arXiv subjects

M. R. Garcia

Publications and source records attributed to M. R. Garcia.

At least 37 records · Page 2Linked to original sources

arXiv2004

Two New X-ray/Optical/Radio Supernova Remnants in M31

We compare a deep (37 ks) Chandra ACIS-S image of the M31 bulge to Local Group Survey narrow-band optical data and Very Large Array (VLA) radio data of the same region. Our precisely registered images reveal two new optical shells with X-ray counterparts. These shells have sizes, [S II]/H-alpha flux ratios, and X-ray spectral properties typical of supernova remnants (SNRs) with ages of 9$^{+3}_{-4}$ and 17$^{+6}_{-9}$ kyr. Analysis of complementary VLA data reveals the radio counterparts, further confirming that they are SNRs. We discuss and compare the properties and morphologies of these SNRs at the different wavelengths.

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arXiv2004

Correlated X-ray and Optical Variability in V404 Cyg in Quiescence

We report simultaneous X-ray and optical observations of V404 Cyg in quiescence. The X-ray flux varied dramatically by a factor of >20 during a 60ks observation. X-ray variations were well correlated with those in Halpha, although the latter include an approximately constant component as well. Correlations can also be seen with the optical continuum, although these are less clear. We see no large lag between X-ray and optical line variations; this implies they are causally connected on short timescales. As in previous observations, Halpha flares exhibit a double-peaked profile suggesting emission distributed across the accretion disk. The peak separation is consistent with material extending outwards to at least the circularization radius. The prompt response in the entire Halpha line confirms that the variability is powered by X-ray (and/or EUV) irradiation.

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arXiv2004

MMT Observations of the Black Hole Candidate XTE J1118+480 near and in Quiescence

We report on the analysis of new and previously published MMT optical spectra of the black hole binary XTE J1118+480 during the decline from the 2000 outburst to true quiescence. From cross-correlation with template stars, we measure the radial velocity of the secondary to derive a new spectroscopic ephemeris. The observations acquired during approach to quiescence confirm the earlier reported modulation in the centroid of the double-peaked Halpha emission line. Additionally, our data combined with the results presented by Zurita et al. (2002) provide support for a modulation with a periodicity in agreement with the expected precession period of the accretion disk of ~52 day. Doppler images during the decline phase of the Halpha emission line show evidence for a hotspot and emission from the gas stream: the hotspot is observed to vary its position, which may be due to the precession of the disk. The data available during quiescence show that the centroid of the Halpha emission line is offset by about -100 km/s from the systemic velocity which suggests that the disk continues to precess. A Halpha tomogram reveals emission from near the donor star after subtraction of the ring-like contribution from the accretion disk which we attribute to chromospheric emission. No hotspot is present suggesting that accretion from the secondary has stopped (or decreased significantly) during quiescence. Finally, a comparison is made with the black hole XRN GRO J0422+32: we show that the Halpha profile of this system also exhibits a behaviour consistent with a precessing disk.

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arXiv2004

Supersoft X-ray Sources in M31: I. A Chandra Survey and an Extension to Quasisoft Sources

We report on very soft X-ray sources (VSSs) in M31. In a survey which was most sensitive to soft sources in four 8'x8' regions covered by Chandra's ACIS-S S3 CCD, we find 33 VSSs that appear to belong to M31. Fifteen VSSs have spectral characteristics mirroring the supersoft X-ray sources studied in the Magellanic Cloud and Milky Way ($k T_{eff} \leq 100$ eV); we therefore call these ``classical'' supersoft sources, or simply supersoft sources (SSSs). Eighteen VSSs may either have small (< 10%) hard components, or slightly higher effective temperatures (but still < 350 eV). We refer to these VSSs as quasisoft sources (QSSs). While hot white dwarf models may apply to SSSs, the effective temperatures of QSSs are too high, unless, e.g., the radiation emanates from only a small portion of surface. Two of the SSSs were first detected and identified as such through ROSAT observations. One SSS and one QSS may be identified with symbiotics, and 2 SSSs with supernova remnants. Both SSSs and QSSs in the disk are found near star-forming regions, possibly indicating that they are young. VSSs in the outer disk and halo are likely to be old systems; in these regions, there are more QSSs than SSSs, which is opposite to what is found in fields closer to the galaxy center. The largest density of bright VSSs is in the bulge; some of the bulge sources are close enough to the nucleus to be remnants of the tidal disruption of a giant by the massive central black hole. By using Chandra data in combination with ROSAT and XMM observations, we find most VSSs to be highly variable, fading from or brightening toward detectability on time scales of months. There is evidence for VSSs with low luminosities ($\sim 10^{36}$ erg s$^{-1}$).

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arXiv2003

Discovery of Radio/X-ray/Optical Resolved Supernova Remnants in the Center of the Andromeda Galaxy

We have detected a spatially resolved supernova remnant (SNR) in the center of the Andromeda Galaxy, in radio, X-ray, and optical wavelengths. These observations provide the highest spatial resolution imaging of a radio/X-ray/optical SNR in that galaxy to date. The multi-wavelength morphology, radio spectral index, X-ray colors, and narrow-band optical imaging are consistent with a shell-type SNR. A second SNR is also seen resolved in both radio and X-ray. By comparing the morphological sturcture of the SNRs in different wavelengths and with that in our own Galaxy, we can study the shock morphologies of SNRs in the Andromeda Galaxy. The proximity of the SNRs to the core suggests high interstellar medium density in the vicinity of the SNRs in the center of the Andromeda Galaxy.

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arXiv2003

Resolved Jets and Long-Period Black Hole X-ray Novae

In this brief note we point out that the four spatially resolved relativistic jets among the 14 dynamically confirmed black hole X-ray novae are all in systems with long orbital periods. Many shorter period systems show transient radio outbursts which are attributed to jets, but these jets have not been spatially resolved. Super-Eddington accretion has been suggested as a requirement for jet formation and may be consistent with our compilation of luminosities, but some super-Eddington outbursts did not form spatially resolved jets. We speculate that some as yet unknown process (or combination of processes) favors formation of substantially larger jets in long period systems. Two short period systems show evidence for extent, but have not been resolved into multiple components as the long period systems have.

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arXiv2003

Time-resolved Spectroscopy of the M15 X-ray Binary AC211/X2127+119

We present time-resolved spectroscopy acquired during two epochs (spaced apart by ~15 days) of the eclipsing Low Mass X-ray Binary AC211/X2127+119 in the globular cluster M15. The spectra show variations in the HeII 4686 emission line not only modulated on the orbital period, but also on time-scales of a few days. During the first epoch of observation, the emission line shows a strong S-wave superimposed on the average double-peaked profile. The line exhibits no evidence of rotational disturbance at the orbital phases when the eclipse is observed in the optical continuum. During the second epoch, no double-peak or S-wave component is present. The HeI absorption lines detected by other authors are not present in our spectra. A Doppler image of the HeII 4686 for the first epoch supports the presence of the accretion disc. No hotspot is detected, although enhanced emission at V_X=30km/s, V_Y=160 km/s is observed. We discuss the implications of this emission in the context of an X-ray heated donor star, in which case a high mass ratio and neutron star primary are implied. Finally, we speculate on the possibility of a misaligned secondary star in AC211.

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arXiv2002

Chandra Studies of the X-ray Point Source Luminosity Functions of M31

Three different M31 disk fields, spanning a range of stellar populations, were observed by Chandra. We report the X-ray point source luminosity function (LF) of each region, and the LF of M31's globular clusters, and compare these with each other and with the LF of the galaxy's bulge. To interpret the results we also consider tracers of the stellar population, such as OB associations and supernova remnants. We find differences in the LFs among the fields, but cannot definitively relate them to the stellar content of the fields. We find that stellar population information, average and maximum source luminosities, X-ray source densities, and slopes of the LF are useful in combination.

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arXiv2002

The Discovery of a Spatially-Resolved Supernova Remnant in M31 with Chandra

Chandra observations of M31 allow the first spatially resolved X-ray image of a supernova remnant (SNR) in an external spiral galaxy. CXOM31 J004327.7+411829 is a slightly elongated ring-shaped object with a diameter of ~11'' (42 pc). In addition, the X-ray image hints that the chemical composition of the SNR is spatial dependent. The X-ray spectrum of the SNR can be well fitted with a Raymond-Smith model or a non-equilibrium ionization model. Depending on the spectral model, the 0.3-7 keV luminosity is between 3.2x10^36 erg/s and 4.5x10^37 erg/s. The age of the SNR is estimated to be 3210-22300 years and the number density of ambient gas is ~0.003-0.3 cm^-3. This suggests that the local interstellar medium around the SNR is low.

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arXiv2002

Spectroscopic observations of the candidate sgB[e]/X-ray binary CI Cam

We present a compilation of spectroscopic observations of the sgB[e] star CI Cam. This includes data from before, during, and after its 1998 outburst. The object shows a rich emission line spectrum originating from circumstellar material, rendering it difficult to determine the nature of either star involved or the cause of the outburst. We collate pre-outburst data to determine the state of the system before this occurred and provide a baseline for comparison with later data. During the outburst all lines become stronger, and hydrogen and helium lines become significantly broader and asymmetric. After the outburst, spectral changes persist for at least three years, with FeII and [NII] lines still a factor of ~2 above the pre-outburst level and HeI, HeII, and NII lines suppressed by a factor of 2-10. We find that the spectral properties of CI Cam are similar to other sgB[e] stars and therefore suggest that the geometry of the circumstellar material is similar to that proposed for the other objects: a two component outflow, with a fast, hot, rarefied polar wind indistinguishable from that of a normal supergiant and a dense, cooler equatorial outflow with a much lower velocity. We suggest that CI Cam is among the hotter members of the class and is viewed nearly pole-on. The nature of the compact object and the mechanism for the outburst remain uncertain, although it is likely that the compact object is a black hole or neutron star, and that the outburst was precipitated by its passage through the equatorial material. We suggest that this prompted a burst of supercritical accretion resulting in ejection of much of the material, which was later seen as an expanding radio remnant. [Abbreviated]

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arXiv2002

The bright Gamma-Ray Burst of February 10, 2000: a case study of an optically dark GRB

The gamma-ray burst GRB000210 had the highest gamma-ray peak flux of any event localized by BeppoSAX as yet but it did not have a detected optical afterglow. It is therefore one of the events recently classified as dark GRBs or GHOST (GRB Hiding Optical Source Transient), whose origin is still unclear. Chandra observations allowed us to localize this GRB within ~1" and a radio transient was detected with the VLA. We identify the likely (P=0.01) host galaxy of this burst at z=0.846. The X-ray spectrum of the afterglow shows intrinsic absorption N_H=5x10**21 cm-2. The amount of dust needed to absorb the optical flux of this object is consistent with the above HI column density, given a dust-to-gas ratio similar to that of our Galaxy. We do not find evidence for a partially ionized absorber expected if the absorption takes place in a Giant Molecular Cloud. We therefore conclude that either the gas is local to the GRB, but is condensed in small-scale high-density (n>~10**9 cm-3) clouds, or that the GRB is located in a dusty, gas-rich region of the galaxy. Finally, if GRB000210 lies at z>5, its X-ray absorbing medium would have to be substantially different from that observed in GRBs with optical afterglows.

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arXiv2002

X-ray Point Sources in The Central Region of M31 as seen by Chandra

We report on \chandra observations of the central region of M31. By combining eight \chandra ACIS-I observations taken between 1999 and 2001, we have identified 204 X-ray sources within the central $\sim 17'\times17'$ region of M31, with a detection limit of $\sim 2\times10^{35}$\lum. Of these 204 sources, 22 are identified with globular clusters, 2 with supernova remnants, 9 with planetary nebula, and 9 as supersoft sources. By comparing individual images, about 50% of the sources are variable on time scales of months. We also found 13 transients, with light curves showing a variety of shapes. We also extracted the energy spectra of the 20 brightest sources; they can be well fit by a single power-law with a mean photon index of 1.8. The spectral shapes of 12 sources are shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at $1.7\times10^{37}$\lum). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star-formation history might be different in different regions.

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arXiv2002

Two Years of the X-ray Sky in M31

We have been monitoring M31 with HRC and ACIS onboard \chandra regularly in the past two years. By combining eight \chandra ACIS-I observations taken between 1999 to 2001, we have identified 204 X-ray sources within the central $\sim 17\arcmin\times17\arcmin$ region of M31, with a detection limit of $\sim 1.6\times10^{35}$\lum. Of these 204 sources, 21 are identified with globular clusters, 2 with supernova remnants (one of them is spatially resolved with \chandra), and 8 with planetary nebula. By comparing individual images, about 50% of the sources are variable in time scales of months. We also found 14 transients. Combining all the available transients found in literatures, there are 25 transients in M31 and M32 detected by \chandra and \xmm; we present some of the long-term lightcurves by using the HRC, ACIS and \xmm data. The spectral shape of 12 sources is shown to be variable, suggesting that they went through state changes. The luminosity function of all the point sources is consistent with previous observations (a broken power-law with a luminosity break at $1.7\times10^{37}$\lum). However, when the X-ray sources in different regions are considered separately, different luminosity functions are obtained. This indicates that the star formation history might be different in different regions.

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arXiv2002

Bright X-Ray Sources in M31 Globular Clusters

We have conducted Chandra observations of ~2560 square arcmin (~131 kpc^2) of of M31, and find that the most luminous X-ray sources in our fields are in globular clusters (GCs). Of the 28 GC X-ray sources in our fields, 17 are newly discovered. Approximately 1/3 of all the sources have L_x([0.5-7] keV) > 10^{37} ergs/s; approximately 1/10 of all the sources have L_x([0.5-7] keV) close to or above 10^{38} erg/s. The most luminous source, in the globular cluster Bo 375, is consistently observed to have L_x greater than 2 x 10^{38} ergs/s. (1) We present data on the spectra and/or light curves of the 5 most luminous M31 GC sources. (2)We explore possible explanations for the high X-ray luminosities of the brightest sources. These include that the X-ray sources may be composites, the radiation we receive may be beamed, metallicity effects could be at work, or the sources may be accreting black holes. We weigh each of these possibilities against the data. In addition, we introduce a neutron star model in which mass transfer proceeds on the thermal time scale of the donor star. Our model can produce luminosities of several times 10^{38} ergs/s, and leads to a set of well-defined predictions. (3) We compute the X-ray luminosity function and the distribution of counts in wavebands that span the range of energies to which Chandra is sensitive. We find the peak X-ray luminosity is higher and that systems with L_x > 10^{37} ergs/s constitute a larger fraction of all GC sources than in the MW. (4) We study the possible reasons for this difference between M31 and Galactic GC X-ray sources and identify three promising explanations.

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arXiv2002

The X-ray Spectra of Black Hole X-ray Novae in Quiescence as Measured by Chandra

We present Chandra observations of black hole X-ray novae V404 Cyg, A0620-00, GRO J1655-40 and XTE J1550-564 in quiescence. Their quiescent spectra can be well fitted by a power-law model with slope $α\sim 2$. While a coronal (Raymond-Smith) model is also a statistically acceptable representation of the spectra, the best fit temperatures of these models is $\sim 5$ times higher than that seen in active stellar coronae. These four spectra of quiescent X-ray novae are all consistent with that expected for accretion via an advection-dominated accretion flow (ADAF) and inconsistent with that expected from a stellar corona. This evidence for continued accretion in quiescence further strengthens the case for the existence of event horizons in black holes. Both A0620-00 and GRO J1655-40 were fainter than in previous observations, while V404 Cyg was more luminous and varied by a factor of 2 in a few ksec. A reanalysis of the X-ray data for XTE J1550-564 shows that (like V404 Cyg and A0620-00) its luminosity exceeds the maximum prediction of the coronal model by a large factor. The 0.3-7 keV luminosity of the four sources studied ranges from $\sim 10^{30}-10^{33}$ erg/s.

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arXiv2001

Optical Spectral Monitoring of XTE J1118+480 in Outburst: Evidence for a Precessing Accretion Disk?

We present spectroscopic observations of the X-ray transient XTE J1118+480 acquired during different epochs following the 2000 March outburst. We find that the emission line profiles show variations in their double-peak structure on time scales longer than the 4.1 hr orbital period. We suggest that these changes are due to a tidally driven precessing disk. Doppler imaging of the more intense Balmer lines and the HeII 4686 line shows evidence of a persistent region of enhanced intensity superposed on the disk which is probably associated with the gas stream, the hotspot or both. We discuss the possible origins of the optical flux in the system and conclude that it may be due to a viscously heated disk.

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arXiv2001

The X-ray afterglow of GRB000926 observed by BeppoSAX and Chandra: a mildly collimated fireball in a dense medium ?

We present X-ray observations of the afterglow of GRB000926 performed around and after the break observed in the optical light curve two days after the burst. The steep X-ray light curve observed around the break confirms the presence of this feature in X-rays. However, the spectral and temporal properties are not consistent with a standard jet scenario based on synchrotron emission, requiring a more complicated model. We find that X-ray and optical data are compatible with a moderately collimated fireball (with opening angle $θ\approx25 °$) expanding in a dense medium ($n\approx 4\times 10^4$ cm$^{-3}$). This produces two breaks in the light curve. The first, at $t\approx$ 2 days, is due to jet behavior. The second, around 5 days, is attributed to the transition of the fireball to a non-relativistic expansion. This transition predicts a{\it flattening} of the light curve, that explains the late X-ray measurement in excess above the extrapolation of the simple jet scenario, and is also consistent with optical data.

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arXiv2001

A Black Hole of > 6 Solar Masses in the X-ray Nova XTE J1118+480

Observations of the quiescent X-ray nova XTE J1118+480 with the new 6.5-m MMT have revealed that the velocity amplitude of the dwarf secondary is 698 +/- 14 km/s and the orbital period of the system is 0.17013 +/- 0.00010 d. The implied value of the mass function, f(M) = 6.00 +/- 0.36 solar masses, provides a hard lower limit on the mass of the compact primary that greatly exceeds the maximum allowed mass of a neutron star. Thus we conclude that the compact primary is a black hole. Among the eleven dynamically established black-hole X-ray novae, the large mass function of XTE J1118+480 is rivaled only by that of V404 Cyg. We estimate that the secondary supplies 34% +/- 8% of the total light at 5900A and that its spectral type is in the range K5V to M1V. A double-humped I-band light curve is probably due to ellipsoidal modulation, although this interpretation is not entirely secure because of an unusual 12-minute offset between the spectroscopic and photometric ephemerides. Assuming that the light curve is ellipsoidal, we present a provisional analysis which indicates that the inclination of the system is high and the mass of the black hole is correspondingly modest. The broad Balmer emission lines (FWHM = 2300-2900 km/s) also suggest a high inclination. For the range of spectral types given above, we estimate a distance of 1.8 +/- 0.6 kpc.

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