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Register a new userFERMI transient J1544-0649: a flaring radio-weak BL Lac
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On May 15th, 2017, the emph{FERMI}/LAT gamma-ray telescope observed a transient source not present in any previous high-energy catalogue: J1544-0649. It was visible for two consecutive weeks, with a flux peak on May 21st. Subsequently observed by a emph{Swift}/XRT follow-up starting on May 26, the X-ray counterpart position was coincident with the optical transient ASASSN-17gs = AT2017egv, detected on May 25, with a potential host galaxy at $z$=0.171. We conducted a 4-months follow-up in radio (Effelsberg-100m) and optical (San Pedro Martir, 2.1m) bands, in order to build the overall Spectral Energy Distribution (SED) of this object. The radio data from 5 to 15 GHz confirmed the flat spectrum of the source, favoring a line of sight close to jet axis, not showing significant variability in the explored post-burst time-window. The Rx ratio, common indicator of radio loudness, gives a value at the border between the radio-loud and radio-quiet AGN populations. The Ca$_{rm{II}}$ H&K break value (0.29$pm$0.05) is compatible with the range expected for the long-sought intermediate population between BL Lacs and FRI radio galaxies. An overall SED fitting from Radio to $gamma$-ray band shows properties typical of a low-power BL Lac. As a whole, these results suggest that this transient could well be a new example of the recently discovered class of radio-weak BL Lac, showing for the first time a flare in the gamma/X-ray bands.
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We report observations of a transient source fermi from radio to grs. fermi was discovered by the {it Fermi-LAT} in May 2017. Follow-up {it Swift-XRT} observations revealed three flaring episodes through March 2018, and the peak X-ray flux is about $10^3$ higher than the {it ROSAT all-sky survey (RASS)} flux upper limit. Optical spectral measurements taken by the {it Magellan 6.5-m telescope} and the {it Lick-Shane telescope} both show a largely featureless spectrum, strengthening the BL Lac interpretation first proposed by citet{Bruni18}. The optical and mid-infrared (MIR) emission goes to a higher state in 2018, when the flux in high energies goes down to a lower level. Our {it RATAN-600m} measurements at 4.8~GHz and 8.2~GHz do not indicate any significant radio flux variation over the monitoring seasons in 2017 and 2018, nor deviate from the archival {it NVSS} flux level. During GeV flaring times, the spectrum is very hard ($Gamma_gammasim$1.7) in the GeV band and at times also very hard (($Gamma_{rm X}lesssim2$) in the X-rays, similar to a high-synchrotron-peak (or even an extreme) BL Lac object, making fermi a good target for ground-based {it Cherenkov telescopes}.
The existence of radio weak BL Lac objects (RWBLs) has been an open question, still unsolved, since the discovery that quasars could be radio-quiet or radio-loud. Recently several groups identified RWBL candidates, mostly found while searching for low energy counterparts of the unidentified/unassociated gamma-ray sources listed in the Fermi catalogs. Confirming RWBLs is a challenging task since they could be confused with white dwarfs (WDs) or weak emission line quasars (WELQs) when there are not sufficient data to precisely draw their broad band spectral energy distribution and their classification is mainly based on a featureless optical spectra. Motivated by the recent discovery that Fermi BL Lacs appear to have very peculiar mid-IR emission, we show that it is possible to distinguish between WDs, WELQs and BL Lacs using the [3.4]-[4.6]-[12]$mu$m color-color plot built using the WISE magnitudes when the optical spectrum is available. On the basis of this analysis, we identify WISE J064459.38+603131 and WISE J141046.00+740511.2 as the first two genuine RWBLs, both potentially associated with Fermi sources. Finally, to strengthen our identification of these objects as true RWBLs, we present multifrequency observations for these two candidates to show that their spectral behavior is indeed consistent with those of the BL Lac population.
We present a new catalogue of the RATAN-600 multi-frequency measurements for BL Lac objects. The purpose of this catalogue is to compile the BL Lac multi-frequency data that is acquired with the RATAN-600 simultaneously at several frequencies. The BL Lac objects emit a strongly variable and polarized non-thermal radiation across the entire electromagnetic spectrum from radio to gamma-rays and represent about one percent of known AGNs. They belong to the blazar population and differ from other blazars featureless optical spectrum, which sometimes have absorption lines, or have weak and narrow emission lines. One of the most effective ways of studying the physics of BL Lacs is the use of simultaneous multi-frequency data. The multi-frequency broadband radio spectrum was obtained simultaneously with an accuracy of up to 1-2 minutes for four to six frequencies: 1.1, 2.3, 4.8, 7.7, 11.2, and 21.7 GHz. The catalogue is based on the RATAN-600 observations and on the data from: equatorial coordinate and redshift, R-band magnitude, synchrotron peak frequency, SED classes and object type, literature. The present version of the catalogue contains RATAN-600 flux densities measurements over nine years (2006-2014), radio spectra at different epochs, and their parameters for more than 300 BL Lacs objects and candidates. The BL Lacs list is constantly updated with new observational data of RATAN-600.
The most elusive and extreme sub-class of active galactic nuclei (AGNs), known as BL Lac objects, shows features that can only be explained as the result of relativistic effects occurring in jets pointing at a small angle with respect to the line of sight. A long standing issue is the identification of the BL Lac parent population, having jets oriented at larger angles. According to the unification scenario of AGNs, radio galaxies with low luminosity and edge-darkened radio morphology are the most promising candidates to be the parent population of BL Lacs. Here we compare the large-scale environment, an orientation independent property, of well-defined samples of BL Lacs with samples of radio-galaxies all lying in the local Universe. Our study reveals that BL Lacs and radio galaxies live in significantly different environments, challenging predictions of the unification scenario. We propose a solution to this problem proving that large-scale environments of BL Lacs is statistically consistent with that of compact radio-sources, known as FR0s, sharing similar properties. This implies that highly relativistic jets are ubiquitous and are the natural outcome of the accretion of gas into the deep gravitational potential well produced by supermassive black holes.
We report on the redshift of the BL Lac object 3FGL J0909.0+2310 based on observations obtained with the OSIRIS Multi Object Spectrograph (MOS) mounted on the 10.4-m Gran Telescopio Canarias. A redshift of 0.432+/-0.002 was obtained by the identification of three absorption features (CaII K&H and G-band) detected in the spectrum of the BL Lac host galaxy. The closest object to the BL Lac at an angular separation of 3.8 (~21 kpc at this distance) has a similar redshift of 0.431+/-0.002. This companion galaxy could be the most likely cause of the nuclear activity as postulated by studies based on more extended data sets and cosmological models. MOS allows us to study the objects neighbourhood within a field of view of approximately 7x2 and we find two small groups of galaxies at redshifts 0.28 and 0.39 which are probably not related to the activity of 3FGL~J0909.0+2310.
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