Disk-Jet Connection in Agns and Microquasars: The Possibility of Thermal Flares in the Center

We discuss the possibility of thermal flares in centers of AGNs and microquasars. We present preliminary results of an ongoing study trying to assess the feasibility of a hypothesis suggesting that certain flares observed in these sources originate in the very centers of the systems and not in the relativistic jets. Using a simple toy model we reproduce optical flares with lightcurves very similar to those observed in the sources. The model suits especially well those cases where only the latter peak of a double-peaked optical flare has a radio counterpart.Comment: 6 pages, 1 figure, to appear in the proceedings of the HEPRO II conference, Buenos Aires, October 26-30 200

GPS Time Disruptions on 26-Jan-2016

On Tuesday, January 26th 2016 Aalto University Metsähovi Radio Observatory discovered that three out of its four GPS receivers on-site (used for monitoring the performance of four on-site hydrogen (H) maser atomic clocks) performed abrupt 13.0 microsecond time jumps in their "1pps" (one pulse per second) time synchronization outputs. The disruptions lasted for approximately 12 hours during which different GPS receiver units outputted erroneous time signal at different times, preventing their use for microsecond-level time synchronization. The event was later acknowledged by the 2nd Space Operations Squadron at the 50th Space Wing, Schriever Air Force Base, Colorado, USA, as an erroneous time correction parameter upload to GPS satellites

Time variability of the core-shift effect in the blazar 3C 454.3

Using VLBI to measure a so-called core shift effect is a common way of obtaining estimates of the jet magnetic field strength. The VLBI core is typically identified as the bright feature at the jet's base, and the position of the core changes with the observed frequency, $r_\mathrm{core} \propto \nu^{-1/k_r}$. In this work, we investigated the time variability of the core-shift effect in the blazar 3C 454.3. We employed self-referencing analysis of multi-frequency (5, 8, 15, 22-24, and 43 GHz) VLBA data covering 19 epochs from 2005 until 2010. We found significant core shift variability ranging from 0.27 to 0.86 mas between 5 and 43 GHz, confirming the core-shift variability phenomenon observed before. Time variability of the core-shift index ($k_r$) was found typically below one, with an average value of $0.85 \pm 0.08$ and a standard deviation of $0.30$. $k_r<1$ values were found during flaring and quiescent states and our results indicate that commonly assumed conical jet shape and equipartition conditions do not always hold simultaneously. Still, these conditions are often assumed when deriving magnetic field strengths from core shift measurements, leading to unreliable results if $k_r$ significantly deviates from unity. Therefore, it is important to verify that $k_r = 1$ holds before using core shift values and the equipartition assumption to derive physical parameters in the jets. When $k_r = 1$ epochs are selected in the case of 3C 454.3, the magnetic field estimates are indeed quite consistent, even though the core shift varies with time. Additionally, our estimations of the jet's magnetic flux in 3C 454.3 show that the source is indeed in the magnetically arrested disk state. Finally, we found a good correlation of the core position with the core flux density, $r_\mathrm{core}\propto S_\mathrm{core}^{0.7}$, which is consistent with increased particle density during the flares.Comment: 53 pages, 6 tables, 57 figures. Article submitted to Astronomy and Astrophysic

Positivity Bias in Customer Satisfaction Ratings

Customer ratings are valuable sources to understand their satisfaction and are critical for designing better customer experiences and recommendations. The majority of customers, however, do not respond to rating surveys, which makes the result less representative. To understand overall satisfaction, this paper aims to investigate how likely customers without responses had satisfactory experiences compared to those respondents. To infer customer satisfaction of such unlabeled sessions, we propose models using recurrent neural networks (RNNs) that learn continuous representations of unstructured text conversation. By analyzing online chat logs of over 170,000 sessions from Samsung's customer service department, we make a novel finding that while labeled sessions contributed by a small fraction of customers received overwhelmingly positive reviews, the majority of unlabeled sessions would have received lower ratings by customers. The data analytics presented in this paper not only have practical implications for helping detect dissatisfied customers on live chat services but also make theoretical contributions on discovering the level of biases in online rating platforms.Comment: This paper will be presented at WWW'18 conferenc

Far-infrared photometry of OJ 287 with the Herschel Space Observatory

Context: The blazar OJ 287 has shown a approximate to 12 year quasi-periodicity over more than a century, in addition to the common properties of violent variability in all frequency ranges. It is the strongest known candidate to have a binary singularity in its central engine.Aims: We aim to better understand the different emission components by searching for correlated variability in the flux over four decades of frequency measurements.Methods: We combined data at frequencies from the millimetric to the visible to characterise the multifrequency light curve in April and May 2010. This includes the only photometric observations of OJ 287 made with the Herschel Space Observatory: five epochs of data obtained over 33 days at 250, 350, and 500 mu m with Herschel-SPIRE.Results: Although we find that the variability at 37 GHz on timescales of a few weeks correlates with the visible to near-IR spectral energy distribution, there is a small degree of reddening in the continuum at lower flux levels that is revealed by the decreasing rate of decline in the light curve at lower frequencies. However, we see no clear evidence that a rapid flare detected in the light curve during our monitoring in the visible to near-IR light curve is seen either in the Herschel data or at 37 GHz, suggesting a low-frequency cut-off in the spectrum of such flares.Conclusions: We see only marginal evidence of variability in the observations with Herschel over a month, although this may be principally due to the poor sampling. The spectral energy distribution between 37 GHz and the visible can be characterised by two components of approximately constant spectral index: a visible to far-IR component of spectral index alpha = -0.95, and a far-IR to millimetric spectral index of alpha = -0.43. There is no evidence of an excess of emission that would be consistent with the 60 mu m dust bump found in many active galactic nuclei.</p

Multi-wavelength temporal variability of the blazar PKS 1510-089

We perform correlation and periodicity search analyses on long-term multi-band light curves of the FSRQ 1510-089 observed by the space-based Fermi--Large Area Telescope in gamma-rays, the SMARTS and Steward Observatory telescopes in optical and near-infrared (NIR) and the 13.7 m radio telescope in Metsahovi Radio Observatory between 2008 and 2018. The z-transform discrete correlation function method is applied to study the correlation and possible time lags among these multi band light curves. Among all pairs of wavelengths, the gamma-ray vs. optical/NIR and optical vs. NIR correlations show zero time lags; however, both the gamma-ray and optical/NIR emissions precede the radio radiation. The Generalized Lomb-Scargle periodogram, Weighted Wavelet Z-transform, and REDFIT techniques are employed to investigate the unresolved-core-emission dominated 37 GHz light curve and yield evidence for a quasi-period around 1540 days, although given the length of the whole data set it cannot be claimed to be significant. We also investigate the optical/NIR color variability and find that this source shows a simple redder-when-brighter behavior over time, even in the low flux state.Comment: Accepted for publication in ApJ; 20 pages, 9 figures, 4 table

Hyvä yliopisto-opettajuus – kysymyksiä ja vastauksia yliopistopedagogisen koulutusvuoden varrelta

Hyvä yliopisto-opettajuus – kysymyksiä ja vastauksia yliopistopedagogisen koulutusvuoden varrelta on julkaisu, joka kuvaa Aalto-yliopiston yliopistopedagogiselle kurssille vuonna 2014 osallistuneiden pedagogisen osaamisen kehittymistä. Oppimisprosessi oli vuoden mittainen ja perustui yhteisölliseen ja tutkivaan oppimiseen. Näin heterogeenisestä osallistujaryhmästä kehittyi yhdessä toimiva oppimis- ja tutkimusryhmä. He kiinnostuivat oman ja työyhteisönsä reflektiivisestä opetuksen kehittämisestä. Julkaisussa Opettaja kehittäjänä –kurssin osallistujat kuvaavat ja avaavat ilmiötä hyvä yliopisto-opettajuus monelta eri näkökulmalta. Kurssin ohjaajat jäsentävät kurssin toteutusta ja arvioivat sen onnistumista pohtien miten tämän tyyppinen kurssi, jonka sisältöjä ei ole ennalta suunniteltu pystyy vastaamaan osallistujien erilaisiin odotuksiin ja osaamistarpeisiin. Julkaisu tarjoaa näkökulmia opetusosaamisen ja opetuksen kehittämiseen. Teos sopii kaikille yliopisto-opetuksesta ja sen kehittämisestä kiinnostuneille. Part of the publication is in English (pages 76-175, title "Integrating transferable skills into teaching at Aalto University"

A Near Magnetic-to-kinetic Energy Equipartition Flare from the Relativistic Jet in AO 0235+164 during 2013-2019

We present the multiwavelength flaring activity of the blazar AO 0235+164 during its recent active period from 2013 to 2019. From a discrete correlation function (DCF) analysis, we find a significant (>95%) correlation between radio and $\gamma$-ray light curves with flares at longer wavelengths following flares at shorter wavelengths. We identify a new jet component in 43 GHz VLBA data that was ejected from the radio core on MJD $57246^{+26}_{-30}$ (2015 August 12), during the peak of the 2015 radio flare. From the analysis of the jet component, we derived a Doppler factor of $\delta_{\rm var}=28.5\pm8.4$, a bulk Lorentz factor of $\Gamma=16.8^{+3.6}_{-3.1}$, and an intrinsic viewing angle of $\theta_{\rm v}=1.42^{+1.07}_{-0.52}\textrm{ degrees}$. Investigation of the quasi-simultaneous radio data revealed a partially absorbed spectrum with the turnover frequency varying in the range of $10-70$ GHz and the peak flux density varying in the range of $0.7-4$ Jy. We find the synchrotron self-absorption magnetic field strength to be $B_{\rm SSA}=15.3^{+12.6}_{-14.0}$ mG at the peak of the 2015 radio flare, which is comparable to the equipartition magnetic field strength of $B_{\rm EQ}=43.6^{+10.6}_{-10.4}$ mG calculated for the same epoch. Additional analysis of the radio emission region in the relativistic jet of AO 0235+164 suggests that it did not significantly deviate from equipartition during its recent flaring activity.Comment: 13 pages, 11 figures, 4 tables; Accepted for publication in MNRA