Abstract. OLAP applications use precomputation of aggregate data to improve query response time. While this problem has been well-studied in the recent database literature, to our knowledge all previous work has focussed on the special case in which all aggregates are computed from a single cube (in a star schema, this corresponds to there being a single fact table). This is unfortunate, because many real world applications require aggregates over multiple fact tables. In this paper, we attempt to fill this lack of discussion about the issues arising in multi-cube data models by analyzing these issues. Then we examine performance issues by studying the precomputation problem for multi-cube systems. We show that this problem is significantly more complex than the single cube precomputation problem, and that algorithms and cost models developed for single cube precomputation must be extended to deal well with the multi-cube case. Our results from a prototype implementation show that for multi-cube workloads substantial performance improvements can be realized by using the multi-cube algorithms.

Variable γ-ray emission has been discovered in five Radio-loud Narrow Line Seyfert 1 (NLSy1) galaxies by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space Telescope. This has clearly demonstrated that these NLSy1 galaxies do have relativistic jets similar to two other cases of γ-ray emitting Active Galactic Nuclei (AGN), namely blazars and radio galaxies. We present here our results on the multi-band analysis of two γ-ray emitting NLSy1 galaxies namely PKS 1502+036 (z = 0.409) and PKS 2004−447 (z = 0.240) using archival data. We generate multi-band long term light curves of these sources, build their spectral energy distribution (SED) and model them using an one zone leptonic model. They resemble more to the SEDs of the flat spectrum radio quasar (FSRQ) class of AGN. We then compare the SEDs of these two sources with two other Fermi detected AGN along the traditional blazar sequence, namely the BL Lac Mrk 421 (z = 0.03) and the FSRQ 3C 454.3 (z = 0.86). The SEDs of both PKS 1502+036 and PKS 2004−447 are found to be intermediate to the SEDs of Mrk 421 and 3C 454.3. In the γ-ray spectral index v/s γ-ray luminosity plane, both these NLSy1 galaxies occupy a distinct position, wherein, they have luminosity between Mrk 421 and 3C 454.3, however steep γ-ray spectra similar to 3C 454.3. Their Compton dominance as well as their X-ray spectral slope also lie between Mrk 421 and 3C 454.3. We argue that the physical properties of both PKS 1502+036 and PKS 2004−447 are in general similar to blazars and intermediate between FSRQs and BL Lac objects and these sources thus could fit into the traditional blazar sequence.

Aims. The nearby (z = 0.031) TeV blazar Mrk 421 was reported to be in a high state of flux activity since November, 2009. We aim to investigate possible changes in the physical parameters of Mrk 421 during its high state of activity using multiwavelength data. Methods. We have observed this source in the bright state using the High Altitude GAmma Ray (HAGAR) telescope array at energies above 250 GeV during February 13-19, 2010. Optical, X-ray and γ-ray archival data were also used to obtain the spectral energy distribution and light curves. Results. Mrk 421 was found to undergo one of its brightest flaring episodes on February 17, 2010 by various observations in X-rays and γ-rays. HAGAR observations during February 13-19, 2010 at energies above 250 GeV show an enhancement in the flux level, with a maximum flux of ∼7 Crab units being detected on February 17, 2010. We present the spectral energy distributions during this flaring episode and investigate the correlation of the variability in X-ray and γ-ray bands. Conclusions. Our multiwavelength study suggests that the flare detected during February 16 and 17, 2010 may have been caused by a passing shock in the jet.

The flat-spectrum radio quasar CTA 102 experienced a prolonged state of enhanced activity across the entire observed electromagnetic spectrum during 2016–2017, most pronounced during a major outburst between 2016 December and 2017 May. Fermi-LAT observed a flux of (2.2 ± 0.2) × 10−5 photons cm−2 s−1 at energies above 100 MeV on 2017 April 19 during a single orbit. We report here the detection of significant (4.7σ) flux variations down to timescales of ∼5 minutes during this orbit. The measured variability timescale is much shorter than the light-travel time across the central black hole (∼70 minutes) indicating a very compact emission region within the jet, similar to that seen in IC 310, Mrk 501, or PKS 1222+21 from MAGIC observations. This short-timescale variability is unexpected since the γ-ray spectrum shows no sign of attenuation due to pair creation in interactions with photons from the broad emission line region, and therefore must be assumed to originate far from the black hole. The observed fast variability could either indicate the dissipation of magnetic islands or protons in a collimated beam from the base of the jet encountering the turbulent plasma at the end of the magnetic nozzle.

Spinning black holes in the centres of galaxies can release powerful magnetised jets. When the jets are observed at angles of less than a few degrees to the line-of-sight, they are called blazars, showing variable non-thermal emission across the electromagnetic spectrum from radio waves to gamma rays. It is commonly believed that shock waves are responsible for this dissipation of jet energy. Here we show that gamma-ray observations of the blazar 3C 279 with the space-borne telescope Fermi-LAT reveal a characteristic peak-in-peak variability pattern on time scales of minutes expected if the particle acceleration is instead due to relativistic magnetic reconnection. The absence of gamma-ray pair attenuation shows that particle acceleration takes place at a distance of ten thousand gravitational radii from the black hole where the fluid dynamical kink instability drives plasma turbulence.