Low-energy Peaked BL Lacs Research Articles

Multiband optical–IR variability of the blazar PKS 0537–441

We have reconsidered the simultaneous and homogeneous optical–IR light curves and the corresponding spectral indices curve of the blazar PKS 0537–441 from January 2011 to May 2015. All the curves show significant fluctuations on various timescales, and the flux variations seem to be more pronounced towards the IR bands. The relation between average fluxes and spectral indices reveals the existence of redder-when-brighter (RWB) and bluer-when-brighter (BWB) trends at different flux levels, along with a long-term achromatic trend and a mild RWB trend on short-term timescales. Cross-correlation analyses present an energy-dependent time delay that the lower-frequency variations follow higher-frequency ones by a few weeks and a hysteresis pattern between spectra and fluxes. Our analysis reveals some potential coherence between low-energy-peaked BL Lacs (LBLs) and FSRQs, and indicates that the observed flux variability and spectral changes could be due to the superposition of a dominant jet emission, an underlying thermal contribution from a more slowly varying disk and/or other geometric effects under the shock-in-jet scenario.

Testing the blazar sequence with the least luminous BL Lacertae objects

In a previous paper, we proposed a new method to select low-power BL Lacs (LPBLs) based on mid-infrared emission and flux contrast through the Ca II spectral break; that study led to the selection of a complete sample formed by 34 LPBLs with 0.05<z<=0.15 and radio luminosities spanning the range log(L_r) = 39.2-41.5 [erg/s]. We now assemble the broadband spectral energy distributions (SEDs) of these sources to investigate their nature and compare them with brighter BL Lacs. We find that the ratios between the X-ray and radio luminosities range from ~20 to ~30000 and that the synchrotron peak frequencies span a wide energy interval, from log(nu_peak)~13.5 to ~20 [Hz]. This indicates a broad variety of SED shapes and a mixture of BL Lac flavors. Indeed, although the majority of our LPBLs are high-energy peaked BL Lacs (HBLs), we find that a quarter of them are low-energy peaked BL Lacs (LBLs), despite the fact that the sample is biased against the selection of LBLs. The analysis of the median LPBL SED confirms disagreement with the blazar sequence at low radio luminosities. Furthermore, if we limit the sample to the LBLs subsample, we find that their median SED shape is essentially indistinguishable from that of the most luminous BL Lacs. We conclude that the observed radio power is not the main driving parameter of the multiwavelength properties of BL Lacs.

Quasi-simultaneous two-band optical variability of the blazars 1ES 1959+650 and 1ES 2344+514

We report the results of quasi-simultaneous two-filter optical monitoring of two high-energy peaked blazars, 1ES 1959+650 and 1ES 2344+514, to search for microvariability and short-term variability (STV). We carried out optical photometric monitoring of these sources in an alternating sequence of B and R passbands, and have 24 and 19 nights of new data for these two sources, respectively. No genuine microvariability (intranight variability) was detected in either of these sources. This non-detection of intranight variations is in agreement with the conclusions of previous studies that high-energy peaked BL Lacs are intrinsically less variable than low-energy peaked BL Lacs in the optical bands. We also report the results of STV studies for these two sources between 2009 July and 2010 August. Genuine STV is found for the source 1ES 1959+650 but not for 1ES 2344+514. We briefly discuss possible reasons for the difference between the intranight variability behaviour of high- and low-energy peaked blazars.

2MASS observation of BL Lac objects II

In this paper, we have searched for the Two Micron All Sky Survey (2MASS) counterparts of 1434 BL Lacs. Eight hundred and thirty-three of 1434 BL Lacs (∼58%) have spatially coincident 2MASS counterparts. Fermi-detected BL Lacs (FBLs) have a much higher 2MASS detection rate than non-Fermi-detected BL Lacs (non-FBLs) (∼95% vs 49%). We compare the near-infrared (NIR) apparent magnitudes, monochromatic luminosities and spectral indices of different subclasses of BL Lacs: (i) FBLs are significantly brighter and more luminous than non-FBLs; (ii) low-energy peaked BL Lacs (LBLs) are significantly more luminous than high-energy peaked BL Lacs (HBLs); (iii) the NIR spectral index, α IR , shows a sequence of decrease from LBLs to intermediate-energy peaked BL Lacs (IBLs) then to HBLs. The K S band luminosity and NIR spectral index are both strongly anti-correlated with the synchrotron peak frequency ( ν peak ). About 66% of BL Lacs in our sample are located close to the power law line in the J- H– H- K S diagram, which implies predominantly non-thermal radiation in the NIR range. We label 218 BL Lacs whose NIR spectra show convex curvatures probably due to the significant contaminations of host-galaxy starlight. Finally we investigate the correlation between NIR and gamma-ray emission using the 2MASS observational data.

Parsec-scale magnetic field structures in HEAO-1 BL Lacs

We present very long baseline interferometry polarization images of an X-ray selected sample of BL Lacertae objects belonging to the first High Energy Astronomy Observatory (HEAO-1) and the ROSAT–Green Bank (RGB) surveys. These are primarily high-energy-peaked BL Lacs (HBLs) and exhibit core-jet radio morphologies on pc-scales. They show moderately polarized jet components, similar to those of low-energy-peaked BL Lacs (LBLs). The fractional polarization in the unresolved cores of the HBLs is, on average, lower than in the LBLs, while the fractional polarizations in the pc-scale jets of HBLs and LBLs are comparable. However a difference is observed in the orientation of the inferred jet magnetic fields – while LBL jets are wellknown to preferentially exhibit transverse magnetic fields, the HBL jets tend to display longitudinal magnetic fields. Although a ‘spine-sheath’ jet velocity structure, along with larger viewing angles for HBLs could produce the observed magnetic field configuration, differences in other properties of LBLs and HBLs, such as their total radio power, cannot be fully reconciled with the different-angle scenario alone. Instead it appears that LBLs and HBLs differ intrinsically, perhaps in the spin rates of their central black holes.

Filling the infrared gap: ISO observations of 1 Jy BL Lacertae objects

The large majority of BL Lacertae objects belonging to the 1 Jy sample, the class prototype for radio-selected sources, are thought to emit most of their synchrotron power in the far IR band. Ironically, this spectral region is very sparsely sampled, with only a minority of the objects having IRAS data (most of them being upper limits or low-quality detections). We aim at filling this IR gap by presenting new, simultaneous ISOCAM and ISOPHOT observations over the 7 - 200 micron range for half the sample. A measurement of the position of the synchrotron peak frequency, nu_peak, can provide information about particle acceleration mechanisms and constrain the inverse Compton radiation that will be detected by up-coming new gamma-ray missions. We have observed 17 1 Jy BL Lacertae objects with the camera and the photometer on board the Infrared Space Observatory (ISO) satellite. The ISOPHOT data reduction was done employing a novel correction, which mitigates the effect of chopping for faint sources. Using our new ISO data, complemented by nearly-simultaneous radio and optical observations for 10 and 4 objects respectively, and other multi-frequency data, we have built the spectral energy distributions of our sources (plus a previously published one) and derived the rest-frame nu_peak. Its distribution is centered at 10^13 Hz (30 micron) and is very narrow, with 60% of the BL Lacs in the 1 - 3 10^13 Hz range. Given our set of simultaneous infrared data, these represent the best determinations available of the synchrotron peak frequencies for low-energy peaked BL Lacs. A comparison with previous such estimates, based on non-simultaneous optical and near IR data, may indicate strong nu_peak variations in a number of sources, possibly associated with large flares as observed in the high-energy peaked BL Lac MKN 501. (abridged)

Six years of BeppoSAX observations of blazars: A spectral catalog

We present a spectral catalog for blazars based on the BeppoSAX archive. The sample includes 44 High-energy peaked BL Lacs (HBLs), 14 Low-energy peaked BL Lacs (LBLs), and 28 Flat Spectrum Radio Quasars (FSRQs). A total of 168 LECS, MECS, and PDS spectra were analyzed, corresponding to observations taken in the period 1996--2002. The 0.1--50 keV continuum of LBLs and FSRQs is generally fitted by a single power law with Galactic column density. A minority of the observations of LBLs (25%) and FSRQs (15%) is best fitted by more complex models like the broken power law or the continuously curved parabola. These latter models provide also the best description for half of the HBL spectra. Complex models are more frequently required for sources with fluxes F_{2-10 keV} > 10^-11 cm-2 s-1, corresponding to spectra with higher signal-to-noise ratio. As a result, considering sources with flux above this threshold, the percentage of spectra requiring those models increases for all the classes. We note that there is a net separation of X-ray spectral properties between HBLs on one side, and LBLs and FSRQs on the other, the distinction between LBLs and FSRQs is more blurry. This is most likely related to ambiguities in the optical classification of the two classes.

The Radio Structure of High-Energy–Peaked BL Lacertae Objects

We present VLA and first-epoch VLBA observations that are part of a program to study the parsec-scale radio structure of a sample of fifteen high-energy-peaked BL Lacs (HBLs). The sample was chosen to span the range of logarithmic X-ray to radio flux ratios observed in HBLs. As this is only the first epoch of observations, proper motions of jet components are not yet available; thus we consider only the structure and alignment of the parsec- and kiloparsec-scale jets. Like most low-energy-peaked BL Lacs (LBLs), our HBL sample shows parsec-scale, core-jet morphologies and compact, complex kiloparsec-scale morphologies. Some objects also show evidence for bending of the jet 10-20pc from the core, suggesting interaction of the jet with the surrounding medium. Whereas LBLs show a wide distribution of parsec- to kpc-scale jet misalignment angles, there is weak evidence that the jets in HBLs are more well-aligned, suggesting that HBL jets are either intrinsically straighter or are seen further off-axis than LBL jets.

Magnetic Field Geometry in `Red' and `Blue' BL Lacs

We compare the systematics of the magnetic field geometry in the red low-energy peaked BL Lacs (LBLs) and blue high-energy peaked BL Lacs (HBLs) using VLBI polarimetric images. The LBLs are primarily radio--selected BL Lacs and the HBLs are primarily X-ray selected. In contrast to the LBLs, which show predominantly transverse jet magnetic fields, the HBLs show predominantly longitudinal fields. Thus, while the SED peaks of core-dominated quasars, LBLs and HBLs form a sequence of increasing frequency, the magnetic field geometry does not follow an analogous sequence. We briefly investigate possible connections between the observed parsec-scale magnetic field structures and circular polarization measurements in the literature on various spatial scales.

X-ray variability in BL Lac objects

X-ray properties of BL Lac objects are classified into two categories. One is low energy peaked BL Lacs (LBLs). The X-ray spectrum of LBLs is a power law type with an index of about 1.7 and is flatter than those in optical and UV bands. X-ray flux varies in correlated way with optical flux for a major outburst. The other catgory is high-energy peaked BL Lacs (HBLs), whose X-ray spectra are either single power law type or power law type with gradual steepening toward higher energy. The spectra are steeper than those of LBLs and can be connected smoothly from optical and UV bands. X-rays from HBLs are characterized by correlated flux variation with spectral index and soft lag. The spectra becomes steeper with decreasing flux, and the peak or bottom of the light curve in the soft band is delayed from those in hard band. These properties are interpreted by a synchrotron-cooling model, and cooling time, escape time, and injection time of synchrotron emitting electrons are determine! d by fitting the light curves with the model. The flux increase and hardening of X-rays during the rise of the outburst was expressed by Fermi acceleration of low energy electrons and hard lag of the start time of the burst was strong support for the acceleration. On the other hand, flat X-ray spectrum of LBLs is attributed to inverse Compton scattering of internal and/or external seed photons.

ASCA observations of blazars and multiband analysis

We present data for 18 blazars observed with ASCA, half of which were also observed contemporaneously with EGRET as parts of multi-wavelength campaigns. the ASCA X-ray spectra of High-energy peaked BL Lacs (HBLs) are soft, and they form the highest energy tail of the low energy (synchrotron) component. The X-ray spectra of the quasar-hosted blazars (QHBs) are hard and consistent with the lowest energy end of the high energy (Compton) component. For Low-energy peaked BL Lacs (LBLs), the X-ray spectra are intermediate. We find that the radiation process responsible for the HE peak for HBLs can be explained solely by Synchrotron-Self-Compton (SSC) emission. For many QHBs, on the other hand, the γ-rays cannot be solely due to the SSC mechanism. We consider an alternative scenario for QHBs where the SSC component dominates in the X-ray band, but it is below the observed γ-ray spectrum. We infer the magnetic field B to be 0.1 – 1 Gauss, and Lorentz factors γ b of electrons radiating at the peak of the ν F(ν) spectrum of ∼ 10 3 for QHBs. This is much lower than γ b ∼ 10 5 for HBLs.

Soft X‐Ray Properties of Flat‐Spectrum Radio Quasars

We report the soft X-ray properties of 41 flat-spectrum radio quasars (FSRQs), derived by cross-correlating literature blazar samples with the ROSAT PSPC archive of pointed observations. The redshifts of the sources span the range z = 0-3. The 0.1-2.4 keV spectra of FSRQs are well described by a single power law modified by column densities of neutral gas consistent with Galactic values. Spectral flattening/steepening at the lower energies is present in only 4/41 and 2/41 sources, respectively. The ROSAT photon indices of FSRQs span a wide range, 0.5 ? 2.5, and have a measurable intrinsic dispersion; only in part is the spread of slopes due to a redshift effect. The rest-frame ROSAT spectral indices of FSRQs are statistically indistinguishable from low-energy peaked BL Lacs and flatter than high-energy peaked BL Lacs, although with several FSRQs having soft X-ray spectra as steep as HBLs. Similarly, while the bulk of FSRQs are intrinsically X-ray brighter than both types of BL Lacs, the X-ray luminosity distributions of the three classes have a large overlap. The ROSAT spectra of 13 FSRQs are steeper than measured with the Einstein IPC (0.3-4.5 keV), indicating a flat tail emerging at ? 1-2 keV at rest-frame energies. We find a trend for blazars with stronger optical emission lines to be more ?-ray-loud and to have flatter X-ray-to-?-ray continua, possibly supporting external Compton scattering models as the primary generation mechanisms of the high-energy continuum in these objects. For several FSRQs repeatedly observed with ROSAT on timescales of weeks to years, flux variations not larger than a factor of 2 have been detected, with more typical variability amplitude on the order of 10%-30%, and with no accompanying spectral changes. This appears to be in contrast to the less luminous BL Lacs, which are generally characterized by large-amplitude X-ray flux changes, usually accompanied by correlated spectral variations.