Optical variability of a newly discovered blazar sample from the BZCAT Catalog

In an optical monitoring program to characterize the variability properties of blazar, we observed 10 sources from the Roma-BZCAT catalogue for 26 nights in V and R bands during October 2014 to June 2015 with two telescopes located in India. The sample includes mainly newly discovered BL Lacs where the redshift of some sources are not known yet. We present the results of flux and color variations of the sample on intraday and short time scales obtained by using the power-enhanced F-test and the nested-ANOVA tests, along with their spectral behavior. We find significant intraday variability in the single FSRQ in our sample, having an amplitude of variation ~12%. Although a few of BL Lacs showed probable variation in some nights, none of them passes the variability tests at 99.9% significance level. We find that 78% of the sample showed significant negative colour--magnitude correlations i.e., a redder-when-brighter spectral evolution. Those which do not show strong or clear chromatism, predominantly exhibit a redder-when-brighter trends. Unlike on hourly timescales, the high synchrotron peaked (HSP) blazars in the sample (BZGJ0656+4237, BZGJ0152+0147 and BZBJ1728+5013) show strong flux variation on days to months timescales, where again we detect a decreasing trend of the spectral slope with brightness. We observe a global steepening of the optical spectrum with increasing flux on intranight timescale for the entire blazar sample. Non-variability in the BL Lacs in our sample could be resulted by distinct contribution from the disk as well as from other components in the studied energy range.

An optical overview of blazars with LAMOST I: Hunting changing-look blazars and new redshift estimates

The extragalactic $gamma$-rays sky observed by Fermi-Large Area Telescope (LAT) is dominated by blazars. In the fourth release of the Fermi-LAT Point Source Catalog (4FGL), are sources showing a multifrequency behavior similar to that of blazars but lacking an optical spectroscopic confirmation of their nature known as Blazar Candidate of Uncertain type (BCUs). We aim at confirming the blazar nature of BCUs and test if new optical spectroscopic observations can reveal spectral features, allowing us to get a redshift estimate for known BL Lac objects. We also aim to search for and discover changing-look blazars (i.e., blazars that show a different classification at different epochs). We carried out an extensive search for optical spectra available in the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) Data Release 5 (DR5) archive. We selected sources out of the 4FGL catalog, the list of targets from our follow-up spectroscopic campaign of unidentified or unassociated $gamma$-ray sources, and the multifrequency catalog of blazars: the Roma-BZCAT. We selected a total of 392 spectra. We also compare some of the LAMOST spectra with those available in the literature. We classified 20 BCUs confirming their blazar-like nature. Then we obtained 15 new redshift estimates for known blazars. We discovered 26 transitional (i.e., changing-look) blazars that changed their classification. Finally, we were able to confirm the blazar-like nature of six BL Lac candidates. All remaining sources analyzed agree with previous classifications. BL Lac objects are certainly the most elusive type of blazars in the $gamma$-ray extragalactic sky.

The composite X-ray spectra of radio-loud and radio-quiet SDSS quasars

We present the study on the X-ray emission for a sample of radio-detected quasars constructed from the cross-matches between SDSS, FIRST catalogs and XMM-Newton archives. A sample of radio-quiet SDSS quasars without FIRST radio detection is also assembled for comparison. We construct the optical and X-ray composite spectra normalized at rest frame $4215,rm AA$ (or $2200,rm AA$) for both radio-loud quasars (RLQs) and radio-quiet quasars (RQQs) at $zle3.2$, with matched X-ray completeness of 19%, redshift and optical luminosity. While the optical composite spectrum of RLQs is similar to that of RQQs, we find that RLQs have higher X-ray composite spectrum than RQQs, consistent with previous studies in the literature. By dividing the radio-detected quasars into radio loudness bins, we find the X-ray composite spectra are generally higher with increasing radio loudness. Moreover, a significant correlation is found between the optical-to-X-ray spectral index and radio loudness, and there is a unified multi-correlation between the radio, X-ray luminosities and radio loudness in radio-detected quasars. These results could be possibly explained with the corona-jet model, in which the corona and jet are directly related.

Properties of radio-loud quasars in Sloan Digital Sky Survey

We present a study of a sample of 223 radio-loud quasars (up to redshift $<$0.3) in order to investigate their spectral properties. Twenty-six of these radio-loud quasars are identified as Flat Spectrum Radio Quasars (FSRQs) and fifty-four are identified as Steep Spectrum Radio Quasars (SSRQs) based on their radio spectral index. We study the [O III] line properties of these quasars to investigate the origin and properties of blue wings (shift of the profile towards lower wavelengths) and blue outliers (shift of the whole spectroscopic feature). Most of the quasars show blue wings with velocities up to 420 km $s^{-1}$. We find that around 17% of the quasars show outliers with velocities spanning 419 to -315 km $s^{-1}$. Finally, we revisit the $it M_{rm BH} - sigma$ relation of our sample using [S II]$lambda$6716, 6731 and [O III] linewidths as surrogates for stellar velocity dispersions, $sigma$, to investigate their location on the $it M_{rm BH} - sigma$ relation for quiescent galaxies. Due to the strong blending of [S II] with $rm H_{alpha}$, we could estimate $sigma_{[rm SII]}$ of only 123 quasars. We find that the radio-loud quasars do not show a relationship between $it M_{rm BH}$ and $sigma_{rm [SII]/[OIII]}$ up to a redshift of 0.3, although they cluster around the local relation. We find an overall offset of 0.12$pm$0.05 dex of our sample of radio-loud quasars from the $it M_{rm BH} - sigma$ relation of quiescent galaxies. Quasars in our highest redshift bin (z=0.25-0.3) show a deviation of $sim$0.33 $pm$ 0.06 dex with respect to the local relation.

The spectral variability of FSRQs

The optical variability of 29 flat spectrum radio quasars in SDSS Stripe 82 region are investigated by using DR7 released multi-epoch data. All FSRQs show variations with overall amplitude ranging from 0.24 mag to 3.46 mag in different sources. About half of FSRQs show a bluer-when-brighter trend, which is commonly observed for blazars. However, only one source shows a redder-when-brighter trend, which implies it is rare in FSRQs. In this source, the thermal emission may likely be responsible for the spectral behavior.

The hard X-ray spectral evolution in X-ray binaries and its application to constrain the black hole mass of ultraluminous X-ray sources

We investigate the relationship between the hard X-ray photon index $Gamma$ and the Eddington ratio ($xi=L_{X}(0.5-25 rm keV)/L_{Edd}$) in six X-ray binaries (XRBs) with well constrained black hole masses and distances. We find that different XRBs follow different anti-correlations between $Gamma$ and $xi$ when $xi$ is less than a critical value, while $Gamma$ and $xi$ generally follow the same positive correlation when $xi$ is larger than the critical value. The anti-correlation and the positive correlation may suggest that they are in different accretion modes (e.g., radiatively inefficient accretion flow (RIAF) and standard disk). We fit both correlations with the linear least-square method for individual sources, from which the crosspoint of two fitted lines is obtained. Although the anti-correlation varies from source to source, the crosspoints of all sources roughly converge to the same point with small scatter($log xi=-2.1pm0.2, Gamma=1.5pm 0.1$), which may correspond to the transition point between RIAF and standard accretion disk. Motivated by the observational evidence for the similarity of the X-ray spectral evolution of ultraluminous X-ray sources (ULXs) to that of XRBs, we then constrain the black hole masses for seven ULXs assuming that their X-ray spectral evolution is similar to that of XRBs. We find that the BH masses of these seven luminous ULXs are around $10^{4}msun$, which are typical intermediate-mass BHs (IMBHs). Our results are generally consistent with the BH masses constrained from the timing properties (e.g., break frequency) or the model fitting with a multi-color disk.

Multi-colour optical monitoring of eight red blazars

We present the observational results of multi-colour optical monitoring of eight red blazars from 2003 September to 2004 February. The aim of our monitoring is to investigate the spectral variability as well as the flux variations at short and long time scales. The observations were carried out using the 1.0 m robotic telescope of Mt. Lemmon Optical Astronomy Observatory, in Arizona, USA, the 0.6 m telescope of Sobaeksan Optical Astronomy Observatory and the 1.8 m telescope of Bohyunsan Optical Astronomy Observatory, in the Republic of Korea. During the observations, all sources show strong flux variations with amplitudes of larger than 0.5 mag. Variations with amplitudes of over 1 mag are found in four sources. Intraday variations with amplitudes larger than 0.15 mag, and a rapid brightness increase with a rate of ~0.2 mag per day in four days, are detected in S5 0716+71. We investigate the relationship between the colour index and source brightness for each source. We find that two out of three FSRQs tend to be redder when they are brighter, and, conversely, all BL Lac objects tend to be bluer. In particular, we find a significant anti-correlation between the V-I colour index and R magnitude for 3C 454.3. This implies that the spectrum became steeper when the source was brighter, which is opposite to the common trend for blazars. In contrast, significant positive correlations are found in 3C 66A, S5 0716+71, and BL Lac. However, there are only very weak correlations for PKS 0735+17 and OJ 287. We propose that the different relative contributions of the thermal versus non-thermal radiation to the optical emission may be responsible for the different trends of the colour index with brightness in FSRQs and BL Lac objects.