- ID:
- ivo://CDS.VizieR/J/ApJS/247/16
- Title:
- Extreme high-frequency-peaked BL Lac objects
- Short Name:
- J/ApJS/247/16
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Extreme high-frequency-peaked BL Lac objects (EHBLs) are blazars that exhibit extremely energetic synchrotron emission. They also feature nonthermal gamma-ray emission whose peak lies in the very high-energy (VHE, E>100GeV) range, and in some sources exceeds 1TeV: this is the case for hard-TeV EHBLs such as 1ES 0229+200. With the aim of increasing the EHBL population, 10 targets were observed with the MAGIC telescopes from 2010 to 2017, for a total of 265hr of good-quality data. The data were complemented by coordinated Swift observations. The X-ray data analysis confirms that all but two sources are EHBLs. The sources show only a modest variability and a harder-when-brighter behavior, typical for this class of objects. At VHE gamma-rays, three new sources were detected and a hint of a signal was found for another new source. In each case, the intrinsic spectrum is compatible with the hypothesis of a hard-TeV nature of these EHBLs. The broadband spectral energy distributions (SEDs) of all sources are built and modeled in the framework of a single-zone, purely leptonic model. The VHE gamma-ray-detected sources were also interpreted with a spine-layer model and a proton synchrotron model. The three models provide a good description of the SEDs. However, the resulting parameters differ substantially in the three scenarios, in particular the magnetization parameter. This work presents the first mini catalog of VHE gamma-ray and multiwavelength observations of EHBLs.
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Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/589/A1
- Title:
- Extreme Horizontal Branch stars in {omega} Cen
- Short Name:
- J/A+A/589/A1
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of an extensive survey for rapid pulsators among Extreme Horizontal Branch (EHB) stars in {omega} Cen. The observations performed consist of nearly 100 hours of time-series photometry for several off-centre fields of the cluster, as well as low-resolution spectroscopy for a partially overlapping sample. We obtained photometry for some 300 EHB stars, for aro und half of which we are able to recover light curves of sufficient quality to either detect or place meaningful non-detection limits for rapid pulsations. Based on the spectroscopy, we derive reliable values of logg, Teff and logN(He)/N(H) for 38 targets, as well as good estimates of the effective temperature for another nine targets, whose spectra are slightly polluted by a close neighbour in the image. The survey uncovered a total of five rapid variables with multi-periodic oscillations between 85 and 125s. Spectroscopically, they form a homogeneous group of hydrogen-rich subdwarf O stars clustered between 48000 and 54000K. For each of the variables we are able to measure between two and three significant pulsations believed to constitute independent harmonic oscillations. However, the interpretation of the Fourier spectra is not straightforward due to significant fine structure attributed to strong amplitude variations. In addition to the rapid variables, we found an EHB star with an apparently periodic luminosity variation of ~2700s, which we tentatively suggest may be caused by ellipsoidal variations in a close binary.
- ID:
- ivo://CDS.VizieR/J/MNRAS/436/718
- Title:
- Extremely faint planetary nebulae in SDSS
- Short Name:
- J/MNRAS/436/718
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using ~1700000 target- and sky-fibre spectra from the Sloan Digital Sky Survey (SDSS), we have carried out a systematic search for Galactic planetary nebulae (PNe) via detections of the [OIII] {lambda}{lambda}4959, 5007 lines. Thanks to the excellent sensitivity of the SDSS spectroscopic surveys, this is by far the deepest search for PNe ever taken, reaching a surface brightness of the [OIII] {lambda}5007 line down to about 29.0mag/arcsec^2^. The search leads to the recovery of 13 previously known PNe in the Northern and Southern Galactic Caps. In total, 44 new PN candidates are identified, including seven candidates of multiple detections and 37 candidates of single detection. The seven candidates of multiple detections are all extremely large (between 21 and 154arcmin) and faint, located mostly in the low Galactic latitude region and with a kinematics similar to disc stars. After checking their images in H{alpha} and other bands, three of them are probably HII regions, one is probably associated with a new supernova remnant, another one is possibly a true PN and the remaining two could be either PNe or supernova remnants. Based on sky positions and kinematics, seven candidates of single detection probably belong to the halo population. If confirmed, they will increase the number of known PNe in the Galactic halo significantly. All the newly identified PN candidates are very faint, with a surface brightness of the [OIII] {lambda}5007 line between 27.0-30.0mag/arcsec^2^, and very challenging to be discovered with previously employed techniques (e.g. slitless spectroscopy, narrow-band imaging), and thus may greatly increase the number of `missing' faint PNe. Our results demonstrate the power of large-scale fibre spectroscopy in hunting for ultrafaint PNe and other types of emission line nebulae. Combining the large spectral data bases provided by the SDSS and other on-going projects (e.g. the LAMOST Galactic surveys), it is possible to build a statistically meaningful sample of ultrafaint, large, evolved PNe, thus, improving the census of Galactic PNe.
- ID:
- ivo://CDS.VizieR/J/A+A/632/A101
- Title:
- Extremely high-velocity jets in Serpens
- Short Name:
- J/A+A/632/A101
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- ALMA observations of four molecular transitions, CO 2-1, SiO 5-4, H_2_CO 3_03_-2_02_ in Band 6 (ALMA project 2013.1.00726.S; PI: C. Hull) and HCN 1-0 observed in Band 3 (ALMA project 2016.1.00710.S; PI: C. Hull) are presented. The synthesized beam of the observations is between 0.3 and 0.6 arcsec, corresponding to 130-260au at the distance to Serpens Main. The largest recoverable scale in the data is 5 arcsec and 12 arcsec (2150 and 4960au) for Band 3 and Band 6, respectively. The spectral resolution of the observations differs between the spectral windows, ranging from 0.04 to 0.3km/s. For both bands, only 12-m array data were used. The Band 6 data were obtained in two configurations (C43-1 and C43-4 with resolutions of 1.1 and 0.3 arcsec, respectively, and the final images are produced from the combined datasets.
- ID:
- ivo://CDS.VizieR/J/MNRAS/469/347
- Title:
- Extremely Isolated Galaxies. II.
- Short Name:
- J/MNRAS/469/347
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have selected a sample of 41 extremely isolated galaxies (EIGs) from the local Universe using both optical and HI Arecibo Legacy Fast ALFA survey redshifts. Narrow-band H{alpha} and wide-band imaging along with public data were used to derive star formation rates (SFRs), star formation histories and morphological classifications for the EIGs. We have found that the extreme isolation of the EIGs does not affect considerably their star formation compared to field galaxies. EIGs are typically 'blue cloud' galaxies that fit the 'main sequence of star-forming galaxies' and may show asymmetric star formation and strong compact star-forming regions. We discovered surprising environmental dependences of the HI content, M_HI_, and of the morphological type of EIGs; the most isolated galaxies (of subsample EIG-1) have lower M_HI_ on average (with 2.5{sigma} confidence) and a higher tendency to be early types (with 0.94 confidence) compared to the less isolated galaxies of subsample EIG-2. To the best of our knowledge, this is the first study that finds an effect in which an isolated sample shows a higher fraction of early types compared to a less isolated sample. Both early-type and late-type EIGs follow the same colour-to-M_*_, SFR-to-M_*_ (main sequence) and M_HI_-to-M_*_ relations. This indicates that the mechanisms and factors governing star formation, colour and the M_HI_-to-M_*_ relation are similar in early-type and late-type EIGs, and that the morphological type of EIGs is not governed by their M_HI_ content, colour or SFR.
- ID:
- ivo://CDS.VizieR/J/MNRAS/475/2480
- Title:
- Extremely-low mass white dwarf star
- Short Name:
- J/MNRAS/475/2480
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The so-called sdA stars are defined by having H-rich spectra and surface gravities similar to hot subdwarf stars, but effective temperature below the zero-age horizontal branch. Their evolutionary history is an enigma: their surface gravity is too high for main-sequence stars, but too low for single evolution white dwarfs. They are most likely byproducts of binary evolution, including blue-stragglers, extremely-low mass white dwarf stars (ELMs) and their precursors (pre-ELMs). A small number of ELMs with similar properties to sdAs is known. Other possibilities include metal-poor A/F dwarfs, second generation stars, or even stars accreted from dwarf galaxies. In this work, we analyse colours, proper motions, and spacial velocities of a sample of sdAs from the Sloan Digital Sky Survey to assess their nature and evolutionary origin. We define a probability of belonging to the main sequence and a probability of being a (pre-)ELM based on these properties. We find that 7 per cent of the sdAs are more likely to be (pre-)ELMs than main-sequence stars. However, the spacial velocity distribution suggests that over 35 per cent of them cannot be explained as single metal-poor A/F stars.
- ID:
- ivo://nasa.heasarc/vlulxcat
- Title:
- Extremely Luminous X-Ray Source Candidates Catalog
- Short Name:
- VLULXCAT
- Date:
- 27 Sep 2024
- Publisher:
- NASA/GSFC HEASARC
- Description:
- Using Chandra archive data, the authors conducted a thorough survey of luminous X-ray sources. They directly analyzed about 9400 Chandra ACIS observations and cross-correlated the detected X-ray sources with 77,000 galaxies within a distance of 250 Mpc. The final catalog includes 119 unique luminous X-ray source candidates with L<sub>X</sub> > 3 x 10<sup>40</sup> erg/s from 93 galaxies or 41 HLX candidates with L<sub>X</sub> > 1 x 10<sup>41</sup> erg/s from 35 galaxies. The authors derive a moderate contamination rate due to foreground or background sources. In the reference paper, they also cross-correlate the catalog with FIRST, perform variability and periodicity tests, and analyze one HLX candidate in particular. This catalog could be a starting point to perform follow-up observations. In order to know whether an X-ray source falls within a particular galaxy, for each galaxy, the authors collected its center's RA, Dec, distance, and D<sub>25</sub> isophotal info, which includes major axis length, minor axis length, and the position angle of the major axis from the PGC2003 Catalog (Paturel et al. 2003, A&A, 412, 45), which includes the full RC3 catalog and has all of the necessary parameters except for distance. The authors restricted the minimum major axis length to be 10 arcseconds, and collected their distances from NED as much as possible. Their final sample includes 77,000 galaxies within 250 Mpc. The authors used all of the Chandra ACIS data in TE mode that were released before 2014, which includes 9400 ObsIDs. A roughly linear relation between the flux and count rate derived by PIMMS 4.6b was established assuming a power-law spectral shape and galactic foreground extinction (Kalberla et al. 2005, A&A, 440, 775). Any source with a PIMMS luminosity larger than 5 x 10<sup>39</sup> erg s<sup>-1</sup> would be recalculated by the CIAO script model flux assuming a power-law index of 1.7 in the 0.3 - 8.0 keV energy band. After the recalculation, 1,809 X-ray sources with L<sub>x</sub> > 3 x 10<sup>40</sup> erg s<sup>-1</sup> falling within 640 D<sub>25</sub> contours covered by 905 ObsIDs were picked out. A large fraction of the 1,809 sources are galactic nuclei and some of them are repeated. Only off-nuclear sources are considered in this paper. In addition, the centers of the galaxies given by PGC2003 are not necessarily precise and the specific environments of the 1,809 sources are different. Therefore, the authors visually checked the Chandra and DSS images simultaneously, since two-band inspection can help to exclude the nuclear sources, bright knots, and extended sources. X-ray sources with clear DSS features would be dropped because, for a source with a visual magnitude <20 and a distance >30 Mpc, its absolute magnitude would be brighter than -12.4, which is beyond the limit of the brightest star clusters. This table was created by the HEASARC in February 2017 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/J/ApJS/222/12">CDS Catalog J/ApJS/222/12</a> file table1.dat, the list of very luminous X-ray source candidates found within the D<sub>25</sub> ellipses of Chandra ACIS-observed PGC2003 galaxies lying within 250 Mpc. Some of the values for the name parameter in the HEASARC's implementation of this table were corrected in April 2018. This is a service provided by NASA HEASARC .
- ID:
- ivo://CDS.VizieR/J/ApJ/819/110
- Title:
- Extremely metal-poor galaxies in SDSS. II.
- Short Name:
- J/ApJ/819/110
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Extremely metal-poor (XMP) galaxies are defined to have a gas-phase metallicity smaller than a tenth of the solar value (12+log[O/H]<7.69). They are uncommon, chemically and possibly dynamically primitive, with physical conditions characteristic of earlier phases of the universe. We search for new XMPs in the Sloan Digital Sky Survey (SDSS) in a work that complements Paper I (Morales-Luis+, 2011, J/ApJ/743/77). This time, high electron temperature objects are selected; metals are a main coolant of the gas, so metal-poor objects contain high-temperature gas. Using the algorithm k-means, we classify 788677 spectra to select 1281 galaxies that have particularly intense [OIII]{lambda}4363 with respect to [OIII]{lambda}5007, which is a proxy for high electron temperature. The metallicity of these candidates was computed using a hybrid technique consistent with the direct method, rendering 196 XMPs. A less restrictive noise constraint provides a larger set with 332 candidates. Both lists are provided in electronic format. The selected XMP sample has a mean stellar mass around 10^8^M_{sun}_, with the dust mass ~10^3^M_{sun}_ for typical star-forming regions. In agreement with previous findings, XMPs show a tendency to be tadpole-like or cometary. Their underlying stellar continuum corresponds to a fairly young stellar population (<1Gyr), although young and aged stellar populations coexist at the low-metallicity starbursts. About 10% of the XMPs show large N/O. Based on their location in constrained cosmological numerical simulations, XMPs have a strong tendency to appear in voids and to avoid galaxy clusters. The puzzling 2%-solar low-metallicity threshold exhibited by XMPs remains.
- ID:
- ivo://CDS.VizieR/J/AJ/128/2402
- Title:
- Extremely metal-poor star candidates abundances
- Short Name:
- J/AJ/128/2402
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present chemical abundances for 110 stars identified in objective-prism surveys as candidates for being very metal-poor. The abundances are derived from high-S/N, intermediate-resolution spectra obtained with the Keck Observatory Echellette Spectrograph and Imager (ESI). An additional 25 stars with well-determined abundances ranging from [Fe/H]=-1.5 to -3.2 were observed and the results used to help calibrate our analysis and determine the accuracy of our abundance determinations. Abundances for the program stars were measured for Fe, Mg, Ca, Ti, Cr, and Ba with an accuracy of approximately 0.3dex. Fifty-three of the stars in our sample have [Fe/H]<=-2, 22 have [Fe/H]<=-2.5, and 13 have [Fe/H]<=-2.9. Surprisingly, approximately one-third of the sample is relatively metal-rich, with [Fe/H]>-1.5. In addition to identifying a number of extremely metal-poor stars, this study also shows that moderate-resolution spectra obtained with the Keck ESI yield relatively accurate abundances for stars as faint as V=14 with modest exposure time (~20 minutes). This capability will prove useful if the so-far elusive stars at [Fe/H]<-4 turn out to be mostly fainter than V=15.
- ID:
- ivo://CDS.VizieR/J/MNRAS/434/1681
- Title:
- Extremely metal-poor stars CaII triplet
- Short Name:
- J/MNRAS/434/1681
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We extend our previous calibration of the infrared CaII triplet (CaT) as a metallicity indicator to the metal-poor regime by including observations of 55 field stars with [Fe/H] down to -4.0dex. While we previously solved the saturation at high metallicity using a combination of a Lorentzian and a Gaussian to reproduce the line profiles, in this paper we address the non-linearity at low metallicity following the suggestion of Starkenburg et al. of adding two non-linear terms to the relation among the [Fe/H], luminosity and strength of the calcium triplet lines. Our calibration thus extends from -4.0 to +0.5 in metallicity and is presented using four different luminosity indicators: V-V_HB_, M_V_, M_I_ and M_K_. The calibration obtained in this paper results in a tight correlation between [Fe/H] abundances measured from high-resolution spectra and [Fe/H] values derived from the CaT, over the whole metallicity range covered.