Nonthermal Optical Emission Research Articles

Evidence of jet-induced optical microvariability in radio-loud narrow-line Seyfert 1 galaxies

ABSTRACT To quantify the role of radio jets for Intra-Night Optical Variability (INOV) in radio-loud narrow-line Seyfert 1 (RLNLSy1) galaxies, we report the first systematic comparative INOV study of 23 RLNLSy1 galaxies, with 15 RLNLSy1s having confirmed detection of jets (jetted) and the remaining 8 RLNLSy1s having no detection of jets (non-jetted) based on their Very Long Baseline Array observations. We have monitored these two samples, respectively, in 37 and 16 sessions of a minimum 3-h duration each. Based upon Fη-test at 99 per cent confidence level with a typical INOV amplitude (ψ) detection threshold of >3 per cent, we find the INOV duty cycles (DC) of 12 per cent for the sample of jetted RLNLSy1s, however, none of the sources showed INOV in the sample of non-jetted RLNLSy1s. Among the jetted RLNLSy1s, we find that the DC for jetted γ-ray detected (γ-ray) RLNLSy1s is found to be 34 per cent in contrast to null INOV detection in the case of non-γ-ray RLNLSy1s. It suggests that instead of the mere presence of a jet, relativistic beaming plays a significant role for INOV in the case of low-luminous high accreting AGNs, such as NLSy1s, in which dilution of the AGN’s non-thermal optical emission by the (much steadier) optical emission contributed by the nuclear accretion disc is quite likely. Our study of jetted γ-ray RLNLSy1s shows more frequent INOV detection for sources with higher apparent jet speed. Further, our results also suggest that among the NLSy1s, only jetted γ-ray RNLSy1 galaxies DC approach blazar-like DC.

Intra-night optical variability of γ −ray detected narrow-line Seyfert1 galaxies

Abstract We report the first attempt to systematically characterise intra-night optical variability (INOV) of the rare and enigmatic subset of Narrow-Line Seyfert1 galaxies (NLSy1s), which is marked by detection in the γ-ray band and is therefore endowed with Doppler boosted relativistic jets, like blazars. However, the central engines in these two types of AGN are thought to operate in different regimes of accretion rate. Our INOV search in a fairly large and unbiased sample of 15 γ-ray NLSy1s was conducted in 36 monitoring sessions, each lasting ≥ 3 hrs. In our analysis, special care has been taken to address the possible effect on the differential light curves, of any variation in the seeing disc during the session, since that might lead to spurious claims of INOV from such AGN due to the possibility of a significant contribution from the host galaxy to the total optical emission. From our observations, a duty cycle (DC) of INOV detection in the γ-ray NLSy1s is estimated to be around 25% - 30%, which is comparable to that known for blazars. This estimate of DC will probably need an upward revision, once it becomes possible to correct for the dilution of the AGN’s nonthermal optical emission by the (much steadier) optical emission contributed not only by the host galaxy but also the nuclear accretion disc in these high Eddington rate accretors. Finally, we also draw attention to the possibility that sharp optical flux changes on sub-hour time scale are less rare for γ-ray NLSy1s, in comparison to blazars.

Nonthermal optical emission spectrometry for simultaneous and direct determination of zinc, cadmium and mercury in spray.

A nonthermal optical emission spectrometric (OES) system is developed for the simultaneous multi-element determination of zinc, cadmium and mercury, with a dielectric barrier discharge (DBD) microplasma generated on the nozzle of a pneumatic micronebulizer as the excitation source. After solution nebulization, zinc, cadmium and mercury are directly atomized and excited in the spray by a DBD microplasma for performing optical emission. The emission spectra are measured with a charge-coupled device (CCD) spectrometer. Various important parameters governing the analytical performance of the nonthermal OES system are scrutinized, encompassing the conditions of sample solution introduction, microplasma formation, signal collection and matrix matching for DBD-OES detection. With the characteristic emission lines of Zn 213.9 nm, Cd 228.8 nm and Hg 253.7 nm as the analytical lines for quantification, the detection limits for the simultaneous determination of Zn, Cd and Hg are 22, 1.6, and 10 μg L-1, respectively, and the corresponding linear ranges are 70-6000, 5-1000, and 35-2000 μg L-1, respectively. The accuracy of the present nonthermal OES system is confirmed by the determination of Zn, Cd and Hg in two certified reference materials, i.e., CRM 176 (city waste incineration ash) and GBW 08608 (water). The proposed nonthermal OES system provides a portable instrument for field analysis in emergency with low cost and low consumption.

VLT/FORS2 observations of the optical counterpart of the isolated neutron star RBS 1774

X-ray observations performed with ROSAT led to the discovery of a group (seven to date) of X-ray dim and radio-silent middle-aged isolated neutron stars (a.k.a. XDINSs), which are characterised by pure blackbody spectra (kT~40-100 eV), long X-ray pulsations (P=3-12 s), and appear to be endowed with relatively high magnetic fields, (B~10d13-14 G). RBS 1774 is one of the few XDINSs with a candidate optical counterpart, which we discovered with the VLT. We performed deep observations of RBS 1774 in the R band with the VLT to disentangle a non-thermal power-law spectrum from a Rayleigh-Jeans, whose contributions are expected to be very much different in the red part of the spectrum. We did not detect the RBS 1774 candidate counterpart down to a 3 sigma limiting magnitude of R~27. The constraint on its colour, (B-R)<0.6, rules out that it is a background object, positionally coincident with the X-ray source. Our R-band upper limit is consistent with the extrapolation of the B-band flux (assuming a 3 sigma uncertainty) for a set of power-laws F_nu ~nu^alpha with spectral indeces alpha<0.07. If the optical spectrum of RBS 1774 were non-thermal, its power-law slope would be very much unlike those of all isolated neutron stars with non-thermal optical emission, suggesting that it is most likely thermal. For instance, a Rayleigh-Jeans with temperature T_O = 11 eV, for an optically emitting radius r_O=15 km and a source distance d=150 pc, would be consistent with the optical measurements. The implied low distance is compatible with the 0.04 X-ray pulsed fraction if either the star spin axis is nearly aligned with the magnetic axis or with the line of sight, or it is slightly misaligned with respect to both the magnetic axis and the line of sight by 5-10 degrees

Rapid Multiwaveband Polarization Variability in the Quasar PKS 0420-014: Optical Emission from the Compact Radio Jet

An 11-day monitoring campaign in late 2005 reveals clear correlation in polarization between the optical emission and the region of the intensity peak (the pseudocore) at the upstream end of the jet in 43 GHz Very Long Baseline Array images in the highly variable quasar PKS 0420-014. The electric-vector position angle (EVPA) of the pseudocore rotated by about 80 degrees in four VLBA observations over a period of nine days, matching the trend of the optical EVPA. In addition, the 43 GHz EVPAs agree well with the optical values when we correct the former for Faraday rotation. Fluctuations in the polarization at both wavebands are consistent with the variable emission arising from a standing conical shock wave that compresses magnetically turbulent plasma in the ambient jet. The volume of the variable component is the same at both wavebands, although only ~20% of the total 43 GHz emission arises from this site. The remainder of the 43 GHz flux density must originate in a separate region with very low polarization. If 0420-014 is a typical case, the nonthermal optical emission from blazars originates primarily in and near the pseudocore rather than closer to the central engine where the flow collimates and accelerates.

Synchrotron Self‐Absorption in Pulsar Magnetospheres: Implications for Optical Emission

Nonthermal optical emission from pulsars has been observed definitively from five objects. The model that most accurately predicts the luminosity of such emissions is that of Pacini and Salvati, a model based on synchrotron radiation originating at a constant fraction of the light cylinder. We parameterize optical emission in a similar way, obtaining the solution to the radiative transfer equation that yields expressions for the expected monochromatic luminosity and synchrotron self-absorption frequency. If due to synchrotron self-absorption, we investigate whether the rollover at IR wavelengths observed for the Crab pulsar is a process likely to be common to all the synchrotron active pulsars. Although the low-frequency turnover in the Crab pulsar spectrum may be due to a low-energy cutoff in the underlying emitting particle population or to particles emitting below their critical frequency, a test of the current self-absorption model is provided by PSR B0540-69 and the 16 ms Crab-like pulsar J0537-69. Our model, scaled relative to the Crab pulsar, predicts that PSR B0540-69 should exhibit a rollover between the L and I bands and that PSR J0537-69 may be self-absorbed at optical wavelengths with mV ~ 24 (dependent on the spectral index, α, where Fν ~ ν-α). Our model is applicable to frequency regions well outside the optical and is limited to young, fast (<100 ms) pulsars.

A Model for Non-thermal Optical Emission from Radio Pulsars

Interpretation of the non-thermal optical emission of pulsars as a spontaneous synchrotron re-emission of the secondary particles involved in the process of resonant absorption of pulsar radio emission is suggested. Our model can explain the non-thermal high-energy luminosities and spectra of both young and middle-aged pulsars. Moreover, it implies a physical link between the radio and higher-energy emission of pulsars.

Location of the nonthermal optical emission from jets in AGN

As part of an approved key science project for the Space Interferometry Mission (SIM), we plan to measure the position offsets between red and blue optical emission from a sample of Active Galactic Nuclei. By using differential measurements of the fringe phase between different color bands, we expect to be able to detect color-dependent astrometric offsets at the micro-arcsecond level. The blue optical continuum is dominated by thermal emission from the inner accretion disk (the source of the Big Blue Bump) in many quasar spectra. The red optical continuum is expected to be dominated by power-law non-thermal emission from the relativistic jet or from a hot corona surrounding the inner accretion disk. Our measurements of position offsets between blue and red light will allow us to determine the location of non-thermal optical emission with respect to the thermal emission from the inner accretion disk. We will also be able to monitor the astrometric stability of the optical emission centroids.

A model for non-thermal optical emission of radio pulsars

Interpretation of the non-thermal optical emission of pulsars as a spontaneous synchrotron reemission of the secondary particles involved in the process of resonant absorption of pulsar radio emission is suggested. In the outer magnetosphere, where the condition of cyclotron resonance is fulfilled, the radio waves are subject to resonant absorption. Although this process is not efficient enough to suppress the integrated radio luminosities dramatically, it causes substantial evolution of the momenta of absorbing particles. The particle pitch angles become ∼θ (where θ is the angle of incidence of resonant photons) and the total momenta can change within a few orders of magnitude. The synchrotron radiation of these particles falls into the optical and higher energy range. Because of strong spatial evolution of the particle momenta, the critical frequency of synchrotron radiation varies markedly inside the emitting volume. Therefore the resultant spectra are very flat over a wide frequency range. Our model is applicable for both middle-aged and young pulsars, providing the optical luminosities ∼10 28 erg s −1 and up to ∼10 34 erg s −1 , respectively. In the case of the Crab pulsar, one can explain the high-energy spectrum up to ∼10 MeV.

On the Origin of Extended Nonthermal Optical Emission in Extragalactic Jets

The extended nonthermal optical emission of extragalactic jets can only be explained by some in situ reacceleration. Since the jets are magnetized plasma flows, the dissipation of the magnetic field via magnetic reconnection seems to be a promising candidate for the reacceleration mechanism. Reconnection converts magnetic energy, which is to be regarded as the only energy supply in the entire jet region, into particle energy via continuous acceleration. We show that inertia-driven reconnection operating in a highly filamentary magnetic flux rope topology can provide a satisfying solution to the reacceleration problem for the collisionless magnetized jet plasma.

Molecular gas in luminous infrared galaxies

view Abstract Citations (456) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Molecular Gas in Luminous Infrared Galaxies Sanders, D. B. ; Scoville, N. Z. ; Soifer, B. T. Abstract CO (1 -> 0) emission profiles are presented for 60 infrared luminous, [L-ir_(8-1000 microns) = 5 x 10^10^-3 x 10^12^ L_sun_, H_0_ = 75 km s^-1^ Mpc^-1^], bright (F_60 micron_ > 5.24 Jy), IRAS galaxies at redshifts 1500-25,000 km s^-1^ Most of these galaxies are sufficiently distant that a single 55" beam measures the total CO luminosity and hence the total molecular gas content. The data for these galaxies are analyzed together with that for a sample of 29 closer IRAS bright galaxies with L_ir_ down to 2 x 10^10^ L_sun_ mapped by Tinney et al. (1990). All of the IRAS bright galaxies are extremely rich in molecular gas with M(H_2_) = 1 x 10^9^-6 x 10^10^ M_sun_, corresponding to 0.5-30 times the mass of H_2_ gas in the Milky Way. The dust masses are evaluated from the IRAS data and the mean ratio of H_2_ gas to "warm dust" is 540 +/- 290. Including the mass of heavy elements and the contribution from H I implies a mean gas-to-dust ratio in the range 900-1100. The discrepancy between this value and the standard Galactic gas-to-dust ratio of ~ 150 could be due to the presence of large quantities of cold dust to which IRAS is insensitive, and/or errors in the assumed dust mass absorption coefficient. The infrared luminosity-to-H_2_ mass ratios, L_ir_/M(H_2_), range from 2-220 L_sun_ M^-1^_sun_, up to a factor of 55 times the mean ratio for molecular clouds in the Galaxy. The ratio L_ir_/M(H_2_) increases with increasing L_ir_, but shows no systematic dependence on M(H_2_). The higher values of L_ir_/M(H_2_) obtained in the most luminous galaxies suggests either an increased star formation efficiency and/or an additional source of luminosity. The kinetic energy of colliding galaxies is ruled out as a significant source of the luminosity in view of the independence of the L_ir_/M(H_2_) ratios on the galactic velocity dispersions and the merging time scales which are too long. If the excess luminosity is supplied by star formation, the implied lifetime of the interstellar medium is <= 3 x 10^8^ yr. However, a significant number of the galaxies with the highest L_ir_/M(H_2_) ratios exhibit non-thermal optical emission lines characteristic of an active galactic nucleus (AGN), and an AGN appears to be an important energy source for these galaxies. Publication: The Astrophysical Journal Pub Date: March 1991 DOI: 10.1086/169800 Bibcode: 1991ApJ...370..158S Keywords: Galactic Structure; Infrared Sources (Astronomy); Luminosity; Molecular Gases; Active Galactic Nuclei; Emission Spectra; Infrared Astronomy Satellite; Interstellar Matter; Line Spectra; Red Shift; Astrophysics; GALAXIES: INTERSTELLAR MATTER; INFRARED: SOURCES full text sources ADS | data products SIMBAD (63) NED (60)

The detection of X-ray emission from the BL Lacertae object Markarian 180

The detection of X-ray emission from Mrk 180, an elliptical galaxy which has a BL Lac object embedded in its core is reported. The X-ray observations were made by the Einstein and HEAO 1 satellites. In addition, new multiaperture UBVR photometric observations are presented which were used to separate the nonthermal optical emission from the galactic emission. Analysis of the Einstein data suggests that the continuous X-ray spectrum of Mrk 180 is a power law with an energy spectral index of -1.3. The deconvolution of the optical magnitudes is consistent with a nonthermal source having a spectral index of -1.2. The continuous spectrum of Mrk 180 is steep from the X-ray to the optical and flattens out in the radio.

Quasars as protoclusters of galaxies

A hypothesis is put forward according to which dense bodies (QSG), which occasionally transform into quasars, decay into components and their subsequent fragmentation results in the formation of groups and clusters of galaxies. Recurrent nonthermal radio emission continues during all the subsequent stages of fragmentation and evolution of the QSG's, and the fraction of nonthermal optical emission decreases. This hypothesis explains the absence of quasars from clusters of galaxies, their complex radio structure, the structural details of clusters, and the distribution of radio galaxies in them. The gravitational redshift component becomes appreciable and the requirements imposed on the energy output per unit mass of the quasars become less demanding. This hypothesis puts into concrete terms Ambartsumyan's idea about the cascade fragmentation of dense material and the dispersion of fragments in the form of disintegrating stellar systems.