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Recently the first all-sky image of the X-ray sky was released by the eROSITA instrument on-board the Spectrum-Roentgen-Gamma” (SRG) mission. It is fascinating that they refer to a million X-ray, which is almost 10 times more than what has been discovered from all X-ray surveys so far, so a bright X-ray future lies ahead! The image is captivating, especially when you try to pinpoint some of the brightest and interesting sources.

The energetic universe as seen with the eROSITA X-ray telescope. The first eROSITA all-sky survey was conducted over a period of six months by letting the telescope rotate continuously, thus providing a uniform exposure of about 150-200 seconds over most of the sky, with the ecliptic poles being visited more deeply. As eROSITA scans the sky, the energy of the collected photons is measured with an accuracy ranging from 2% – 6%. To generate this image, in which the whole sky is projected onto an ellipse (so-called Aitoff projection) with the centre of the Milky Way in the middle and the body of the Galaxy running horizontally, photons have been colour-coded according to their energy (red for energies 0.3-0.6 keV, green for 0.6-1 keV, blue for 1-2.3 keV). The original image, with a resolution of about 10”, and a corresponding dynamic range of more than one billion, is then smoothed (with a 10’ FWHM Gaussian) in order to generate the above picture.The red diffuse glow away from the galactic plane is the emission of the hot gas in the vicinity of the solar system (the Local Bubble). Along the plane itself, dust and gas absorb the lowest energy X-ray photons, so that only high-energy emitting sources can be seen, and their colour appears blue in the image. The hotter gas close to the galactic centre, shown in green and yellow, carries imprinted the history of the most energetic processes in the life of the Milky Way, such as supernova explosions, driving fountains of gas out of the plane, and, possibly, past outburst from the now dormant supermassive black hole in the centre of the galaxy. Piercing through this turbulent, hot diffuse medium, are hundreds of thousands of X-ray sources, which appear mostly white in the image, and uniformly distributed over the sky. Among them, distant active galactic nuclei (including a few emitting at a time when the Universe was less than one tenth of its current age) are visible as point sources, while clusters of galaxies reveal themselves as extended X-ray nebulosities. In total, about one million X-ray sources have been detected in the eROSITA all-sky image, a treasure trove that will keep the teams busy for the coming years.

Credit: Jeremy Sanders, Hermann Brunner and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI)

Source: Max Planck Institute for Extraterrestrial Physics, Our deepest view of the X-ray sky, (accessed 26 June 2020)

Annotated version of the eROSITA First All-Sky image. Several prominent X-ray features are marked, ranging from distant galaxy clusters (Coma, Virgo, Fornax, Perseus) to extended sources such as Supernova Remnants (SNRs) and Nebulae to bright point sources, e.g. Sco X-1, the first extrasolar X-ray source to be detected. The Vela SNR is to the right of this image, the Large Magellanic Cloud in the bottom right quadrant, the Shapley supercluster in the upper right (though not easily visible in this projection).

Credit: Jeremy Sanders, Hermann Brunner, Andrea Merloni and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI)

Source: Max Planck Institute for Extraterrestrial Physics, Presskit for the eROSITA First All-Sky Survey (accessed 26 June 2020)

Building upon our previous ATel #12224 we managed to obtain an optical spectrum of the counterpart. The full text follows:

ATel #12237; G. Maravelias (NOA), V. Antoniou (TTU/SAO), K. Boutsia (LCO), A. Zezas (UoC/SAO), A. Z. Bonanos (NOA), F. Haberl (MPE), D. Hatzidimitriou (UoA/NOA)

on 21 Nov 2018; 23:36 UT /

In ATel #12224 we reported the Hα emission, derived from a wide-field photometric survey of the Small Magellanic Cloud (Maravelias et al. 2017, IAUS 329, 373; Maravelias et al. 2019, in prep.), of the proposed optical counterpart source [M2002] 20671 to the X-ray transient XTE J0052-723 pulsar (SXP 4.78; Swift J005139.2-721704; ATel #12209). In addition to this clear detection, the work of Bonanos et al. (2010, AJ, 140, 416) showed that the IR colors of this source ([2dFS]0811; J=15.54 mag, J-[3.6]=0.56 mag) are indicative of a “photometric” Be star, defined as sources with an intrinsic color of JIRSF-[3.6]>0.5 mag.

However, to confirm the nature of the counterpart optical spectroscopy is needed. We obtained optical spectra using the LDSS-3 spectrograph on the 6.5m Clay/Magellan telescope (Las Campanas Observatory). The observations were performed on Nov. 20, 2018, acquiring two exposures of 600s each with a 1″ slit using the VPH-All grism (resulting in a 2630 — 10859 Å wavelength range, at a nominal dispersion of 1.9 Å/pix). The spectrum shows a clear single-peaked Hα line in emission, with a corresponding equivalent width of -10.65±0.14 Å. This is the first optical spectroscopic confirmation of the presence of Hα in emission. We note that Hβ appears in emission as well.

The spectral classification was based on the blue part of the spectrum and the classification scheme used in Maravelias et al. (2014, MNRAS, 438, 2005). The presence of the OII+CIII 4640-4650 blend and the strong HeI 4471 line point to an early-type star (spectral type up to B2), which is consistent with the absence of the MgII 4481 line and the weak SiII 4552 triplet line. The HeII 4200 and 4686 lines, which are indicative of B0.5 and earlier-spectral types, are absent, thus pointing to later types.

Combining these criteria we constrain the optical classification to a B1-2e source (with an error of 0.5 subtype). This is consistent with the early-type B star classification proposed in ATel #12229, as well as the previous classification of B0-B3 by Evans et al. (2004, MNRAS, 351, 601; source ID [2dFS]0811).

Thus, we provide the first optical spectrum with Hα emission of the optical counterpart of XTE J0052-723, further confirming its Be/X-ray binary nature.

The ATel #12224 was released on 17 Nov 2018 (19:01 UT) regarding the identification of the proposed optical counterpart to the source XTE J0052-723 (SXP 4.78; Swift J005139.2-721704) as an Hα emitting OB star. The full text follows.

ATel #12224; G. Maravelias (NOA), V. Antoniou (TTU/SAO), A. Zezas (UoC/SAO), A. Strantzalis (UoA), D. Hatzidimitriou (UoA), F. Haberl (MPE)

on 17 Nov 2018; 19:01 UT /

ATel #12209 reported the detection of a new X-ray transient in the Small Magellanic Cloud (SMC), Swift J005139.2-721704, exhibiting outbursting activity. The system has been classified as a new SMC high-mass X-ray binary based on its identification with the B-type star [M2002]20671. NICER followed up this source immediately (ATel #12219), reporting an absorption-corrected luminosity of LX ~7×1037erg s-1 (0.5-8 keV; 60 kpc). Temporal analysis of NICER and Fermi/GBM observations (ATel #12222) identified Swift J005139.2-721704 with the known X-ray pulsar XTE J0052-723 (SXP 4.78). Evans et al. (2004, MNRAS, 353, 601) obtained a refined B0-3(III) spectral type for [M2002]20671 (= [2dFS]0811) based on optical spectroscopy, but they do not report Hα emission.

Here, we report the identification of the SMC star [M2002]20671, and thus XTE J0052-723, with an Hα emitting source, which confirms XTE J0052-723 as a Be/X-ray binary (Be-XRB) pulsar in the SMC. This source is identified in Hα and R-band observations covering the SMC almost completely (Maravelias et al. 2017, IAUS 329, 373) using the Wide Field Imager on the 2.2m MPG/ESO telescope at La Silla (16/17 November 2011) and the MOSAIC-II camera on the 4m Blanco telescope at Cerro Tololo (15/16 December 2011).

The Hα emitting source is found at RA=00:51:38.78 and DEC=-72:17:04.7 (better than 0.2″ relative astrometry with respect to the 2MASS catalog). It is located 2.2″ away from the Swift position (ATel #12209), with Hα and R-band magnitudes equal to 15.476±0.006 and 15.613±0.008, respectively. The resulting Hα-R color is -0.137±0.010 mag with an SNR~13 above the continuum and an Hα excess significance of ~6σ above the Hα-R baseline value. The only nearby source with significant Hα-R excess is much fainter (Hα and R ~22 mag), has a lower SNR (~4) and is located at a larger distance from the X-ray source (~6″).

Observations with the IMACS f/2 camera on the 6.5m Magellan Telescope (4 October 2004) give B=15.652±0.009 mag and I=15.524±0.017 mag (Strantzalis et al. 2018, IAUS 344, 453; Strantzalis et al. 2019, in prep.), compatible with an OB star.

All the identifications described above secure the nature of Swift J005139.2-721704 = XTE J0052-723 = SXP 4.78 as Be-XRB pulsar in the SMC.

Optical spectra of 5 new Be/X-ray Binaries in the Small Magellanic Cloud and the link of the supergiant B[e] star LHA 115-S 18 with an X-ray source

Grigoris Maravelias, Andreas Zezas, Vallia Antoniou, Despoina Hatzidimitriou

The Small Magellanic Cloud (SMC) is well known to harbor a large number of High-Mass X-ray Binaries (HMXBs). The identification of their optical counterparts provides information on the nature of the donor stars and can help to constrain the parameters of these systems and their evolution. We obtained optical spectra for a number of HMXBs identified in previous Chandra and XMM-Newton surveys of the SMC using the AAOmega/2dF fiber-fed spectrograph at the Anglo-Australian Telescope. We find 5 new Be/X-ray binaries (BeXRBs; including a tentative one), by identifying the spectral type of their optical counterparts, and we confirm the spectral classification of an additional 15 known BeXRBs. We compared the spectral types, orbital periods, and eccentricities of the BeXRB populations in the SMC and the Milky Way and we find marginal evidence for difference between the spectral type distributions, but no statistically significant differences for the orbital periods and the eccentricities. Moreover, our search revealed that the well known supergiant B[e] star LHA 115-S 18 (or AzV 154) is associated with the weak X-ray source CXOU J005409.57-724143.5. We provide evidence that the supergiant star LHA 115-S 18 is the optical counterpart of the X-ray source, and we discuss different possibilities of the origin of its low X-ray luminosity (Lx ~ 4 x 10^33 erg/s).

arXiv:1312.0593

In the following tables are given the transformation factors of the fluxes from a specific X-ray energy band to the 0.5-7 band, assuming Gamma 1.7 with Nh 6e20 and 0.

Table 1 >>>

Gamma    Nh   MinE MaxE   ModeledFx  Fx_in_0.5-7_band
1.7   6.0E20  2.0  25.0   7.4353E-9  1.524470492137473
1.7   6.0E20  0.1  2.0    2.177E-9   0.44635349756468984
1.7   6.0E20  3.0  10.0   3.234E-9   0.6630717726446831
1.7   6.0E20  2.0  30.0   8.2332E-9  1.6880650974147609
1.7   6.0E20  2.0  10.0   4.0686E-9  0.8341910736421079
1.7   6.0E20  0.3  10.0   6.2155E-9  1.2743731397684144
1.7   6.0E20  0.7  10.0   5.6647E-9  1.1614417444752492
1.7   6.0E20  0.2  10.0   6.245E-9   1.2804215595974016
1.7   6.0E20  0.2  12.0   6.836E-9   1.401595187981426
1.7   6.0E20  0.5  7.0    4.8773E-9  0.9999999789407894
1.7   6.0E20  0.1  10.0   6.2456E-9  1.2805445712067978
1.7   6.0E20  0.2  0.5    2.8144E-10 0.05770405966125076
1.7   6.0E20  0.5  1.0    7.3497E-10 0.1506919758411837
1.7   6.0E20  1.0  2.5    1.6072E-9  0.32952659047456384
1.7   6.0E20  0.5  2.0    1.895E-9   0.3885346266886183
1.7   6.0E20  1.0  2.0    1.16E-9    0.23783650481957802
1.7   6.0E20  2.0  4.5    1.7909E-9  0.36719086048976335
1.7   6.0E20  4.5  12.0   2.8688E-9  0.5881943055328245
1.7   6.0E20  2.5  7.0    2.5352E-9  0.5197957988828394
1.7   6.0E20  0.5  10.0   5.9636E-9  1.222725654804594
1.7   6.0E20  2.5  10.0   3.6215E-9  0.742521474746644

Table 2 >>>

Gamma Nh   MinE MaxE  ModeledFx  Fx_in_0.5-7_band
1.7   0.0  2.0  25.0  7.4574E-9  1.4222179900505934
1.7   0.0  0.1  2.0   3.9046E-9  0.7446553253977446
1.7   0.0  3.0  10.0  3.2422E-9  0.618327465238828
1.7   0.0  2.0  30.0  8.2554E-9  1.5744063925281622
1.7   0.0  2.0  10.0  4.0902E-9  0.7800515263759988
1.7   0.0  0.3  10.0  6.9496E-9  1.3253742741638779
1.7   0.0  0.7  10.0  5.8724E-9  1.119938952929684
1.7   0.0  0.2  10.0  7.3779E-9  1.407056368582311
1.7   0.0  0.2  12.0  7.9692E-9  1.519824622996969
1.7   0.0  0.5  7.0   5.2435E-9  0.999999972639108
1.7   0.0  0.1  10.0  7.9948E-9  1.5247068517737434
1.7   0.0  0.2  0.5   1.0472E-9  0.1997139302730967
1.7   0.0  0.5  1.0   1.0005E-9  0.19080766971838653
1.7   0.0  1.0  2.5   1.696E-9   0.32344807992770214
1.7   0.0  0.5  2.0   2.2406E-9  0.42730998909437085
1.7   0.0  1.0  2.0   1.2401     2.3650234120924503E8
1.7   0.0  2.0  4.5   1.8094E-9  0.34507484756230744
1.7   0.0  4.5  12.0  2.872E-9   0.547725750200595
1.7   0.0  2.5  7.0   2.547E-9   0.4857442441663391
1.7   0.0  0.5  10.0  6.3307E-9  1.2073424171358944
1.7   0.0  2.5  10.0  3.6342E-9  0.6930866886631255 

[Tables calculated through XSPECv12 by Vallia Antoniou]