Optical Spectral Index Research Articles

The optical spectral index vs. luminosity relation in AGN and one of its implications

Spectroscopic optical data (∼4200-6300 Å in the rest-frame) acquired during the monitoring of some QSOs seem to confirm that AGN spectra normally harden at higher luminosities. Focusing only on the global trend (i.e. comparing different objects), the implications in deriving luminosities using optical photometric data (through the K-correction) is briefly outlined here, as well as the result of its parameterization for calculating BH masses through the M (FWHM Hβ)2 virial relation.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Multiband Optical Follow-up Observations of GRB 020813 at the Kiso and Bisei Observatories

Observations were made of the optical afterglow of GRB 020813 (Fox, Blake, & Price) with the Kiso observatory 1.05 m Schmidt telescope and the Bisei astronomical observatory 1.01 m telescope. Four-band (B, V, R, and I) photometric data points were obtained on 2002 August 13 (10:52-16:46 UT), or 0.346-0.516 days after the burst. In order to investigate the early-time (<1 day) evolution of the afterglow, four-band light curves were produced by analyzing the data taken at these two astronomical observatories as well as the publicly released data taken using the Magellan Baade telescope (Gladders & Hall). The light curves can be approximated by a broken power law, of which the indices are approximately 0.46 and 1.33 before and after a break at ~0.2 days, respectively. The optical spectral index stayed approximately constant at ~0.9 over 0.17-4.07 days after the burst. Since the temporal decay index after the break and the spectral index measured at that time are both consistent with those predicted by a spherical expansion model, the early break is unlikely to be a jet break but is likely to represent the end of an early bump in the light curve, as was observed in the optical afterglow of GRB 021004.

The Katzman Automatic Imaging Telescope Gamma‐Ray Burst Alert System, and Observations of GRB 020813

ABSTRACTWe present the technical details of the gamma‐ray burst (GRB) alert system of the Katzman Automatic Imaging Telescope (KAIT) at Lick Observatory and the successful observations of the GRB 020813 optical afterglow with this system. KAIT responds to GRB alerts robotically, interrupts its prearranged program, and takes a sequence of images for each GRB alert. A grid‐imaging procedure is used to increase the efficiency of the early‐time observations. Different sequences of images have been developed for different types of GRB alerts. With relatively fast telescope slew and CCD readout speed, KAIT can typically complete the first observation within 60 s after receiving a GRB alert, reaching a limiting magnitude of ∼19. Our reduction of the GRB 020813 data taken with KAIT shows that unfiltered magnitudes can be reliably transformed to a standard passband with a precision of ∼5%, given that the color of the object is known. The GRB 020813 optical afterglow has an exceptionally slow early‐time power‐law decay index, although other light‐curve parameters and the optical spectral index are fairly typical of GRBs.

HST optical spectral index map of the jet of 3C 273

We present HST images at 622 nm and 300 nm of the jet in 3C 273 and determine the run of the optical spectral index at 0: 00 2 along the jet. The smoothness of spectral index changes shows that the physical conditions are varying smoothly across the jet. There is no correlation between the optical flux and spectral index, as would be expected for relativistic electrons suering strong cooling due to synchrotron emission. We nd no evidence for localized acceleration or loss sites. This suggests that the spectral shape is not changing much throughout the jet. We show that relativistic beaming and/or sub-equipartition magnetic elds cannot remove the discrepancy between light-travel time along the jet and the lifetime of electrons emitting optical synchrotron radiation. We consider this further evidence in favour of a distributed electron acceleration process.

Colors of 2625 Quasars at 0 &lt; [ITAL][CLC]z[/CLC][/ITAL] &lt; 5 Measured in the Sloan Digital Sky Survey Photometric System

We present an empirical investigation of the colors of quasars in the Sloan Digital Sky Survey (SDSS) photometric system. The sample studied includes 2625 quasars with SDSS photometry: 1759 quasars found during SDSS spectroscopic commissioning and SDSS follow-up observations on other telescopes, 50 matches to FIRST quasars, 573 matches to quasars from the NASA Extragalactic Database, and 243 quasars from two or more of these sources. The quasars are distributed in a 25 wide stripe centered on the celestial equator covering ~529 deg2. Positions (accurate to 02) and SDSS magnitudes are given for the 898 quasars known prior to SDSS spectroscopic commissioning. New SDSS quasars, which range in brightness from i* = 15.39 to the photometric magnitude limit of the survey, represent an increase of over 200% in the number of known quasars in this area of the sky. The ensemble average of the observed colors of quasars in the SDSS passbands are well represented by a power-law continuum with αν = -0.5 (fν ∝ να) and are close to those predicted by previous simulations. However, the contributions of the small blue (or λ3000) bump and other strong emission lines have a significant effect upon the colors. The color-redshift relation exhibits considerable structure, which may be of use in determining photometric redshifts for quasars from their colors alone. The range of colors at a given redshift can generally be accounted for by a range in the optical spectral index with a distribution αν = -0.5 ± 0.65 (95% confidence), but there is a red tail in the distribution. This tail may be a sign of internal reddening, especially since fainter objects at a given redshift tend to exhibit redder colors than the average. Finally, we show that there is a continuum of properties between quasars and Seyfert galaxies, and we test the validity of the traditional dividing line (MB = -23) between the two classes of active galactic nuclei.

The Optical–Near‐Infrared Spectrum of the M87 Jet fromHubble Space TelescopeObservations

We present 1998 Hubble Space Telescope observations of M87 that yield the first single-epoch optical and radio-optical spectral index image of the jet at 015 resolution. We find αro ≈ 0.67, comparable to previous measurements, and αo ≈ 0.9 (Fν ∝ ν-α), slightly flatter than previous workers. Reasons for this discrepancy are discussed. These observations reveal a large variety of spectral slopes. Bright knots exhibit significantly flatter spectra than interknot regions. The flattest spectra (αo ~ 0.5-0.6, comparable to or flatter than αro) are found in the two inner jet knots (D-East and HST-1), which contain the fastest superluminal components. The flux maximum regions of other knots have αo ~ 0.7-0.9. The maps of αo and αro appear poorly correlated. In knots A, B, and C, αo and αro are essentially anticorrelated with one another. Near the flux maxima of two inner jet knots (HST-1 and F), changes in αro appear to lag changes in αo, but in two other knots (D and E), the opposite relationship is observed. This is further evidence that the radio and optical emissions of the M87 jet come from substantially different physical regions. The delays observed in the inner jet are consistent with localized particle acceleration in the knots, with tacc tcool for optically emitting electrons in knots HST-1 and F, and tacc ~ tcool for optically emitting electrons in knots D and E. Synchrotron models fit to the radio-optical data yield νB 1016 Hz for knots D, A, and B, and somewhat lower values, νB ~ 1015-1016 Hz, in other regions of the jet. If the X-ray emissions from knots A, B, and D are cospatial with the optical and radio emission, we can strongly rule out the continuous injection model, which overpredicts the X-ray emissions by large factors. Because of the short lifetimes of X-ray synchrotron-emitting particles, the X-ray emission likely traces sites of particle acceleration and fills volumes much smaller than the optical emission regions.

Quasar Variability in the Framework of Poissonian Models

In this paper we review the basic Poissonian formulation of quasar variability, using it as a mathematical tool to extract relevant parameters such as the energy, rate and lifetimes of the flares through the analysis of observed light curves. It is shown that in this very general framework the well established anti-correlation between variability amplitude and wavelength can only be understood as an effect of an underlying spectral component which remains stable on long time-scales, and is redder than the variable component. The formalism is applied to the B and R light curves of 42 PG quasars collected by the Wise Observatory group. Variability indices for these data are obtained with a Structure Function analysis. The mean number of living flares is constrained to be in the range between 5 and 100, while their rates are found to be of order 1--100 per yr. Monochromatic optical flare energies of 10^{46-48} erg/A and life-times of 0.5 to 3 yr are derived. Lower limits of typically 25% are established for the contribution of a non-variable component in the R band. The substantial diversity in these properties among quasars invalidates simple versions of the Poissonian model in which flare energies, lifetimes and the background contribution are treated as universal invariants. The good correlation between the EW(H_beta) and the long term variability amplitude is interpreted in a scenario where only the variable component participates in the ionization of the line emitting gas. This idea is consistent with the observed trends of the variability amplitude with lambda, EW(HeII) and the X-ray to optical spectral index (abridged).

Infrared photometry of z 1 3C quasars

We present JHKL' photometry of a complete sample of steep-spectrum radio-loud quasars from the revised 3CR catalogue in the redshift range 0.65 < z < 1.20. After correcting for contributions from emission lines and the host galaxies, we investigate their spectral energy distributions (SEDs) around 1 micron. About 75% of the quasars are tightly grouped in the plane of optical spectral index, alpha_lo, versus near-infrared spectral index, alpha_hi, with the median value of alpha_lo close to the canonical value, and the median alpha_hi slightly flatter. We conclude that the fraction of moderately-obscured, red quasars decreases with increasing radio power, in accordance with the `receding torus' model which can also explain the relatively flat median near-infrared spectra of the 3CR quasars. Two of the red quasars have inverted infrared spectral indices, and we suggest that their unusual SEDs might result from a combination of dust-scattered and transmitted quasar light.

Visible and Near‐Infrared Spectroscopy of Seyfert 1 and Narrow‐Line Seyfert 1 Galaxies

This paper studies the continuum and emission-line properties of a sample composed of 16 normal Seyfert 1 and seven narrow-line Seyfert 1 (NLS1) galaxies using optical and near-IR CCD spectroscopy. The continuum emission of the galaxies can be described in terms of a combination of stellar population, a nonstellar continuum of power-law form, and Fe II emission. A significative difference in the optical spectral index between NLS1's and normal Seyfert 1's is observed; the latter is steeper. Most NLS1's show Fe II/Hβ ratios larger than those observed in the other Seyfert 1's. In the IRAS band, both groups of galaxies have very similar properties. We have searched for the presence of optically thin gas in the broad-line region (BLR) of the galaxies by comparing the broad O I λ8446 and Hα emission-line profiles. Our analysis show that in the NLS1's, both profiles are similar in shape and width. This result contradicts the hypothesis of thin gas emission in the high-velocity part of the BLR to explain the narrowness of broad optical permitted lines in these objects. Evidence of narrow O I λ8446 emission is found in six galaxies of our sample, implying that this line is not restricted to a pure BLR phenomenon. In the narrow-line region, we find similar luminosities in the permitted and high-ionization lines of NLS1's and normal Seyfert 1's. However, low-ionization lines such as [O I] λ6300, [O II] λ3727, and [S II] λλ6717, 6731 are intrinsically less luminous in NLS1's. Physical properties derived from density- and temperature-sensitive line ratios suggest that the [O II] and [S II] emitting zones are overlapping in normal Seyfert 1's and separated in NLS1's.

Multi-colour optical monitoring of the intra-day variable blazar S5 0716+71

We report multi-colour optical monitoring of the intra-day variable blazar S5 0716+71 with a dense temporal coverage over a total duration of 4 weeks in February/March 1994. Four lters (BV Johnson and RI Cousins) were used on a majority of the nights. The light curves of the blazar were derived relative to two compar- ison stars present on the CCD frames. Intensity varia- tions of 0.05 mag from night-to-night were observed in half the cases and a good correlation between the light curves in the dierent passbands was noticed (as also for the observed intra-night variations). There seems to be no correlation between flux and optical spectral index of the blazar. In all, three prominent events of intra-night variability were detected, each with a rate of 0.1 mag per hour, lasting for at least 3 hours. Each of these flaring segments of the blazar light curves can be well tted with an exponential intensity prole whose rate of variation is essentially the same for all the passbands. No clear event of ultra-rapid fluctuations of the blazar on time scale 1 hour was detected in these multi-colour data. Present ob- servations have been carried out during a low brightness level of the blazar and they have no large amplitude vari- able events.

Discovery of an Optical Synchrotron Jet in 3C 15

Observations with the Faint Object Camera on board the Hubble Space Telescope have revealed a new optical jet in the core of the elliptical galaxy NGC 3862 (3C 264). Morphologically, this jet is similar to the synchrotron jets seen in other galaxies, as it shows knots and bifurcations. The optical spectral index is also similar to that found in other jets. Thus, the nucleus of NGC 3862 appears to contain the fifth known example of an optical synchrotron jet. Since NGC 3862 is a typical radio-loud elliptical galaxy, it seems likely that many nonthermal jets found in the radio continuum may also have optical counterparts.

Analysis of Ly absorption lines in the vicinity of QSOs

We have compiled, from the literature, a sample of Ly ${\alpha}$ forest lines in the spectra of 69 QSOs, all observed with a resolution between 60 to 100 km s$^{-1}$. The sample is studied for proximity effect. We have tried to account for the effect of blending which is inherent in the intermediate resolution sample, by calculating the column density distribution, using an effective velocity dispersion parameter, from the observed equivalent width distribution. The use of this column density distribution in the proximity effect analysis reduces the background intensity values by a factor of 2 to 3 compared to the values obtained by using the column density distribution obtained from high resolution observations. Evidence is presented for a weak correlation between the effective velocity dispersion parameter and equivalent width. Such a correlation if present can increase the background values by a factor of up to 1.5. Considerations of proximity in the spectra of 16 QSOs, from our sample, exhibiting damped Ly ${\alpha}$ lines gives a background intensity which is 3 times smaller than the values obtained from the whole sample, confirming the presence of dust in the damped Ly $\alpha$ systems. Lines close to the QSOs are shown to be marginally stronger and broader compared to lines away from the QSOs. The Ly $\alpha$ lines with absorption redshift larger than emission redshift are shown to be uncorrelated with QSO luminosity, radio loudness or optical spectral index. These lines occur more frequently at high redshifts. Their presence is correlated weakly with the presence of associated metal line systems. The possibilities that the QSO emission redshift is considerably higher and that either the Ly $\alpha$ clouds or the QSOs have peculiar velocities are considered. It is argued that a combination of both these

3C 65: old galaxy or buried quasar?

3C 65 is one of the reddest known radio galaxies at z ≈ 1. We have taken optical and near-infrared spectra, and a K-band image in an attempt to assess whether this red colour is produced by an underlying old stellar population. We find that the broad- and intermediate-band fluxes are consistent with a power-law spectrum in the optical/near-infrared, with no evidence for a 4000-Å break, showing that the K-band light is dominated by a reddened quasar nucleus. By comparing the optical spectral index with the mean for optically selected quasars, we estimate an AV ≈ 2 towards the nucleus and an unreddened quasar magnitude of V ≈ 19. Our spectra, however, do indicate the presence of a 4000-Å break in the off-nuclear light, consistent with either a formation redshift zf > 4 for a stellar population with a standard IMF, or the presence of young supergiant stars, which we have previously argued are present in the off-nuclear knot of 3C 356.

The properties of X-ray selected active galactic nuclei. 3: The radio-quiet versus radio-loud samples

view Abstract Citations (68) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Properties of X-Ray-selected Active Galactic Nuclei. III. The Radio-quiet versus Radio-loud Samples della Ceca, Roberto ; Lamorani, Giovanni ; Maccacaro, Tommaso ; Wolter, Anna ; Griffiths, Richard ; Stocke, John T. ; Setti, Giancarlo Abstract Using a sample of 406 active galactic nuclei (AGNs), extracted from the Einstein Observatory Extended Medium Sensitivity Survey (EMSS), for which complete radio information exists, we present and discuss some properties of the radio-loud and radio-quiet objects, and compare them with those of radio-observed, optically selected AGNs. The radio to optical spectral index of the EMSS AGN sample shows a bimodal distribution with a minimum at α_ro_~0.35. A bimodal distribution is observed also in samples of optically selected AGNs, making this characteristic a general property of the total AGN population. This feature allows us to split, almost unambiguously, the EMSS AGN sample into radio-loud (RL: α_ro_ > 0.35: 43 objects) and radio-quiet (RQ: α_ro_ < 0.35:363 objects) objects. An abrupt change in the radio properties of the X-ray selected AGN population is observed at M_B_ ~ -24: in the optically bright subsample (30 +/- 4)% of the objects are RL, while in the optically faint subsample (3 +/- 1)% of the objects are RL. This difference is statistically confirmed at a high confidence level by the comparison of the α_ro\_ distributions of the two subsamples. These results have been compared with those obtained from a compilation of optically selected AGNs. The G(> α_ro_) distributions for optically and X-ray selected optically faint (M_B_ > - 24) AGN are not statistically different. On the contrary, the same distributions for optically bright AGNs are statistically different at a high confidence level. As expected from previous results, the fraction of RL AGNs among optically bright X-ray selected AGNs is ~2.5 times larger than among the optically selected ones. We have determined, for the first time, the X-ray luminosity functions (XLF) at different redshifts, the cosmological evolution and the devolved XLF of radio-quiet and radio-loud objects separately. There is no evidence that the evolutionary properties of radio-loud objects differ significantly from those of radio-quiet ones: the best-fit evolution parameter of the RL AGNs is higher than, but consistent within errors with, that of the RQ AGNs. On the contrary, the shape of the devolved XLF of the two classes appears to be different and a flattening of the XLF of the RL subsample is visible for L_x_(z = 0) <~ 3 x 10^44^ ergs s^-1^. Similar results were obtained, in the optical domain, from an analysis of the radio properties of the PG sample, the only sizeable complete sample of optically selected AGNs with full radio information. This measured difference adds to the already significant number of reasons for postulating the two distinct classes of AGNs: RL and RQ. Using the devolved XLF and the best-fit evolution parameters for the two classes of AGN, we have computed the expected fraction of RL objects as a function of the X-ray flux: this fraction ranges from ~9% for f_x_ ~5 x 10^12^ ergs cm^-2^ s^-1^, to ~13% for f_x_~2 x 10^-13^ ergs cm^-2^ s^-1^, and then decreases to ~2.5% for f_x_~ 2 x 10^-15^ ergs cm^-2^ s^- 1^. Recent deep radio observations of a deep ROSAT field seem to be in agreement with our calculations. Publication: The Astrophysical Journal Pub Date: August 1994 DOI: 10.1086/174428 Bibcode: 1994ApJ...430..533D Keywords: Active Galactic Nuclei; Cosmic X Rays; Galactic Evolution; Galactic Radio Waves; Radio Emission; Astronomical Models; Continuums; Cosmology; Luminosity; Red Shift; Astrophysics; GALAXIES: ACTIVE; GALAXIES: EVOLUTION; GALAXIES: LUMINOSITY FUNCTION; MASS FUNCTION; GALAXIES: NUCLEI; RADIO CONTINUUM: GALAXIES; X-RAYS: GALAXIES full text sources ADS | data products SIMBAD (11) Related Materials (3) Part 1: 1991ApJ...374..117M Part 2: 1992ApJ...389..491D Part 4: 1996ApJ...465..650D

Planetary Camera observations of the M87 stellar cusp

Analysis of V and I band HST Planetary Camera images of the giant elliptical galaxy M87 show that its central starlight distribution is consistent with the black hole M_• = 2.6 X 10^9 M_☉ cusp model proposed for M87 by Young et al. [ApJ, 221, 721 (1978)]. A combined approach of image deconvolution and modeling is used to investigate the starlight distribution into limiting radii of ≈0.04 (3 pc at 16 Mpc). The central structure of M87 can be described by three components: a power-law starlight profile of the form µ(r)∝a:r^(-1/4) for r<3, a central nonthermal point source, and optical counterparts of the jet knots N1 and M identified by VLBI observations. M87 lacks a constant surface brightness core, and its central starlight luminosity density exceeds 10^3L_☉ pc^(-3) (I band) for r< 10 pc. The profile strongly resembles a stellar cusp associated with a massive black hole. A review of existing velocity dispersion observations suggests that the Young et al. black hole mass can be accommodated to the observations with minor adjustments of dynamical models. The central luminosity spike itself remains unresolved at HST resolution, with r_c < pc. The spike has optical spectral index ɑ= -0.46 ± 0.20 and is at least as blue if not bluer than the rest of the M87 jet. The total nonthermal flux in the inner 1 of M87 agrees well with the central radio flux and the radio-optical spectral index of the rest of the jet. It is also consistent with the spectral-line dilution seen by Dressler & Richstone [ApJ, 348, 120 ( 1990)]; we thus argue that the spike is completely nonthermal.

The elusive optical jets

Imaging observations in the U band of eight radio galaxies are presented. We find no optical counterpart to the radio jets. For the three radio galaxies 3C 147, 3C 279 and 3C 433, we show that the radio to optical spectral index of the jet is significantly higher than the typical values found in the three best known optical jets (M87, 3C 273 and 3C 66B). We conclude that the cut-off frequencies are lower than $10^{14}$Hz in these cases. For the 3C 31 jet, our data are consistent with the radio to optical spectral index being comparable to the typical values. This result is in contradiction with the detection of the optical jet in the B band by Butcher et al (1980). Finally, the lower limit on the radio to optical spectral index we obtain for the four other radio jets of our sample is still consistent with the typical values.

Comparison of Radio and Optical Polarization in Extended Extragalactic Radio Sources: I. Observations

We presented new observations in various wavebands of those extragalactic radio sources for which the emission of optical synchrotron light is established by polarimetric investigations (jets of M 87 and 3C 273, hot spots in the lobes of 3C 20, 3C 33 and Pictor A). In addition to optical spectral index and polarization maps, we collected flux measurements at radio, millimetre, and near-infrared frequencies. Our observations result in accurate synchrotron spectra between 109 and 1015 Hz. For some sources a detailed comparison between radio and optical polarization is possible.

Radio to optical spectral index variations along the M87 jet

CCD photometry and MERLIN observations of M87 (Virgo A) have been used to study the variations of the radio to optical spectral index along the jet. In general, the radio spectrum is flatter than the optical one and both become steeper with distance from the nucleus. The optical fluxes of the bright knots lie at or below the extrapolation of the radio spectrum. These properties can be explained by power-law spectra at low frequencies that turn down gradually beyond a cut-off frequency. The location of the local cut-off frequency is estimated by fitting the measured fluxes to the theoretical spectrum of a truncated power-law electron energy distribution. The results are interpreted in terms of models of synchrotron emission from shock waves in extragalactic jets.

X-ray properties of optically selected QSOs

The dependence of the X-ray-to-optical luminosity ratio on optical luminosity and redshift for optically selected QSOs is studied, largely on the basis of two, complete, magnitude-limited samples (Bright Quasar Survey /1983/ and Braccesi Faint /1984/) which were observed with the Einstein Observatory. Heterogeneous samples are established as adequate for the study of that dependence. Optimal choices for increasing the size of the data set for such a study are pointed out. The previous results of Avni and Tananbaum for alpha sub 0, x(z, L sub opt) are confirmed and strengthened, and the numerical sensitivity to changes in the values of q sub 0 and of the optical spectral index is evaluated. It is shown that the large majority, probably all, of optically selected QSOs are X-ray loud; no more than a few percent can be X-ray quiet. Thus X-ray emission appears to be a universal property of QSOs. It is shown that comparisons of optically selected QSOs with X-ray selected QSOs are numerically sensitive to the details of the input ingredients. A residual discrepancy of about a factor of 2 between calculated and observed X-ray number counts is found. Directions for further research that are important for understanding the full bivariate optical-X-ray evolution and luminosity function for QSOs are discussed.

Four-point optical energy distributions for faint BL Lacertae objects

view Abstract Citations (59) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Four-point optical energy distributions for faint BL Lacertae objects. Tapia, S. ; Craine, E. R. ; Johnson, K. Abstract Broad-band photoelectric observations of 10 radio sources with continuous optical spectra have been reduced to monochromatic flux densities in order to study the shape and variations of their optical energy distributions. The observed spectral indices range between 1 and 3. It is suggested that sources with a large optical spectral index may have detectable nebulosity. Publication: The Astrophysical Journal Pub Date: January 1976 DOI: 10.1086/154078 Bibcode: 1976ApJ...203..291T Keywords: Astronomical Photometry; Astronomical Spectroscopy; Quasars; Radiant Flux Density; Radio Sources (Astronomy); Spectral Energy Distribution; Ubv Spectra; Line Shape; Luminous Intensity; Monochromatic Radiation; Spectrophotometry; Spectrum Analysis; Tables (Data); Astronomy full text sources ADS | data products NED (12)