arXiv subjects

N. Mirabal

Publications and source records attributed to N. Mirabal.

67 records · Page 4Linked to original sources

arXiv2003

Search for a Point-Source Counterpart of the Unidentified Gamma-Ray Source TeV J2032+4130 in Cygnus

We have made a multiwavelength study of the overlapping error boxes of the unidentified gamma-ray sources TeV J2032+4130 and 3EG J2033+4118 in the direction of the Cygnus OB2 association (d = 1.7 kpc) in order to search for a point-source counterpart of the first unidentified TeV source. Optical identifications and spectroscopic classifications for the brighter X-ray sources in ROSAT PSPC and Chandra ACIS images are obtained, without finding a compelling counterpart. The classified X-ray sources are a mix of early and late-type stars, with one exception. The brightest source in the Chandra observation is a new, hard absorbed source that is both transient and rapidly variable. It lies 7' from the centroid of the TeV emission, which places it outside of the claimed 2 sigma location (r = 4.8'). A possible eclipse or "dip" transition is seen in its light curve. With a peak 1-10 keV luminosity of 7 x 10^(32) (d/1.7 kpc})^2 ergs s^(-1), this source could be a quiescent low-mass X-ray binary that lies beyond the Cyg OB2 association. A coincident, reddened optical object of R = 20.4, J = 15.4, H = 14.2, and K = 13.4 is observed, but not yet classified due to the lack of obvious emission or absorption features in its spectrum. Alternatively, this Chandra} and optical source might be a considered a candidate for a proton blazar, a long hypothesized type of radio-weak gamma-ray source. More detailed observations will be needed to determine the nature of this variable X-ray source, and to assess the possibility of its connection with TeV J2032+4130.

astro-ph↗

arXiv2002

High Resolution Grating Spectroscopy of GRB 020405 with CHANDRA LETGS

We present high-resolution X-ray spectroscopy of GRB 020405 obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board the Chandra X-ray Observatory starting 1.68 days after the burst. The spectrum appears featureless, with no evidence for emission lines, absorption edges, or narrow radiative recombination continua. The continuum can be fitted by a power law of photon index Gamma = 1.72 +/- 0.21 and temporal decay index alpha = 1.87 +/- 0.1, with a marginally significant excess column density of cold gas N_{H}=(4.7 +/- 3.7) x 10^{21} cm^{-2} at the redshift of the host galaxy. The absence of iron lines indicates that the density of nearby surrounding material was unlikely to be very dense (n < 5 x 10^{12} cm^{-3}) at the time of the Chandra observation. In the case of recombination following photoionization in an optically thin medium, most ionic species would be completely stripped at lower gas densities than this. In the case of a power-law spectrum reflecting off a ``cold'', opaque medium of low density, negligible emission features would be produced. Alternative to these possible explanations for the lack of emission features, any X-ray line emission taking place in a dense medium in a ``nearby reprocessor'' scenario might have been overwhelmed by the bright afterglow continuum. Although the absence of discrete features does not unambiguously test for a connection between GRB 020405 and nucleosynthesis, it emphasizes the need for high-resolution X-ray spectroscopy to determine the exact emission mechanism responsible for the reported discrete lines in other GRB afterglows.

astro-ph↗

arXiv2002

Time-Dependent Optical Spectroscopy of GRB 010222: Clues to the GRB Environment

We present sequential optical spectra of the afterglow of GRB 010222 obtained one day apart using the Low Resolution Imaging Spectrometer (LRIS) and the Echellette Spectrograph and Imager (ESI) on the Keck telescopes. Three low-ionization absorption systems are spectroscopically identified at z1=1.47688, z2=1.15628, and z3=0.92747. The higher resolution ESI spectrum reveals two distinct components in the highest redshift system at z1a=1.47590 and z1b=1.47688. We interpret the z1b=1.47688 system as an absorption feature of the disk of the host galaxy of GRB 010222. The best fitted power-law optical continuum and [Zn/Cr] ratio imply low dust content or a local gray dust component near the burst site. In addition, we do not detect strong signatures of vibrationally excited states of H_2. If the GRB took place in a superbubble or young stellar cluster, there are no outstanding signatures of an ionized absorber, either. Analysis of the spectral time dependence at low resolution shows no significant evidence for absorption-line variability. This lack of variability is confronted with time-dependent photoionization simulations designed to apply the observed flux from GRB 010222 to a variety of assumed atomic gas densities and cloud radii. The absence of time dependence in the absorption lines implies that high-density environments are disfavored. In particular, if the GRB environment was dust free, its density was unlikely to exceed nH=10^2 cm^-3. If depletion of metals onto dust is similar to Galactic values or less than solar abundances are present, then nH > 2 x 10^4 cm^-3 is probably ruled out in the immediate vicinity of the burst.

astro-ph↗

arXiv2002

Discovery of a 136 millisecond radio and X-ray pulsar in SNR G54.1+0.3

We report the discovery of a pulsar with period P = 136 ms and dispersion measure 308 pc/cc in a deep observation of the supernova remnant (SNR) G54.1+0.3 with the Arecibo radio telescope. Timing measurements of the new pulsar, J1930+1852, reveal a characteristic age of Pdt/2dP = 2900 yr and spin-down luminosity dE/dt = 1.2e37 erg/s. We have subsequently searched archival ASCA X-ray data of this SNR, and detect pulsations with a consistent period. These findings ensure that PSR J1930+1852 is the pulsar that powers the "Crab-like" SNR G54.1+.3. Together with existing Chandra observations of the SNR, we derive an X-ray pulsed fraction (2-10 keV) of ~27%. We also find that the cooling efficiency of the pulsar wind nebula (PWN) is intermediate between those of the Vela and Crab PWNe: Lx (2-10 keV) ~0.002 dE/dt. PSR J1930+1852 is a very weak radio source, with period-averaged flux density at 1180 MHz of 60 microJy. For a distance of 5 kpc, its luminosity, ~1 mJy kpc^2, is among the lowest for known young pulsars.

astro-ph↗

arXiv2002

Multiwavelength Searches of Unidentified EGRET Sources

3EG J1835+5918 is the brightest of the so-called unidentified EGRET sources at intermediate galactic latitude (l,b)=(89,25). We obtained complete radio, optical, and X-ray coverage of its error box, discovering a faint ultrasoft X-ray source in the ROSAT All-Sky Survey. Deep optical imaging at the location of this source, as pinpointed by an observation with the ROSAT HRI, reveals a blank field to a limit of V > 25.2. The corresponding lower limit on f_X/f_V is 300, which signifies that the X-ray source 3EG J1835+5918 is probably a thermally emitting neutron star. Here we report on recent Chandra and HST observations that strengthen this identification. 3EG J1835+5918 may thus become the prototype of an hypothesized population of older pulsars, born in the Gould belt, that can account for as many as 40 local EGRET sources. In addition to 3EG 1835+5918, we review the ongoing multiwalength effort by members of our group to study other unidentified EGRET sources using X-ray, optical, and radio data.

astro-ph↗

arXiv2002

The Next Geminga: Deep Multiwavelength Observations of a Neutron Star Identified with 3EG J1835+5918

We describe Chandra, HST, and radio observations that reveal a radio-quiet but magnetospherically active neutron star in the error circle of the high-energy gamma-ray source 3EG J1835+5918, the brightest of the unidentified EGRET sources at high Galactic latitude. A Chandra ACIS-S spectrum of the ultrasoft X-ray source RX J1836.2+5925, suggested by Mirabal & Halpern as the neutron star counterpart of 3EG J1835+5918, requires two components: a blackbody of T~3x10^5 K and a hard tail that can be parameterized as a power law of photon index Gamma~2. An upper limit of d < 800 pc can be derived from the blackbody fit under an assumption of R = 10 km. Deep optical imaging with the HST STIS CCD failed to detect this source to a limit of V > 28.5, thus f_X/f_V > 6000 and d > 250 pc assuming the X-ray fitted temperature for the full surface. Repeated observations with the 76 m Lovell telescope at Jodrell Bank place an upper limit of < 0.1 mJy on the flux density at 1400 MHz for a pulsar with P > 0.1 s, and < 0.25 mJy for a ~10 ms pulsar at the location of RX J1836.2+5925. All of this evidence points to an older, possibly more distant version of the highly efficient gamma-ray pulsar Geminga, as the origin of the gamma-rays from 3EG J1835+5918.

astro-ph↗

arXiv2002

Is the EGRET source 3EG J1621+8203 the radio galaxy NGC 6251?

We discuss the nature of the unidentified EGRET source 3EG J1621+8203. In an effort to identify the gamma-ray source, we have examined X-ray images of the field from ROSAT PSPC, ROSAT HRI, and ASCA GIS. Of the several faint X-ray point sources in the error circle of 3EG J1621+8203, most are stars or faint radio sources, unlikely to be counterparts to the EGRET source. The most notable object in the gamma-ray error box is the bright FR I radio galaxy NGC 6251. If 3EG J1621+8203 corresponds to NGC 6251, then it would be the second radio galaxy to be detected in high energy gamma rays, after Cen A, which provided the first clear evidence of the detection above 100 MeV of an AGN with a large-inclination jet. If the detection of more radio galaxies by EGRET has been limited by its threshold sensitivity, there exists the exciting possibility that new high energy gamma-ray instruments, with much higher sensitivity, will detect a larger number of radio galaxies in the future.

astro-ph↗

arXiv2002

The 2001 Superoutburst of WZ Sagittae

We report the results of a worldwide campaign to observe WZ Sagittae during its 2001 superoutburst. After a 23-year slumber at V=15.5, the star rose within 2 days to a peak brightness of 8.2, and showed a main eruption lasting 25 days. The return to quiescence was punctuated by 12 small eruptions, of ~1 mag amplitude and 2 day recurrence time; these "echo outbursts" are of uncertain origin, but somewhat resemble the normal outbursts of dwarf novae. After 52 days, the star began a slow decline to quiescence. Periodic waves in the light curve closely followed the pattern seen in the 1978 superoutburst: a strong orbital signal dominated the first 12 days, followed by a powerful /common superhump/ at 0.05721(5) d, 0.92(8)% longer than P_orb. The latter endured for at least 90 days, although probably mutating into a "late" superhump with a slightly longer mean period [0.05736(5) d]. The superhump appeared to follow familiar rules for such phenomena in dwarf novae, with components given by linear combinations of two basic frequencies: the orbital frequency omega_o and an unseen low frequency Omega, believed to represent the accretion disk's apsidal precession. Long time series reveal an intricate fine structure, with ~20 incommensurate frequencies. Essentially all components occurred at a frequency n(omega_o)-m(Omega), with m=1, ..., n. But during its first week, the common superhump showed primary components at n (omega_o)-Omega, for n=1, 2, 3, 4, 5, 6, 7, 8, 9 (i.e., m=1 consistently); a month later, the dominant power shifted to components with m=n-1. This may arise from a shift in the disk's spiral-arm pattern, likely to be the underlying cause of superhumps. The great majority of frequency components ... . (etc., abstract continues)

astro-ph↗

arXiv2001

GRB 000418: A Hidden Jet Revealed?

We report on optical, near-infrared and centimeter radio observations of GRB000418 which allow us to follow the evolution of the afterglow from 2 to 200 days after the gamma-ray burst. In modeling these broad-band data, we find that an isotropic explosion in a constant density medium is unable to simultaneously fit both the radio and optical data. However, a jet-like outflow with an opening angle of 10-20 degress provides a good description of the data. The evidence in favor of a jet interpretation is based on the behavior of the radio light curves, since the expected jet break is masked at optical wavelengths by the light of the host galaxy. We also find evidence for extinction, presumably arising from within the host galaxy, with A(V)=0.4 mag, and host flux densities of F_R=1.1 uJy and F_K=1.7 uJy. These values supercede previous work on this burst due to the availability of a broad-band data set allowing a global fitting approach. A model in which the GRB explodes into a wind-stratified circumburst medium cannot be ruled out by these data. However, in examining a sample of other bursts (e.g. GRB990510, GRB000301C) we favor the jet interpretation for GRB000418.

astro-ph↗

arXiv2001

Multi-Color Observations of the GRB000926 Afterglow

We present multi-color light-curves of the optical afterglow of GRB 000926. Beginning ~1.5 days after the burst, the light-curves of this GRB steepen measurably. The existence of such achromatic breaks are usually taken to be an important observational signature that the ejecta are not expanding isotropically, but rather have a collimated jet-like geometry. If we interpret the data in this context, we derive an opening angle of 5 degrees, which reduces the energy release compared to an isotropic model by a factor of ~275, to 1.7 x 10^51 erg. To fit the data with a simple jet model requires extinction along the line of sight. The derived A_V is in the range 0.11 - 0.82 mag, depending on the adopted extinction law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expected extinction from our Galaxy, we attribute this to the GRB host. We note that this extinction is typical of a galactic disk, and therefore the event likely took place in the disk of its host.

astro-ph↗

arXiv2000

A Neutron-Star Identification for the High-Energy Gamma-ray Source 3EG J1835+5918 Detected in the ROSAT All-Sky Survey

In the error box of 3EG J1835+5918, the brightest as-yet unidentified EGRET source at intermediate Galactic latitude, we find a weak, ultrasoft X-ray source at energies E < 0.3 keV in the ROSAT All-Sky Survey. Deep optical imaging at the location of this source, as pinpointed by an observation with the ROSAT HRI, reveals a blank field to a limit of V > 25.2. The corresponding lower limit on f_X/f_V is 300, which signifies that the X-ray source is probably a thermally emitting neutron star. Considering our previous complete multiwavelength survey of the 3EG J1835+5918 region which failed to find any other notable candidate for identification with 3EG J1835+5918, we propose that this X-ray source, RX J1836.2+5925, is a rotation-powered gamma-ray pulsar which is either older or more distant than the prototype Geminga. We see marginal evidence for variability between two ROSAT HRI observations. If real, this would indicate that the X-ray emission has an external origin, perhaps due to intermittent heating of the polar caps by a variable particle accelerator. RX J1836.2+5925 could even be an old, recycled pulsar, which may nevertheless have a high gamma-ray efficiency.

astro-ph↗

arXiv2000

GRB 991216 Joins the Jet Set: Discovery and Monitoring of its Optical Afterglow

The optical light curve of the energetic gamma-ray burst GRB 991216 is consistent with jet-like behavior in which a power-law decay steepens from t**(-1.22 +/- 0.04) at early times to t**(-1.53 +/- 0.05) in a gradual transition at around 2 d. The derivation of the late-time decay slope takes into account the constant contribution of a host or intervening galaxy which was measured 110 d after the event at R = 24.56 +/- 0.14, although the light curve deviates from a single power law whether or not a constant term is included. The early-time spectral energy distribution of the afterglow can be described as F_nu ~ nu**(-0.74 +/- 0.05) or flatter between optical and X-ray, which, together with the slow initial decay, is characteristic of standard adiabatic evolution in a uniformly dense medium. Assuming that a reported absorption-line redshift of 1.02 is correct, the apparent isotropic energy of 6.7 x 10**53 erg is reduced by a factor of ~ 200 in the jet model, and the initial half-opening angle is ~ 6 deg. GRB 991216 is the third good example of a jet-like afterglow (following GRB 990123 and GRB 990510), supporting a trend in which the apparently most energetic gamma-ray events have the narrowest collimation and a uniform ISM environment. This, plus the absence of evidence for supernovae associated with jet-like afterglows, suggests that these events may originate from a progenitor in which angular momentum plays an important role but a massive stellar envelope or wind does not, e.g., the coalescence of a compact binary.

astro-ph↗

arXiv2000

Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-ray Source

The EGRET source 3EG J1835+5918 is the brightest and most accurately positioned of the as-yet unidentified high-energy gamma-ray sources at high Galactic latitude (l,b=89,25). We present a multiwavelength study of the region around it, including X-ray, radio, and optical imaging surveys, as well as optical spectroscopic classification of most of the active objects in this area. The identified X-ray sources in or near the EGRET error ellipse are radio-quiet QSOs, a galaxy cluster, and coronal emitting stars. The radio sources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. In addition there are no flat-spectrum radio sources in the vicinity. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, 3EG J1835+5918 must be lacking one or more of the physically essential attributes of these known classes of gamma-ray emitters. If it is an AGN it lacks the beamed radio emission of blazars by at least a factor of 100 relative to identified EGRET blazars. If it is an isolated neutron star, it lacks the steady thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. If a pulsar, 3EG J1835+5918 must be either older or more distant than Geminga, and probably an even more efficient or beamed gamma-ray engine.

astro-ph↗