ABSTRACT We searched for quasi-periodicities on year-like timescales in the light curves of six blazars in the optical-near-infrared bands and we made a comparison with the high energy emission. We ...obtained optical/NIR light curves from Rapid Eye Mounting photometry plus archival Small & Moderate Aperture Research Telescope System data and we accessed the Fermi light curves for the γ-ray data. The periodograms often show strong peaks in the optical and γ-ray bands, which in some cases may be inter-related. The significance of the revealed peaks is then discussed, taking into account that the noise is frequency dependent. Quasi-periodicities on a year-like timescale appear to occur often in blazars. No straightforward model describing these possible periodicities is yet available, but some plausible interpretations for the physical mechanisms causing periodic variabilities of these sources are examined.
ABSTRACT We have considered the Fermi γ-ray light curve of the blazar PKS 0537-441 during a high state extending from 2008 August 10 to 2011 August 27. The periodogram exhibits a peak at T ∼ 280 ...days, with a significance of ∼99.7%. A peak of similar relevance at is found in the optical light curves. Considering the entire duration of the Fermi light curve 2008-2015, no significant peak is revealed, while the optical one remains meaningful. Comparing with recent observations of PKS 2155-304 and PG 1553+113 it seems that month-year oscillations can characterize some blazars. Month-scale oscillations can also show up only during phases of enhanced or bursting emission, such as in the case of PKS 0537-441.
Context. We present the light curves of six BL Lac objects, PKS 0537-441, PKS 0735+17, OJ 287, PKS 2005-489, PKS 2155-304, and W Comae, and of the flat spectrum radio quasar PKS 1510-089, as a part ...of a photometric monitoring program in the near-infrared/optical bands started in 2004. All sources are Fermi blazars. Aims. Our purpose is to investigate flux and spectral variability on short and long time scales. Systematic monitoring, independent of the activity of the source, guarantees large sample size statistics, and allows an unbiased view of different activity states on weekly or daily time scales for the whole timeframe and on nightly time scales for some epochs. Methods. Data were obtained with the REM telescope located at the ESO premises of La Silla (Chile). Light curves were gathered in the optical/near-infrared VRIJHK bands from April 2005 to June 2012. Results. Variability ≳3 mag is observed in PKS 0537-441, PKS 1510-089 and PKS 2155-304, the largest ranges spanned in the near-infrared. The color intensity plots show rather different morphologies. The spectral energy distributions in general are well fitted by a power law, with some deviations that are more apparent in low states. Some variability episodes during a night interval are well documented for PKS 0537-441 and PKS 2155-304. For the latter source the variability time scale implies a large relativistic beaming factor.
Abstract
This paper describes the database of optical spectra of BL Lacertae (BLL) objects (Z BLL objects) available at
https://web.oapd.inaf.it/zbllac/
. At present, it contains calibrated spectra ...for 295 BLL. For about 35% of them, we report a firm measure of redshift
z
, while for 35 sources we set a lower limit on
z
based on the detection of intervening absorption systems, mainly ascribed to Mg
ii
(
λ
2800 Å). We report here on the architecture of the database and on its website front-end that permits us to filter, query, and interactively explore the data. We discuss some properties of the objects in the present data set by giving the distribution of the redshifts and reporting on the detected emission lines, which turn out to be mainly forbidden and ascribed to O
ii
(
λ
3737 Å) and O
iii
(
λ
5007 Å). Finally, we discuss on intervening absorption systems detected in 35 BLLs that allow us to set lower limits to their distance.
Abstract
Isolated black holes and neutron stars can be revealed through the observation of long-duration gravitational microlensing events. A few candidates have been found in surveys of stars in the ...direction of the Galactic bulge. Recently, thanks to the addition of astrometric information at milliarcsecond level, it has been possible to reduce the uncertainties in the masses and distances for some of these “dark” gravitational lenses and select the most promising candidates. These isolated compact objects might emit X-rays powered by accretion from the interstellar medium. Using data of the Chandra, XMM-Newton, and INTEGRAL satellites, we searched for X-ray emission in the isolated black hole candidate OGLE-2011-BLG-0462, and in several other putative collapsed objects found with gravitational microlensing. OGLE-2011-BLG-0462 has been recently interpreted as a 7.1
M
⊙
black hole at a distance of 1.6 kpc, although a different group obtained a mass range (1.6–4.4
M
⊙
) that cannot exclude a massive neutron star. We have derived upper limits on the flux from OGLE-2011-BLG-0462 of 9 × 10
−15
erg cm
−2
s
−1
in the 0.5–7 keV range and ∼2 × 10
−12
erg cm
−2
s
−1
in the 17–60 keV range. The implied X-ray luminosity is consistent with the small radiative efficiency expected for a black hole and disfavors a neutron star interpretation. Limits down to a factor of about five lower are obtained for the soft X-ray flux of other candidates, but their interpretation is affected by larger uncertainties in the masses, distances, and spatial velocities.
We have searched for periodicities in our VRIJHK photometry of PKS 2155-304, which covers the years 2005-2012. A peak of the Fourier spectrum with high significance is found at T ~ 315 days, ...confirming the recent findings by Zhang et al. The examination of the gamma-ray light curves from the Fermi archives yields a significant signal at ~2T, which, while nominally significant, involves data spanning only ~6T. Assuming a black hole mass of 10 super(9) M sub(middot in circle), the Keplerian distance corresponding to the quasi-period T is ~10 super(16) cm, about 50 Schwarzschild radii.
We report on 16 BL Lacertae objects that were proposed to be at z > 1. We present spectroscopic observations secured at the 10.4 m GTC that allowed us to assess the redshift of these sources. In ...particular, for five objects, we disprove the previous value of the redshift reported in the literature and found that they lie at z < 1. Moreover, two of them exhibit broad emission lines that are not characteristic of BL Lacertae objects. On the other hand, for eight targets, we improve the tentative value of z, previously based on only one feature, by detecting a number of emission lines. Finally, in three cases, we detect the onset of the Ly forest at z > 2.50. Based on the new high quality spectra, we found that only half of the observed objects can be classified as bona-fide BL Lacs.
We study the properties of the broad-line region in blazars by comparing the virial estimate of black hole masses with that derived from the mass of the host galaxies. The former is sensitive to the ...width of broad lines, i.e. to the projection of the velocity of line-emitting clouds along the line of sight; the latter is not. This comparison allows us to constrain the deprojection factor f, thus revealing general properties of the geometry of the broad-line region. We show that blazars tend to have (1) higher f values than the quasars of our reference sample: 〈f
BLLacs〉= 6.9 ± 2.3; 〈f
blazars〉= 5.6 ± 1.3 and 〈f
quasars〉= 2.0 ± 0.3, (2) relatively narrow broad emission lines and (3) modest equivalent widths, as expected because of the occurrence of jet emission at very low inclination angles. In a disc-like sketch of the broad-line region, these results indicate a pole-on view of a flat geometry in blazars. This consistently extends the orientation-dependent unified model of active nuclei to the geometry of the broad-line region.
We review the reports of possible year-long quasi-periodicities of BL Lac objects in the γ-ray and optical bands, and present a homogeneous time analysis of the light curves of PKS2155−304, ...PG1553+113, and BL Lac. Based on results from a survey covering the entire Fermi γ-ray sky we have estimated the fraction of possible quasi-periodic BL Lac objects. We compared the cyclical behaviour in BL Lac objects with that derived from the search of possible optical periodicities in quasars, and find that at z ≲ 1 the cosmic density of quasi-periodic BL Lac objects is larger than that of quasi-periodic quasars. If the BL Lac quasi-periodicities were due to a supermassive binary black hole (SBBH) scenario, there could be a tension with the upper limits on the gravitational wave background measured by the pulsar timing array. The argument clearly indicates the difficulties of generally associating quasi-periodicities of BL Lac objects with SBBHs.
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