georgios Leloudas (et al.). Astronomy and Astrophysics 574

    We construct spectra of supernovae (SNe) interacting strongly with a circumstellar medium (CSM) by adding SN templates, a black-body continuum, and an emission-line spectrum. In a Monte Carlo simulation we vary a large number of parameters, such as the SN type, brightness and phase, the strength of the CSM interaction, the extinction, and the signal to noise ratio (S/N) of the observed spectrum. We generate more than 800 spectra, distribute them to ten different human classifiers, and study how the different simulation parameters affect the appearance of the spectra and their classification. The SNe IIn showing some structure over the continuum were characterized as SNe IInS to allow for a better quantification. We demonstrate that the flux ratio of the underlying SN to the continuum f(v) is the single most important parameter determining whether a spectrum can be classified correctly. Other parameters, such as extinction, S/N, and the width and strength of the emission lines, do not play a significant role. Thermonuclear SNe get progressively classified as Ia-CSM, IInS, and IIn as f(v) decreases. The transition between Ia-CSM and IInS occurs at f(v) similar to 0.2-0.3. It is therefore possible to determine that SNe Ia-CSM are found at the (un-extincted) magnitude range -19.5 > M > -21.6, in very good agreement with observations, and that the faintest SN IIn that can hide a SN Ia has M = -20.1. The literature sample of SNe Ia-CSM shows an association with 91T-like SNe Ia. Our experiment does not support that this association can be attributed to a luminosity bias (91T-like being brighter than normal events). We therefore conclude that this association has real physical origins and we propose that 91T-like explosions result from single degenerate progenitors that are responsible for the CSM. Despite the spectroscopic similarities between SNe Ibc and SNe Ia, the number of misclassifications between these types was very small in our simulation and mostly at low S/N. Combined with the SN luminosity function needed to reproduce the observed SN Ia-CSM luminosities, it is unlikely that SNe Ibc constitute an important contaminant within this sample. We show how Type II spectra transition to IIn and how the H alpha profiles vary with f(v). SNe IIn fainter than M = -17.2 are unable to mask SNe IIP brighter than M = -15. A more advanced simulation, including radiative transfer, shows that our simplified model is a good first order approximation. The spectra obtained are in good agreement with real data.

  • Rahman Amanullah (et al.). Astrophysical Journal Letters 788 (2), L21

    The wavelength dependence of the extinction of Type Ia SN 2014J in the nearby galaxy M82 has been measured using UV to near-IR photometry obtained with the Hubble Space Telescope, the Nordic Optical Telescope, and the Mount Abu Infrared Telescope. This is the first time that the reddening of an SN Ia is characterized over the full wavelength range of 0.2-2 mu m. A total-to-selective extinction, R-V >= 3.1, is ruled out with high significance. The best fit at maximum using a Galactic type extinction law yields R-V = 1.4 +/- 0.1. The observed reddening of SN 2014J is also compatible with a power-law extinction, A(lambda)/A(V) = (lambda/lambda(V))(p) as expected from multiple scattering of light, with p = -2.1 +/- 0.1. After correcting for differences in reddening, SN 2014J appears to be very similar to SN 2011 fe over the 14 broadband filter light curves used in our study.

  • Ariel Goobar (et al.). Astrophysical Journal Letters 784 (1), L12

    We report on the discovery of SN 2014J in the nearby galaxy M82. Given its proximity, it offers the best opportunity to date to study a thermonuclear supernova (SN) over a wide range of the electromagnetic spectrum. Optical, near-IR, and mid-IR observations on the rising light curve, orchestrated by the intermediate Palomar Transient Factory, show that SN 2014J is a spectroscopically normal Type Ia supernova (SN Ia), albeit exhibiting high-velocity features in its spectrum and heavily reddened by dust in the host galaxy. Our earliest detections start just hours after the fitted time of explosion. We use high-resolution optical spectroscopy to analyze the dense intervening material and do not detect any evolution in the resolved absorption features during the light curve rise. Similar to other highly reddened SNe Ia, a low value of total-to-selective extinction, R-V less than or similar to 2, provides the best match to our observations. We also study pre-explosion optical and near-IR images from Hubble Space Telescope with special emphasis on the sources nearest to the SN location.

Visa alla publikationer av Tanja Petrushevska vid Stockholms universitet

Senast uppdaterad: 20 juni 2017

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