Search

We report the results of a search for small-amplitude (A less than 0.3 mag) long-period (P longer than 100 days) sources with periodic variability in SDSS Stripe 82 region. The SDSS coverage of Stripe 82 enables such a search because there are on average 20 observations per band in ugriz bands for about 1 million stars, collected over about 6 years, with a faint limit of r ∼ 22 mag and precisely calibrated 1-2% photometry. We calculated the periods of candidate variable sources in this sample using the LombScargle periodogram and considered the three highest periodogram peaks in each of the gri filters as relevant. Only those sources with gri periods consistent to within 0.1% were later studied. We focus our search on periodic variables with periods 100 < P < 600 days and uniform distribution of data points in phased light curves. We present 10 plausible candidates out of which seven have available archival spectra (two consistent with stars and five consistent with quasars). We have thoroughly analised the periodicity and photometry of the five plausibly periodically variable quasars, specifically focusing on possible false periods originating from the stochastic quasar variability. Furthermore, we extended the observational baseline of the SDSS with additional epochs from the ZTF and Pan-STARRS. The new data are in agreement with our model. Multiple explanations for the observed periodic behaviour are found in the literature: radio jet precession, tilted or warped accretion disks, but most of the explanations include binary super-massive black hole (BSMBH) systems. We single out one quasar with a period of P = 278 days whose X-ray variability is also detected by Chandra. This source is our pick for the most likely periodically variable quasar and follow-up efforts are in progress.

I will report the results of a search for long-period P = [100, 600] days sources with small-amplitude (A larger than 0.03 mag) periodic variability in SDSS Stripe 82 region. The SDSS coverage of Stripe 82 enables such a search because there are on average 20 observations per band in ugriz bands for about 1 million sources, collected over about 6 years, with a faint limit of r~22 mag and precisely calibrated 1-2% photometry. Lomb-Scargle periodograms of candidate variable sources from this sample were calculated and carefully scrutinized in order to select only those sources with the highest likelihood of being true periodic variables and avoid the possibility of false periodicity caused by stochastic variability. In addition to periodogram analysis, we cross-matched the candidate periodically variable sources to other surveys across the EM spectrum (photometry and spectroscopy) in order to confirm their variability, its origin and the type of the source. Our analysis yields 11 candidate periodically variable sources and is reinforced by Pan-STARRS and ZTF time series extending the observational baseline to more than 20 years. Furthermore, most of the candidates have optical spectra that allowed determination of the type of the source. The candidate list is dominated by quasars (inferred from the spectroscopy). The highest-ranked candidate has a spectrum consistent with a quasar and a period of 278 days. It is furthermore flagged as a variable source in the Chandra X-ray catalogue. Multiple explanations for the observed periodic behaviour of quasars are found in the literature: radio jet precession, tilted or warped accretion disks, tidal disruption remnants and other accretion-related effects. While these effects can be caused by a single black hole, most models require super-massive binary black holes (SMBBH). If this object indeed proves to be a SMBBH -- one of the loudest sources of gravitational waves, it would become the SMBBH with the shortest known period and therefore likely most compact orbit and remaining time before the merger and emission of GWs. In order to ascertain the nature of this object, and hopefully prepare for longer-term monitoring of it and similar systems, we have received 2.18hrs of Director's Discretionary Time on Gemini telescope’s GMOS-N instrument. This will allow us to study the conspicuous Mg II line visible in extant spectra of lower resolution and signal-to-noise ratio for the presence of asymmetries and additional peaks that could indicate the presence of SMBBH. I would also like to put these finding into the context of future Gaia data releases and time-domain surveys such as Vera C. Rubin Legacy Survey of Space and Time. It has been shown that full 10-year Gaia data could astrometrically resolve orbital motion of final-stage (sub-pc separation) SMBBHs up to z=0.02 and in the magnitude range V = [10, 13] (D’Orazio and Loeb, 2019). On the other hand, LSST’s etendue and expected exquisite photometric precision will allow searches in a much larger volume.

A galaxy’s spectral energy distribution (SED) enables the estimation of different galactic properties. In contrast to the better-understood, but harder to interpret, infrared SED, the radio spectrum below 30 GHz offers a dust-unbiased way of constraining properties of galaxies. We have constructed the average radio spectral energy distribution for 1.4 GHz-selected samples of highly star-forming galaxies (SFR > 100M_⊙/ yr, HSFGs), and a sample of sources exhibiting a 3σ excess of radio luminosity from that expected from star formation (RxAGN) in the COSMOS field. To achieve a broad rest-frame frequency range, we combined previously published VLA observations at 1.4 GHz and 3 GHz with unpublished GMRT observations at 325MHz and 610MHz by employing survival analysis to account for non-detections in the GMRT maps caused by their higher RMS values. For HSFGs, by fitting a broken power-law to the SED, we find the spectral index to change from α1 = 0.42 ± 0.06 below 4.3 GHz to α2 = 0.94 ± 0.06 above 4.3 GHz. Our results are in line with previous low-redshift studies of star-forming galaxies with an SFR > 10M_⊙/ yr that show that their SED differs from the one found in normal star-forming galaxies by having a steeper spectral index around 10 GHz, which could imply a smaller thermal fraction than in normal star-forming galaxies. For RxAGN, we find that a single power-law model fits best with a spectral index of 0.64 ± 0.07. However, such a model does not capture all the features of the RxAGN radio SED which can be better described by a broken power-law with a spectral index of 0.28 ± 0.03, below 4 GHz, and 1.16 ± 0.04, above 4 GHz. We have established the different SED shapes for highly star-forming galaxies and RxAGN at radio frequencies around 1-10 GHz in the COSMOS field. Further work needs to be done to assess the possible differences in SEDs above 10 GHz, which would give better estimates of the free-free contribution in star-formation processes at high redshift, providing a major step forward in understanding the average radio SEDs affecting the prevalence of different galaxy populations in samples detected in present and future deep radio surveys. Svojstva galaksija mogu se odrediti koristeći njihove spektralne energetske distribucije (SED). Za razliku od bolje istraženih SED-ova u infracrvenom području, radio spektar na frekvencijama manjim od 30 GHz je lakši za interpretirati jer nije pod utjecajem prašine te stoga pruža nepristran način određivanja svojstava galaksija. Konstruirali smo prosječne spektralne energetske distribucije za uzorke galaksija selektirane po opažanjima na 1:4 GHz dvaju populacija galaksija. Uzorak galaksija koje intenzivno stvaraju zvijezde (skraćeno HSFG), izabran je prema kriteriju brzine stvaranja zvijezda SFR > 100M_⊙/ yr, dok je uzorak aktivnih galaktičkih jezgri sa suviškom radio zračenja (Rx- AGN) identificiran 3σ suviškom radio luminoziteta u odnosu na očekivani radio luminozitet s obzirom na procese povezane sa stvaranjem zvijezda. Kako bismo postigli širok raspon frekvencija u sustavu mirovanja izvora spojili smo već prethodno objavljena opažanja VLA interferometrom na 1.4 GHz i 3 GHz s prethodno neobjavljenim GMRT opažanjima na 325MHz i 610MHz. VLA opažanja imaju veću dubinu, mjerenu srednjim kvadratnim odstupanjem radio mapa, od GMRT opažanja. Taj nesrazmjer smo razriješili koristeći analizu preživljavanja. ZaHSFGgalaksije, koristeći prilagodbu slomljenog zakona potencije na radio SED, pronalazimo da im se spektralni indeks mijenja od α1 = 0.42 ± 0.06, ispod 4.3 GHz, do α2 = 0.94 ± 0.06, iznad 4.3 GHz. Ovi rezultati se slažu s prethodnim studijama SFR > 10M_⊙/ yr galaksija izvedenima pri malom crvenom pomaku. Te studije ukazivale su da HSFG galaksije pokazuju strmije spektre (veći spektralni indeks) oko 10 GHz u odnosu na normalne galaksije u kojima se stvaraju zvijezde, što upućuje na manji doprinos termalnog zračenja. Za RxAGN uzorak pronalazimo kako uzorak možemo opisati običnim zakonom potencije sa spektralnim indeksom od 0.64 ± 0.07, no također smo uvidjeli kako tim jednostavnim modelom ne možemo obuhvatiti sve karakteristike RxAGN spektra. Ponovno koristeći slomljeni zakon potencije, dobivamo spektralni indeks od 0.28 ± 0.03 ispod 4 GHz te spektralni indeks od 1.16 ± 0.04 iznad 4 GHz. Ukratko, odredili smo oblik spektralnih energetskih distribucija dvaju uzoraka galaksija (HSFG i RxAGN) u rasponu od 1 GHz do 10 GHz u COSMOS predjelu neba. Potrebna su daljnja istraživanja kako bi se odredile razlike u obliku SED-a iznad 10 GHz u svrhu dobivanja bolje ocjene doprinosa zakočnog zračenja pri procesu stvaranja zvijezda u galaksijama ii na velikom crvenom pomaku te kako bi se bolje razumjeli odnosi između različitih populacija galaksija u uzorcima u sadašnjim i budućim pregledima neba.

We have analyzed milimeter-wave spectra of the hot cores G327.3-0.6 and NGC 6334 I, which were taken by the SHeFI instrument at the APEX telescope in Chile. Rotational and population diagrams were used to derive the gas kinetic temperatures and column densities of the detected molecules in the sources. We have identified the following molecular species in the observed hot cores: acetone, carbonyl sulfide, dimethyl ether, ethanol, ethyl cyanide, formic acid, ketene, methanol, methyl acetylene, methylamine, methyl cyanide, methyl formate, sulfur dioxide, and vinyl cyanide along with some of their isotopologues. Analizirali smo spektre vrućih jezgri G327.3-0.6 i NGC 6334 I u milimetarskom području koje su bile opažane pomoću SHeFI instrumenta na APEX teleskopu u Čileu. Rotacijski i populacijski dijagrami su korišteni pri odredivanju kinetičkih temperatura plina i stupčanih gustoća detektiranih molekula u izvorima. Identificirali smo sljedeće molekularne vrste u promatranim vrućim jezgrama: aceton, karbonil sulfid, dimetil eter, etanol, etil cijanid, mravlju kiselinu, keten, metanol, metil acetilen, metilamin, metil cijanid, metil format, sumporov dioskid, vinil cijanid kao i neke od njihovih izotopologa.